Planetary and Space Science 59 (2011) 810–834

Contents lists available at ScienceDirect

Planetary and Space Science

0032-06

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/pss

Review Article

State studies of Earth’s plasmasphere: A review

A.K. Singh a,n, R.P. Singh b, Devendraa Siingh c

a Physics Department, University of Lucknow, Lucknow 226007, Indiab Space Physics Laboratory, Physics Department, B.H.U., Varanasi 221005, Indiac Indian Institute of Tropical Meteorology, Pune 411008, India

a r t i c l e i n f o

Article history:

Received 29 July 2010

Received in revised form

6 January 2011

Accepted 21 March 2011Available online 2 April 2011

Keywords:

Whistlers

Plasmasphere

Plasmapause

Erosion/recovery

33/$ - see front matter & 2011 Elsevier Ltd. A

016/j.pss.2011.03.013

esponding author. Tel.: þ91 9415371523.

ail address: aksphys@gmail.com (A.K. Singh).

a b s t r a c t

The plasmasphere sandwiched between the ionosphere and the outer magnetosphere is populated by

up flow of ionospheric cold (�1 eV) and dense plasma along geomagnetic field lines. Recent

observations from various instruments onboard IMAGE and CLUSTER spacecrafts have made significant

advances in our understanding of plasma density irregularities, plume formation, erosion and refilling

of the plasmasphere, presence of thermal structures in the plasmasphere and existence of radiation

belts. Still modeling work and more observational data are required for clear understanding of

plasmapause formation, existence of various sizes and shapes of density structures inside the plasma-

sphere as well as on the surface of the plasmapause, plasmasphere filling and erosion processes; which

are important in understanding the relation of the process proceeding in the Sun and solar wind to the

processes observed in the Earth’s atmosphere and ionosphere.

& 2011 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 810

2. Plasmasphere dynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 811

2.1. Convection–corotation model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 812

2.2. Plasmasphere dynamics associated with convection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813

2.3. Plasmapause formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814

2.4. Dayside bulge and structures at the plasmapause . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 815

3. Erosion and recovery of plasmasphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

3.1. Erosion of plasmasphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

3.2. Refilling of plasmasphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 819

4. Waves in plasmasphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 821

5. Plasma density distribution in the plasmasphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824

5.1. Plasma density observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824

5.2. Models of plasmaspheric density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 825

6. Radiation belts and plasmasphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 826

7. Thermal structure of plasmasphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 827

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 829

1. Introduction

The plasmasphere is the torus of cold (�1–2 eV), dense(102–104 cm�3) plasma containing Hþ (�80%), Heþ (�10–20%)and Oþ (5–10%) ions and trapped by geomagnetic field (Lemaire

ll rights reserved.

and Gringauz, 1998). It is essentially an extension of the iono-sphere at high altitudes. In this region magnetic field lines areclosed and approximately dipolar, permitting filling from thedayside ionosphere; the plasma expands upward from the day-side ionosphere into space along the geomagnetic field lines,slowly filling the dayside flux tube with cold plasma. The east-ward rotation of the Earth’s magnetic field along with daysidefilling produces torus of cold plasma of ionospheric origin.The plasmasphere demonstrates the influence of both the

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 811

magnetosphere and ionosphere (Singh and Singh, 2005). Theplasmasphere acts as a plasma reservoir by maintaining thenighttime ionosphere through inward flow. During prolongedperiods of very low geomagnetic activity, filling from the iono-sphere is dominant and plasmasphere becomes quite large. Theouter region may cross geosynchronous orbit having no distinctouter boundary (Goldstein et al., 2002).

The existence of plasma in the plasmasphere for the first timewas conclusively proved by Storey (1953) on the basis of whistlerobservations, contrary to the belief that plasma density ratherrapidly decrease above the ionospheric F-region (Marasigan,1958). Since then whistler studies were used to study electrondensity distribution in the equatorial altitude as well as along thegeomagnetic field lines under different geomagnetic condition(Morgan and Allcock, 1956; Helliwell et al., 1956; Smith andHelliwell, 1960; Smith, 1961; Pope, 1961; Carpenter, 1962).Carpenter (1963) detected a sharp decrease in the electrondensity in the equatorial plane and called it ‘‘Knee’’, which wasalso observed by the Soviet satellite/Lunik moon probes(Gringauz, 1963; Carpenter, 1965).

In the early sixties, the basic concept of plasmasphere emergedas the region of the inner magnetosphere where corotationdominates over the cross tail electric fields. Carpenter (1966)subsequently proposed the term plasmasphere and plasmapausein analogy with the terms magnetosphere and magnetopause.

The plasmaspheric studies have been reviewed by Lemaire andGringauz (1998) and Lemaire and Storey (2001). Ganguli et al.(2000) stressed the need of more detailed understanding in thearea of plasmaspheric dynamics during geomagnetic storms,refilling of plasmasphere after a geomagnetic storm and couplingbetween the plasmasphere and ionosphere. These areas are beingcontinuously explored both by refinements in measuring toolsand advances in modeling and numerical simulations. For exam-ple, Calvert et al. (1995) presented a method of satellite-basedradio sounding of plasmapause, and Meier et al. (1998) proposedan image inversion technique for remote sensing in the EUVrange, which was confirmed by Nakamura et al. (2000). Plasmadensity structures/irregularities have also been measured usingsatellite-beacon radio-interferometer arrays (Jacobson et al.,1996; Hoogeveen and Jacobson, 1997). The ground-based mag-netometer data and signals from GPS satellites provided proto-tomographic capabilities of plasmasphere (Dent et al., 2003;Foster et al., 2002). In-situ measurements onboard various satel-lites such as magnetospheric plasma analyzer (MPA)-Los AlmosGeosynchronous satellites (Moldwin et al., 1995), CRRES sweepfrequency receiver (Carpenter et al., 2000; Summers et al., 2008),Alebono (Kimura et al., 1997), INTERCOSMOS-24 (Bankov et al.,1993), MAGION-2 (Boskova et al., 1993) OGO-4 (Taylor et al.,1971); OGO-5 (Chappell et al., 1970a) and Hinotori (Tanaka,1983) have refined and augmented our knowledge of the globalelectron density distribution within the plasmasphere.

The above measurements could not provide a global perspec-tive of plasmasphere, although some results were available usingmodel and simulations. Magnetospheric imaging from spacecould achieve this aim and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite (Burch et al., 2001)containing RPI (radio plasma instrument) (Reinisch et al., 2000)and EUV imager (Sandel et al., 2001) was launched whichincluded a low power radar with three dipole antennas coveringas frequency range 3 kHz to 3 MHz, falling within the domain ofthe free space L–O and R–X wave modes at the satellites altitudesabove �20,000 km. Masson et al. (2009) was able to locateregions of various plasma densities, irregularities by observingreflected radar echoes from the plasma. EUV detected extremeultraviolet radiation at 30.4 nm of solar origin resonantlyscattered by thermal Heþ and provided global images of

plasmasphere at a time cadence of 10 min with a nominal spatialresolution of 0.1RE in the equatorial plane from orbit apogee orbetter. EUV images could provide temporal evolution of plasma-spheric dynamics from storm onset and erosion through recoveryand refilling. RPI and EUV observation could provide electrondensity distribution in the equatorial plane as well as alonggeomagnetic field lines including density structures of variousscale sizes. Also, plasmapause and its structures have been wellcharacterized (Darrouzet et al. (2009) and references there in).

CLUSTER spacecraft provide a meridian view of plasmasphericdensity in contrast with IMAGE which provides global twodimensional view of the plasmasphere. Four spacecrafts in theCLUSTER cross the plasmasphere near perigee around the fourEarth radii every 57 h from southern to northern hemisphere(Escoubet et al., 1997). Electron density is derived from WHISPERobservations of low frequency cut-off of natural plasma emis-sions, which operates between 2 and 80 kHz with a frequencyresolution of 162 Hz. WHISPER contains a pulse transmitter, awave receiver and a wave spectrum analyzer and provides betterspatial resolution, accurate measurements, 3-D view of plasma-sphere and the lifetimes of density structures, global dynamics ofstructures including resolution and lifetimes have already beenmeasured in the plasmasphere (Decreau et al., 2005; Darrouzetet al., 2008).

CLUSTER and IMAGE based instruments added new dimen-sions in the study of plasmaspheric density structure, erosion andrefilling. The small scale, medium scale and the large scale densitystructures along the field lines have been observed. Field-alignedirregularities with scale widths in the range 200–800 m are oftenobserved in the plasmasphere beyond L¼2.5 (Carpenter et al.,2002). The density levels within the irregularities remainedwithin a range 1–30% of the background. Decreau et al. (2005)have studied morphology and dynamics of small scale irregula-rities. Even density structures of various shapes have beenobserved at the plasmapause and outside the plasmapause(Darrouzet et al., 2009). The plasmasphere erosion initiates nearmidnight and propagates eastward and westward with a finitespeed and covers entire nightside plasmasphere within few hours(Spasojevic et al., 2003; Goldstein and Sandel, 2005; Gallagherand Adrian, 2007). Global view of refilling model has beendeveloped for the disturbed plasmasphere (Gallagher et al.,2005; Sandel and Denton, 2007; Galvan et al., 2008) as well asquiet plasmasphere (Yoshikawa et al., 2003; Tu et al., 2007). Theplasmasphere rarely fills to saturation level i.e. in diffusiveequilibrium with the ionosphere.

In this paper, we have reviewed some characteristic features ofplasmasphere based on observations including IMAGE and CLUSTERmissions. Section 2 briefly discusses plasmaspheric dynamicsincluding corotation, convection, dayside bulge and structures atplasmapause. Plasmaspheric erosion and refilling are presented inSection 3. Section 4 deals with the waves in plasmasphere. Plasmadensity distribution in the plasmasphere, observations and modelsare described in Section 5. Radiation belts and thermal structure ofthe plasmasphere are discussed in Sections 6 and 7, respectively.Brief conclusion is given in Section 8.

2. Plasmasphere dynamics

The plasmasphere is a dynamic system and contains lowtemperature (a few eV) plasma embedded by geomagnetic fieldwhich is closed dipolar in nature. The plasmasphere is populatedby ionospheric plasma, warm ring current particles and energeticradiation belt particles. The heavier ions of ionospheric origindominate the plasma composition. The densities of these popula-tion vary over several orders of magnitude and the plasma

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834812

composition is also variable (Goldstein, 2006). The dayside iono-spheric plasma leaks up into space along geomagnetic field lines,slowly filling dayside flux tubes. The eastward rotation of theEarth’s magnetic field combined with dayside filling produces atorus of cold plasma of ionospheric origin. The plasmasphereduring prolonged quiet periods (when ionospheric filling is thedominant effect) can become quite large (reaching beyondL¼6.62, geosynchronous orbit). In such a situation outer bound-ary becomes indistinguishable (Goldstein et al., 2003).

The plasmasphere experiences forcing at low altitude by theionosphere and at high altitudes by the solar wind-inducedmagnetospheric electric fields. As a result plasmasphere under-goes cyclic evolution. The solar wind disturbance increases theflank-to-flank electric potential difference across the magneto-sphere and the dawn-to-dusk electric field becomes stronger,consequently the outer region of the plasmasphere is erodedaway. The plasmasphere develops a sharp density gradient at theouter region, called plasmapause. During strong disturbance, theeroded material can form a plume in the afternoon local timesector with nightside edge of the plume footpoint consideringwith the intense electric fields associated with subauroral iono-spheric ion drifts or subauroral polarization streams. Afterdisturbance is subsided, the electric field recovers and theplasmasphere starts refilling from the ionospheric plasma. Therefilling time scale can be hours to days.

The rotation of the Earth and magnetospheric convectionproduces electric fields, which complicate scenario of physicalprocesses in the plasmasphere. Solar and geomagnetic activitiesinfluences morphological features of plasmaspheric plasma andalso affect the shape of the plasmasphere. The first model ofplasmasphere was proposed as convection–corotation model,which could specify the shape of the plasmasphere and locationof the plasmapause. Deformation in shape and various structuresobserved inside the plasmasphere and at the plasmapauserequired further modification and detail studies. Some of theseproblems have been studied with the data available from IMAGEand CLUSTER spacecrafts (Keyser et al., 2009). An extreme ultra-violet imager onboard IMAGE produces global pictures of theplasmasphere and radio sounder remotely senses the environ-ment of the spacecraft. The four spacecraft configuration of theCLUSTER mission provides observations at four nearby points inthe plasmasphere. These non-local measurements provide newinsight in the morphology of plasmaspheric plumes or notches(Darrouzet et al., 2009) which challenges current model for thedynamic evolution of plasmasphere (Pierrard et al., 2009).

Fig. 1. Equipotential map of total electric field including convection and corotation ef

values of the potential are in kV and are given on the contour drawn every 2 kV. The

(credit—Pierrard et al., 2008).

2.1. Convection–corotation model

The corotation of geomagnetic field along with the Earthproduces n�B electric field in the low-altitude ionosphere whichpenetrates into the high altitude plasmasphere and forces thecold trapped plasma to corotate with the angular velocity of theEarth. In the equatorial plane, the corotation electric field is givenby (Kivelson and Russell, 1995)

ER ¼B0

L3

� �LREw

where w¼7.272�10�5 s�1 is the angular rotation frequency ofthe Earth, B0¼3.1�10�5 T is the equatorial magnetic field at thesurface of the Earth, RE¼6371 km is the Earth radius. The othersource of large scale electric field is the convection electric fieldcontrolled by the solar wind condition and the level of geomag-netic activity. This electric field directed from dawn to dusk, tendsto produce equipotentials parallel to the Sun–Earth axis. In theconvection–corotation model, the outer boundary of the plasma-sphere, the plasmapause is approximately located where thesetwo fields (i.e. corotation and cross tail fields) are equal. For atypical plasmapause location of L¼4RE, cross tail field¼1 mV/m(Nishida, 1966). The toroidal surface of the plasmasphere inter-sects the Earth’s surface at high latitudes (for L¼4, MLAT¼601),roughly corresponding to the equatorward boundary of theauroral zone. This model suggests a teardrop shape that willchange in the size with changes in solar/geomagnetic activity.Beyond the plasmasphere, magnetic field lines are drawn west-ward and stretched down the magnetotail on the nightside.

The convection electric fields, which depend on geomagneticactivity, modify the plasma corotation velocity. Thus plasmacorotation depends on the geomagnetic activity. For KPo2,corotation is observed along all field lines whose L-value issmaller than about 4. For KP42, departure from corotation isobserved (Pierrard et al., 2009; Reinisch et al., 2009). Thedeparture from corotation is prominent in the post-midnightsector, where large azimuthal convection velocities in the east-ward direction are observed (Alcayde et al., 1986). This causeslarger angular convection velocity in this local time sector(Lemaire, 1985).

The above observations lead to modifications in the simpleconvection–corotation model. Stern–Volland (Volland, 1973;Stern, 1974) proposed a model in which a semi-empirical shield-ing effect was included. The polarization electric fields (i.e., partial

fects for KP¼6 and using VSMC (left panel) and E5D (right panel) are shown. The

dotted circle corresponds to L¼6.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 813

ring current from tail) from the plasmasheet could penetrate intothe plasmasphere and shield it from convection electric field.

2.2. Plasmasphere dynamics associated with convection

Plasmaspheric studies show dynamic behavior in response tosolar activity and geomagnetic conditions. Semi-empirical,empirical and mathematical models of convection electric fieldsincluding dependence of KP and IMF intensity and direction withvarious degrees of sophistications have been developed (Nishida,1966; Volland, 1973; Stern, 1974; Sojka et al., 1986; Ebihara andEjiri, 2002). The Volland–Stern model is a simple mathematicalmodel considering a uniform dawn–dusk electric potential dis-tribution across the magnetosphere. The potential in corotatingframe of reference is represented by a scalar potential. UsingOGO-3 and OGO-5 satellite data, Maynard and Chen (1975)

Fig. 2. Equipotential map of the Weimer convection–corotation field using solar wind p

magnetospheric magnetic field is used to map electric potential from the ionospheric al

drawn every 4 kV.

(after—Reinisch et al., 2009).

obtained KP dependence of the empirical model, is known asVolland–Stern–Maynard–Chan (VSMC) model.

Another analytical representation of convection electric poten-tial depending on KP index was proposed by McIlwain (1986) andconstants of the model were deduced from ATS-5 and ATS-6particle flux measurements at geosynchronous altitude. Themodel known as E5D depend on the three hourly KP index. Themodel is derived by fitting the observed position of the injectionboundary. Pierrard et al. (2008) compared the two electricpotential models for different KP-values and showed that theE5D model is less sensitive to KP than VSMC model. Fig. 1 showsthe equipotential contour maps of the equipotentials for the E5Dand VSMC models for KP¼6 (Pierrard et al., 2008). It is seen thatthe last closed equipotential is found to be everywhere closer tothe Earth for the VSMC model than for the E5D model. E5D modelproduces stronger and showed that the E5D model has lessshielding effect in the dusk sector as compared to dawn sector.

arameters corresponding to April 17, 2002. Tsyganenko and Stern (1996) model of

titude to the equator. The values of the potential (in kV) are given on the contours

Fig. 3. Electric potential patterns in corotating frame at IEF¼0.2 and 1.7 mV m�1 are shown. Contour intervals are 1 and 5 kV for thin and thick lines, respectively.

(credit—Reinisch et al., 2009).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834814

Unlike the above two models, Weimer (1996) proposed anelectric field model driven by solar wind parameters (IMF magni-tude, solar wind velocity and dipole tilt angle) in which electricpotential is represented by an expansion in spherical harmonics as afunction of the geomagnetic co-latitude and magnetic local time(MLT). The coefficients were derived by a least error fit from multiplesatellite measurements of the ionospheric convection velocity. Fig. 2shows the equatorial contour maps of Weimer’s model derived fromsolar wind parameters during the geomagnetic event of April 17,2002 (Reinisch et al., 2009). This model generally produces strongerelectric field than E5D and VSMC. Also in this model shielding is lessefficient in the dawn sector as compared to dusk sector. There areother complex ionospheric and magnetospheric electric field modelsused to study the effects of inner magnetospheric convection on ringcurrent dynamics (Jordanova et al., 2004) and on storm-time particleenergization (Khazanov et al., 2004).

All the above electric field models are based on the assumptionthat magnetic field lines are equipotentials, which is not validduring geomagnetic storm periods and in the regions where field-aligned currents exist. Further, these models are also based on theionospheric measurements. In order to overcome these difficul-ties, a model based on measurements in the inner magnetospherenear the equator has been developed, which is known as UNH-IMEF model (Matsui et al., 2008). The model is based on a largedata base collected from Electric Drift Instrument (EDI) andElectric Field and Waves (EFW) onboard CLUSTER spacecrafts.The model is being studied by comparing stimulated and mea-sured values. The model is available on website- http://edi.sr.unh.edu/unh-imef/. Fig. 3 shows contours of electric potential patternsin the corotating frame for IEF (Interplanetary Electric Field)0.2 and 1.7 mV m�1 (Reinisch et al., 2009). It is noted that theelectric field strength increases as the IEF increases. Further, theequipotential contours often rotate around the Earth on the dusk-side which indicates ionospheric shielding of the electric field.

2.3. Plasmapause formation

The convective electric field plays key role in the formation ofthe plasmapause. During high geomagnetic activity period, theouter region of the plasmasphere is stripped and removed and theplasmapause location moves closer to the Earth surface (Lemaire,1987). The response in the plasmapause movement is delayed onthe order of hours relative to KP index. Based on whistler data,Carpenter and Park (1973) proposed KP-dependent empirical

relation for the plasmapause location LPP¼5.70–0.47KP, valid forthe midnight dawn sector. The maximum value of KP index duringthe preceding 12 h has to be used in the computation of LPP.O’Brien and Moldwin (2002) studied degree of correlationbetween LPP and KP, Dst, and AE indices for various time delays.The EUV data from IMAGE have been used to define the locationof plasmapause position which changes with variation in solarwind conditions. Larsen et al. (2007) have studied the location ofplasmapause LPP using data from EUV and solar wind conditionsprovided by ACE mission, which is given by

LPP ¼ 4:38þ0:0374Bz�1:05� 10�4f ð1Þ

where Bz is the southward component of IMF and f is the

magnetic merging proxy defined by f¼nB sin2(y/2), n is the solar

wind speed, B is the total IMF field strength and y is IMF clockangle and can be defined in terms of IMF By and Bz as

y¼ arccosðBz=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiBy

2þBz

2q

Þ) (Larsen et al., 2007). The best fit for

LPP with the observed value was obtained for separately delayed

Bz and f where f is also a function of solar wind. The delay times

are 155 and 275 min for Bz and f, respectively. Delay time isrequired because solar wind parameters are being measured byACE and events will take time to reach the magnetosphere fromthere. The time-dependent simulation of plasmapause formationusing convection electric field models were successfully com-pared with observations of plasmapause using EUV/IMAGE data(Pierrard and Cabrera, 2005, 2006). Fig. 4 depicts the differencesbetween the equatorial location of plasmapause obtained withthe different mechanisms (MHD and flux-tube interchange) afterthe geomagnetic storm of April 17, 2002 (Pierrard et al., 2009). Inthe upper panel, plasmapause simulated results using the E5D,VSMC and Weimer models of electric potential in the MHDmechanisms are presented. VSMC and Weimer models producea plasmapause position closer to the Earth than E5D model. Thecomputations are made for the models at 21:00 UT. The variationof KP, Dst and Bz corresponding to storm period from April 16,2002, 00:00 UT up to April 18, 2002, 24:00 UT are shown in themiddle panel of the figure. Interchange simulated results usingE5D model are shown in the bottom panel for the sake ofcomparison, The IMAGE EUV observation of the equatorial plas-mapause position at 21:07 UT is also shown in the bottom rightpanel. The white circle corresponds to L�2, 4, 6 and 8, respec-tively. It is clearly seen that in the midnight sector, VSMC andWeimer electric field models lead to a plasmapause position too

Fig. 4. Upper panel—the equatorial cross section of plasmasphere derived from MHD simulations using E5D (left), VSMC (middle) and Weimer (right) after the magnetic

storm of April 17, 2002. Middle panel—Bz, Dst and KP-values from April 16, 2002, 00:00 UT to April 18, 2002, 24:00 UT are given. Bottom panel—interchange simulations

with E5D (left) and observations of IMAGE EUV at 21:07 UT (right) are shown. The white circle on EUV-IMAGE corresponds to L¼2, 4, 6 and 8, respectively.

(credit—Pierrard et al., 2008, 2009).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 815

close to the Earth as compared to the EUV observations. Theresults obtained with E5D model are close to the EUV observa-tions. In the noon sector, EUV results are reproduced when E5Dmodel is used in the interchange mechanism. CLUSTER observa-tions have also been used to study the plasmapause formation(Schafer et al., 2007, 2008). Pierrard and Stegen (2008) presenteda complete three dimensional model of plasmasphere combiningdynamic simulations of plasmapause formation and kinetic modelof the plasmasphere developed by Pierrard and Lemaire (2001).

2.4. Dayside bulge and structures at the plasmapause

The simple model of plasmapause shape driven by combinedcorotation and convection electric fields suggests a teardropshape with the dusk-side bulge. The substorm depolarizationinjects plasma into the ring current which inflates the geomag-netic field leading to an induced electric field which pulls theplasmapause outward to form a 1–2RE bulge. The bulge isobserved to exhibit more dynamical behavior than predicted bythe above simple model. After the formation of the bulge, theionospheric closure of the partial ring current may generate awestward subauroral polarization stream (SAPS) flow which

removes the bulge. The net global effect is an outward-then-inward motion that propagates westward along the plasmapause.The phenomenon is known as plasmapause undulation (Goldsteinet al., 2005, 2007).

Fig. 5 shows the shape of plasmasphere in the equatorial planewith a bulge in the dusk sector. The average size of plasmapauseis larger for dusk-side than for a dawn-side magnetic local time.Early models (Grebowsky, 1970) offered explanations for theseobservations. Sunward convection erodes the outer layers of theplasmasphere, removing plasma and creating a steep plasma-pause boundary. In the model, L-value is inversely dependentupon KP value and MLT shape is influenced by a dusk-sidestagnation region where sunward convection and eastward cor-otation are oppositely directed. Steepening of plasmapause andan inward motion of plasma has also been associated with theconsequences of the dynamic balance between centrifugal andother forces (Lemaire, 1974).

One of the interesting feature of the plasmasphere is theformation of detached plasma regions in the afternoon sector,which can be either detached clouds (Chappell, 1974), or fila-mentary extensions of the bulge (tail or streamers) (Chen andGrebowsky, 1974). The IMAGE and CLUSTER satellites provided

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834816

considerably advance observations of the plasmasphere dynamicsand revealed new degree of structure both within the plasma-sphere and at the plasmapause. The three EUV imagers, withoverlapping fields of view onboard IMAGE satellite are sensitiveto the 30.4 nm line scattered by Heþ (Sandel et al., 2000). Sincethe plasmasphere contains about 5–10% Heþ , this permits directimaging of the plasmasphere when the IMAGE is near apogee.Using such images when apogee occurs well above/below theequatorial plane, the images may be reprojected into the plane ofmagnetic equator and the location of plasmapause extracted(Fig. 6) (Green et al., 2000).

The EUV images showing different structures such as plumes,notches, shoulders, fingers, channels, and crenulations are pre-sented in Fig. 7. Plumes are produced even with small increaseof geomagnetic activity. Plumes are formed and developed inthree phases (sunward surge, plume narrowing, plume rotating)(Goldstein and Sandel, 2005). During an intense geomagneticactivity, a strong negative solar wind electric field (Bz being

Fig. 6. Schematic illustration of the IMAGE spacecraft orbit in the magnetosphere. The m

RPI on IMAGE is able to simultaneously perform radio sounding upward toward the m

(credit—Green et al., 2000).

Fig. 5. The shape of the plasmasphere in the equatorial plane showing distinct

bulge region.

(credit—Carpenter, 1966).

southward) develops which initiates a sunward surge of plasma-spheric plasma. As a result the nightside plasmapause movesinward whereas dayside plasmapause moves outward and abroad sunward pointing plume is formed. The development ofplume formation for the event of June 18, 2001 using EUV imagesare shown in Fig. 8b–d) (Goldstein and Sandel, 2005). The daysideplume maintains its sunward orientation during continued highactivity but becomes progressively narrower in local times(Fig. 8e–h). The weakening of geomagnetic activity relaxes theplume’s sunward orientation and the plume begins to rotateeastward with the rest of the plasmasphere (Fig. 8i–l). Thesephases of plume formation have been observed in numerousstudies (Sandel et al., 2003; Goldstein et al., 2002, 2003, 2005;Goldstein and Sandel, 2005; Abe et al., 2006; Kim et al., 2007).Denser plumes are observed at small L-value (around 5–7) and inall MLT sectors except morning sector. Plasmaspheric plumesignatures have also been seen in the ionosphere (Yizengawet al., 2008) and they play a crucial role in the mid-latitudeionospheric density enhancements (Yizengaw et al., 2006). Thisenhancement may result from a strengthening of the equatorialion fountain due to electric fields in vicinity of the South AtlanticAnomaly (Foster et al., 2005). Spasojevic et al. (2004) have alsodiscussed plume’s association with enhanced wave growth thatcan lead to pitch-angle scattering and energization of particles.

Notches are characterized by the radial density depletion inthe outer plasmasphere and may also include earlier observeddensity cavities (Carpenter et al., 2002). Time development ofnotches observed by IMAGE and CLUSTER reveal departure fromcorotation in the plasmasphere. The departure in corotation isexplained in terms of corresponding motion of plasma in theionosphere (Burch et al., 2004). Sometimes notches include acentral prominence of enhanced plasma density. This also pro-vides evidence that the explanation provide by Burch et al. (2004)for the departure from corotation does not always work(Gallagher et al., 2005). Decreau et al. (2004) showed that notcheswere often associated with both continuum radiation observed atthe high end of WHISPER frequency range and intense electro-static emissions. Electrostatic emissions are primary source ofcontinuum radiation. Green et al. (2004) demonstrated thata notch structure is typically a critical condition for the genera-tion of kilometric continuum radiation. However, notches do not

agnetospheric Ne cavity extends from the plasmapause to the magnetopause. The

agnetopause and downward toward the plasmapause.

Fig. 7. Structures observed by the EUV instrument onboard IMAGE and new morphological nomenclature: examples of shoulders, plumes, fingers, channels, crenulations

and notches. The direction to the Sun is shown as a yellow dot for each image (for interpretation of the references to color in this figure legend, the reader is referred to the

web version of this article).

(credit—Darrouzet et al., 2009) (from http://image.gsfc.nasa.gov/poetry/discoveries/N47big.jpg).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 817

always provide the conditions necessary for the generation of theemission (Masson et al., 2009). More details are required tounderstand the relation between waves and notches. The trackingof notches for long duration (30–60 h) provides evidence ofdeparture of plasmaspheric plasma corotation with the Earth(Sandel et al., 2003). The departures from corotation may bedriven by corresponding motions of plasma in the ionospherewhere departures from corotation are often observed (Sandelet al., 2003; Burch et al., 2004; Galvan et al., 2008). Gallagher et al.(2005) suggested that a dawn–dusk asymmetry in the electricpotential resulting from gradients in the Hall conductance at theterminator may lead to sub-corotational drift where amplitudedepends on storm phase. At present clear picture is not availablein favor of one or the other mechanism. Further studies arerequired to pin-point the probable mechanism.

Low density channels are usually located between the plasma-sphere and the plume. Its formation could be explained in termsof differential rotation of the western edge of a plume in L andstagnation of the eastern edge (Spasojevic et al., 2003). Thiswould lead to the wrapping of the plume around the mainplasmasphere. This is clearly seen in Fig. 9 where channeldevelopment on June 10, 2001 in the pre-midnight sector whereplume wrapped around the atmosphere (Sandel et al., 2003).

The shoulders appear as a sharp azimuthal gradient in plasmausually observed during sharp increases in geomagnetic activity.Goldstein et al. (2002) explained the formation of shoulder by theresidual of over shielding of the convection electric field following thesudden weakening of convection during the turning of the IMF fromsouthward to northward. Goldstein and Sandel (2005) studied globalpattern of evolution of plasmaspheric drainage plumes and foundmodulation of plasmapause location by a few tenths of RE betweenthe dawn-side terminator and the west edge of the plume duringstorm period. The events are termed as crenulations. Even duringdeep quiet periods irregular features called fingers are observed at theplasmapause. They are attributed to arise from some type ofresonances of ultra-low frequency waves (Adrian et al., 2004).

Medium scale spatial structures were observed more fre-quently during quieter intervals in the data of IMAGE and

CLUSTER. Goldstein and Sandel (2005) suggested that the increasein structural complexity during early and deep recovery phasescould be explained by the consideration of streamline effect offlow. In the case of strong flow streamlines are closer, leading todecreased transverse scale size. In such a situation plasmapausedensity gradient will be steeper and plasmapause shape will belaminar. Streamlines with weaker flow will be widely spaced andtransverse size of medium scale structure would be larger. In thiscase plasmapause will become more uneven.

IMAGE and CLUSTER spacecrafts have presented more com-plex features of plasmasphere and plasmapause. Plasmasphericirregularities of small, medium and large sizes during stormperiods and deep quiet periods have been observed, some ofwhich are still unexplained. This reflects an incomplete under-standing of plasmaspheric electric fields and quantitative effectsof ULF waves and plasma instabilities on the distribution of coldplasma in the plasmasphere and at the plasmapause.

3. Erosion and recovery of plasmasphere

3.1. Erosion of plasmasphere

The plasma density at the outer boundary of plasmaspheredecreases by about two orders of magnitude. The region withdecreased density is known as plasmapause first observed byCarpenter (1963) using whistler data and by Gringauz (1963)using direct measurement. The plasmapause density gradient isformed by peeling-off the relatively dense plasmasphere flux dueto variable geomagnetic conditions (Chen and Wolf, 1972).Plasmaspheric flux tubes are dominated by Earth’s corotationalelectric field and are continually supplied by fluxes from thedayside ionosphere until a diffusive equilibrium is reached. Day-side filling combined with the eastward rotation of the Earthproduces a torus of cold plasma of ionospheric origin.

The plasma experiences gravitational and centrifugal forces.The latter arises due to corotation. The gravitational forcedecreases with altitude and there is a limit beyond which

Fig. 8. Figure shows different phases of plume development obtained from EUV images on June 18, 2001 mapped to the equatorial plane in SM coordinates. Sun is on the

right and dashed circles are at L¼2, 4, 6, 6.6, respectively. In the bottom panel variation of solar wind electric field is shown. The electric field is negative when the IMF is

southward.

(credit—Goldstein and Sandel, 2005).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834818

gravitational force is balanced by centrifugal force. The limit iscalled Zero Radial Force (ZRF) surface. Beyond ZRF any externalforce acting on plasma will be directed away from the Earth(Lemaire, 1987). During intense geomagnetic activity the convec-tion electric field intensifies leading to enhanced centrifugal effectsand as a consequence blocks of plasma may be detached in thenight local time sector due to the development of plasma inter-change instability. The electric conductivity of the lower iono-sphere limits the growth rate of plasma interchange instability.

Nishida (1966) modeled the plasmasphere erosion duringgeomagnetic activity in terms of convection electric field. Butdynamical details of erosion remains a major challenge toscientists because complex electric fields that develop at subaur-oral latitudes during the process of plasmasphere erosion are notwell understood. Density structures observed in the plasmapauseregion suggest that waves and instabilities play significant role inthe erosion/recovery cycle which remain unaccessed. Electrondensity interior to a new formed plasmapause boundary tend tobe reduced by a factor of up to three in association with theerosion process, so that refilling during recovery occurs there aswell as in the more deeply depleted plasma-trough region andbeyond (Carpenter and Lemaire, 1997). The number of electron

lost from this interior region through interchange with the iono-sphere, can be of the order of 50% of the number lost from beyondthe new boundary through flow perpendicular to the geomag-netic equatorial field.

In the absence of direct global scale or mesoscale observations,various effects of plasmasphere erosion are seen through anec-dotal data. We often see the density profile of an eroded plasma-sphere as well as outlying dense features in a data taken alongindividual satellite orbits, but do not know how some originalplasmasphere configuration evolved so far as to appear this way.IMAGE and CLUSTER spacecrafts have provided data considerablyimproved in temporal and spatial resolution.

IMAGE observations show that initial erosion starts in theplasmasphere near midnight that widens and spreads eastwardand westward encompassing the entire nightside plasmaspherewithin a few hours (Goldstein and Sandel, 2005; Gallagher andAdrian, 2007). Removal of plasma occurs at different times fordifferent MLTs, so that the effects of erosion propagate with afinite speed from the initial position. This propagation effect hasbeen observed in all the analyzed events of erosion observed byEUV/IMAGE (Goldstein and Sandel, 2005). RPI/IMAGE observa-tions also exhibit the dramatic loss of plasma along the magnetic

Fig. 9. Top—EUV images showing channel development and wrapping of main body of the plasmasphere recorded on June 10, 2001. Sun is to the left. Bottom—figure

shows mapping of the prominent brightness gradients on to the geometric equator panel in (L, MLT) space. The yellow fill makes the channel (for interpretation of the

references to color in this figure legend, the reader is referred to the web version of this article).

(credit—Sandel et al., 2003).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 819

field lines. Reinisch et al. (2004) analyze the data of 31 March2001 storm, showed that outer plasmaspheric flux tubes lostmore than 66% of their plasma in less than 14 h and refillingoccurred over a period of 10 days. Using data from both IMAGEand ground-based magnetometers, Dent et al. (2006) found thehistory of local conditions to be important in erosion and refillingand they also observed limited emptying effects from shortintervals of enhanced convection.

A similar finite propagation of plasma erosion also occursduring transient disturbances of the plasmapause produced bybursts of convection associated with substorms (Goldstein et al.,2005, 2007). This may be explained by considering the plasmainjection into the ring current by substorm depolarization. Geo-magnetic field is inflated and induces an electric field which pullsthe plasmapause outward to form a bulge of 1–2RE. Ionosphericclosure of the partial ring current then generates a westwardsubauroral polarization stream flows that removes the bulge(Goldstein et al., 2007). The net result is an outward-then-inwardmotion that propagates westward along the plasmapause.

3.2. Refilling of plasmasphere

The concept of refilling of the plasmasphere was introduced byNishida (1966). He proposed that within the plasmapause, theflux tubes corotate with the Earth under the influence of Earth’scorotation electric field and flux tubes outside the plasmapauseare dominated by the mangnetospheric convection electric fieldand intersected the dayside magnetopause. These flux tubes atthe time of intersection with the magnetopause become open andvent their plasma into interplanetary space and hence plasmadensity into these flux tubes become quite less as compared tothat of flux tubes within the plasmapause. When the intensity ofconvection decreases from a pre-existing level, the region ofcorotating flux tubes is expanded beyond the ‘‘original plasma-pause’’ and this outer region of depleted but newly corotating fluxtubes undergoes the processes of refilling from the ionosphericplasma. The process refilling continues until plasma flux reachesinto diffusive equilibrium prior to intersection with magneto-pause. Diffusive equilibrium is rarely reached. During the earlystage of refilling, plasma density in the flux tube is very low

(o1 cm�3) and the plasma flows from the top of the ionospherefrom the both hemisphere. The plasma flow is supposed to besupersonic initially (Banks et al., 1971; Grebowsky, 1972; Chenand Wolf, 1972; Schulz and Koons, 1972). The plasma flowingfrom the both hemispheres interact in the equatorial region.In a qualitative refilling scenario, Lemaire (1989) proposed thatballistic and escaping particles from the ionosphere enter empty/partially empty flux tubes and setup a filled-aligned densitydistribution exceeding that of equatorial exosphere in less thanfour hours along all refilling flux tubes.

The plasmasphere refilling processes were studied using obser-vations and theoretical techniques. Direct observations of plasmadensity accumulations (Park, 1974; Horwitz et al., 1984, 1986;Sojka and Wrenn, 1985; Decreau et al., 1986), global structures ofplasmasphere (Nagai et al., 1985; Gallagher et al., 1988; Farrugiaet al., 1989; Horwitz et al., 1990; Carpenter and Anderson, 1992)and field-aligned flow in the outer regions of plasmasphere(Hoffman and Dodson, 1980; Sojka et al., 1983; Chandler andChappel, 1986; Olsen et al., 1985; Menietti et al., 1988; Saxton andSmith, 1989) during refilling processes were made. In addition tothese phenomena, an important aspect refilling is the trapping andthermalization of field-aligned flowing ions in the equatorialregion. Plasma observations near the equator in the vicinity ofthe plasmapause often reveal plasma pitch-angle distributionswith peaks near 901 pitch angle (pancake or trapped distributions)and characteristic energies of several eV’s and above (Horwitz andChappell, 1979; Comfort and Horwitz, 1981; Olsen et al., 1987;Sagawa et al., 1987). Early observations relevant to plasmasphererefilling have been reviewed by Singh and Horwitz (1992).

The interactions of plasma up flowing from the ionosphere ofboth hemispheres in the equatorial region are modeled usingsingle stream and two streams fluid models and semi-kineticmodels. In the single fluid model, the plasma flows from con-jugate ionosphere were treated as a single fluid and a set oftransport equations including the thermal anisotropies of bothelectrons and ions are solved (Khazanov et al., 1984; Singh et al.,1986). An inherent feature of this model is that an electrostaticshock pair is automatically forms when the plasma flowing fromboth hemispheres merges. The shocks propagate downward andaffect refilling processes. The flow becomes subsonic when the

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834820

shocks reach the critical points near the top side conjugateionosphere. In this mode refilling takes place from the equator(top) to the ionosphere (bottom).

Various plasma processes involved in the refilling phenomenaare schematically shown in Fig. 10 (Singh and Hwang, 1987). Theplasma flow from the ionosphere along the flux tube is character-ized by large scale phenomena. The processes dealing withtrapping and thermalization (from supersonic or subsonic flow)are small scale phenomena and involve plasma–plasma andwave–particle interactions. In schematic diagram extended per-pendicular ion heating is shown by the thick arrow, which occurswhen the interactions cause energy gain in the degree of freedomperpendicular to the magnetic field. Intense equatorial ion heat-ing and the resultant downward parallel electric fields andformation of a pair of electromagnetic shocks propagating down-ward with a velocity vs are also depicted. These processes havebeen discussed by Singh and Horwitz (1992). The downwardelectric fields reflect the plasma streams coming from the con-jugate ionosphere and retard refilling processes.

The formation of electrostatic shocks at the equator cannot beexamined in the single fluid model (Rasmussen and Schunk, 1988).Therefore, the plasma originating from the conjugate ionosphereswere treated as separate fluids and two stream model was developed(Rasmussen and Schunk, 1988) in which interstream interactionsoccur through Coulomb collisions and polarization electric fields.Under suitable plasma conditions, inter penetrating streams excitethe ion–ion instability which can effectively couple the ion beamsforming a pair of electrostatic shocks (Singh, 1988, 1990).

In the semi-kinetic model the ions are treated as kineticgyrocenters and the electrons are treated with a Boltzmanndescription (Wilson et al., 1992; Lin et al., 1992). This modelincludes the effects of Coulomb collisions (Lemaire, 1990). In thismodel filling occurs from the ionosphere (bottom) to the equator(top) and the predicted plasma behavior is quite different fromthat seen in hydrodynamic model (single stream and twostreams). Even shocks are not observed either at the equator ornear the ionospheric mirror locations. The trapping/accumulating

Fig. 10. Schematic diagram showing a flux tube and some plasma processes such as (a)

arrow), (b) formation of a pair of electrostatic shocks at the equator, vs is the velocity o

plasma), and (c) equatorial ion heating and the resultant downward parallel electric fi

(credit—Singh and Hwang, 1987).

effects of Coulomb collision allow the development of highdensities in the refilling flux tubes. The semi-kinetic modelcontains information on the evolution of the ion distributionfunctions under the influence of velocity distribution functionsunder the influence of collisions, wave–particle interaction andvelocity dispersions (Lin et al., 1992), which are difficult to studyfrom the hydrodynamic model.

At lower altitudes, collisions with the neutral gas dampen theflow and enhance the Hþ temperature (Guiter et al., 1991). Alsoion temperature is enhanced in the shocked plasma. Since shock’svelocity is proportional to the square root of temperature, theshocks move faster and shorten the lifetime of supersonic flows inthe flux tubes (Guiter and Gombosi, 1990; Singh, 1991) and hencerestrain the initial refilling process. During refilling processestemperature anisotropy evolves (Singh and Torr, 1990) which isformed to have profound effect on refilling processes through aforce proportional to T?�TJ (Comfort, 1988), where T? and TJ areion temperatures perpendicular and parallel to the magnetic field,respectively. This anisotropic force is upward for T?4TJ anddownward for T?oTJ. Observations from DE-1 and SCATHAsatellites have shown the presence of equatorially trapped Hþ

(predominant) ions confined within 31 of magnetic equator in theouter plasmaspheric flux tubes having perpendicular temperaturein the range 5–100 eV and temperature anisotropy (T?/TJ)�10–50(Olsen et al., 1987). These ions can be produced by perpendicularheating of the thermal ions by the waves (Curtis, 1985). Theperpendicular heating of field-aligned ions flowing in the equator-ial region inhibits the inter-hemispheric plasma exchange due tothe mirror force on the heated ions.

In the hydrodynamic models of shock formation near theequator and propagating to higher latitudes, higher density nearthe equator should be observed. Reinisch et al. (2004) analyzedEUV data corresponding to the magnetic storm of March 31, 2001but they did not find any evidence for a higher density near theequator. Also they did not observe the significance of minimumdensity at the equator as required by non-shock formation model(Schulz and Koons, 1972).

extended perpendicular heating of the ions in the plasma stream (shown by thick

f shocks moving away from the equator (and leaving behind a dense thermalized

eld reflects the plasma streams coming from the conjugate ionosphere.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 821

The wave–particle interaction may lead to either pitch-anglescattering or energization of ions. The energization of ions in theequatorial regions produces a variety of pitch-angle distributionsranging from broadened magnetic field-aligned beams and ionconics to the pancake types of distributions depending on thenature and location of interactions (Lin et al., 1992). Some of thesedistributions have been observed (Comfort and Horwitz, 1981;Sagawa et al., 1987; Sojka et al., 1983). The difference betweenthe pitch-angle anisotropies of electrons and ions generateselectric fields (Chiu and Schulz, 1978). This has been supportedby the numerical simulations of equatorial trapping of ions. Thisdownward electric field can reflect the field-aligned flow andretard the rate of refilling. Even in the absence of wave–particleinteraction, the hot protons can enhance the electromagneticfluctuation level that can critically affect the low energy upflowing thermal ions (Singh and Hwang, 1987). A quantitativeestimate of these effects on the rate of refilling has not beenassessed.

Guglielmi et al. (1995) invoked that the presence of Alfvenwaves in the plasmasphere can enhance the outflow of iono-spheric ions because of pondermotive force effect and hence mayenhance the refilling processes. Based on the interaction, Feyginet al. (1997) calculated refilling rates for various wave intensitiesand found the effect to be positive.

Krall et al. (2008) studied dynamical refilling model in whichrefilling occurs from the ionosphere to the equator and heobtained dynamical field-aligned density distribution for particlesof ionospheric origin. For this type of refilling model, Lemaire(2001) suggested that during refilling of flux tubes, the decelera-tion deflection of the upwelling plasma streams may generateintense electrostatic emissions. The same have been observed byWHISPER onboard CLUSTER, on partially depleted flux tubesbeyond the plasmapause confined between 721 latitude closeto the equatorial surface (El-Lemdani Mazouz et al., 2006).

Sandel and Denton (2007) using EUV for six consecutiveIMAGE orbits derived radial profiles of Heþ column abundanceand showed an orderly increase in column abundance with time.Fig. 11 shows the abundance profiles increase steadily withincreasing time, reflecting refilling of the plasmasphere. Forunambiguous cases of refilling, Gallagher et al. (2005) foundaverage refilling rates at the equator of 3.8 Heþ cm�3 h�1 atL¼2.75 and 2.7 Heþ cm�3 h�1 at L¼3.25. These rates are basedon limited sample in space and time. Galvan et al. (2008)

Fig. 11. Radial profiles of Heþ column abundance for six times during the refilling

study. The abundance profiles increase steadily with increasing time, reflecting

refilling of the plasmasphere. The black lines without points show column

abundances calculated from SUPIM for the same observing geometry.

(after—Sandel and Denton, 2007).

investigated diurnal variation in Heþ abundance extending refill-ing studies to shorter time scales and found that Hþ abundance isdominated by up flow from the sunlit ionosphere and down flowsinto the night ionosphere. Further measured variations show nodependence on geomagnetic activity. In fact extended quietperiods (KPo1þ) are required for refilling to significantly pro-ceed. Recent observation showed significant refilling in less than28 h near 2.5RE (Reinisch et al., 2004) but still insufficient to reachsaturation level, which is rarely achieved. By saturation, we meanthe plasmasphere to be in diffusive equilibrium with the iono-sphere. Comparing the measured plasma densities with theoreti-cally (multi-species kinetic model) predicted value, Reynoldset al. (2003) showed that the plasmasphere could be filled atmost 25% of saturation value within three days of very quietgeomagnetic activity, and density was still increasing after thisperiod. These examples suggested that the plasmasphere refillingrate depends in a very complex way on the ionosphere, magneto-sphere and solar wind conditions.

These are various physical processes involved in the plasma-sphere erosion and refilling which are not yet understood (Singhand Horwitz, 1992; Lemaire and Gringauz, 1998). For instance it isnot known, how and where a plasmapause boundary is formed ata new location during a period of enhanced disturbance activity?One wants to know what kinds of erosive effects occur at or nearthe nightside interface between the cold plasma of plasmasphereand hot plasma of inner edge of the plasmasheet. Anotherimportant question is: are subauroral ion drifts (SAIDs) importantin plasmasphere erosion (Anderson et al., 1993; Carpenter et al.,1993; Ober et al., 1997)? Recent observations show that in theaftermath of geomagnetic disturbances, complex density struc-tures are developed in the region of plasmapause density gradi-ents and in the outer plasmasphere (Darrouzet et al., 2009). Tools(both theory and simulations) capable of investigating on a globalscale and at various stages of erosion and recovery and theinterplay between solar wind-induced magnetospheric convec-tion and Earth’s rotation is required.

4. Waves in plasmasphere

Waves play an important role in the overall dynamics of theplasmasphere by influencing its transport properties. Particularly,they are key to understand the way mass and energy aretransferred from the magnetotail to the plasmasphere, the iono-sphere and finally to the atmosphere. They also act as a usefuldiagnostic tool of the plasma state and the relevant physicalprocesses (Singh, 2010). Small scale waves can dissipate energyand influence the thermal state of the plasmasphere, whereaslarge scale waves can reflect the global morphology of theplasmasphere (Khazanov et al., 1996). Waves propagating in theplasmasphere interact with the charged particles present inthe medium and can accelerate or decelerate them. Particles canalso be diffused into the loss cone and precipitate to loweraltitudes. In turn waves can also be amplified or damped.

Various waves are found in this region from a few mH to afew MHz, both electrostatic and electromagnetic. Ground-basedobservation and space mission since the 1950s have collected awealth of information about waves in the plasmasphere andderived valuable information about plasmaspheric propertiesand processes (Lemaire and Gringauz, 1998). Low frequencywhistler mode waves were the first observed waves in spaceplasma (Parks, 1991) and were used for diagnostics (Storey,1953). Most of the early information on the plasmasphericparameters like electron density, total electron content of a fluxtube, downward transport of electron flux, properties of ductspresent in the medium, large scale electric field, global shape of

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834822

the plasmasphere, location of the plasmapause and on the factthat plasma density undergoes cycles of erosion and recoveryduring magnetically disturbed periods were derived from theground-based whistler observations (Carpenter, 1963, 1966,1970; Park, 1972, Park et al., 1978; Singh and Singh, 1997,1999; Singh et al., 1998a; Carpenter, 2004). The discovery of abulge-like extension in the plasmasphere in the dusk sector(Carpenter, 1966, 1970, 1983) stimulated early efforts to interpretthe shape of plasmasphere in terms of the interplay between themotion of plasma imposed by the Earth’s rotation and sunwardflow driven by solar wind interaction with the magnetosphere(Nishida, 1966; Brice, 1967). The analysis of long time collectedwhistler data identified major temporal variations in the plasmadensity ranging from hours to eleven years (Park et al., 1978;Clilverd et al., 1991; Singh, 1995). Whistlers also made it possibleto study the interchange of plasma between the ionosphere andthe plasmasphere and to compute the rate at which upwardfluxes from the ionosphere refilled/depleted overlying regions ofplasmasphere (Park, 1974) and also to classify the role of plasma-sphere as a reservoir for the decaying nighttime ionosphere (Parkand Banks, 1974).

Reinisch et al. (2000) developed tools to extract informationabout whistler mode waves from data obtained by the RPIsounder onboard IMAGE. These tools include (a) magnetosphericreflections (MR) at the location where the wave frequency rthelocal lower hybrid resonance frequency, (b) specular reflection(SR) from the steep density gradients at the bottom side of theionosphere, and (c) multipath propagation and scattering due tothe presence of field-aligned density irregularities. The presenceof field-aligned irregularities are seen on the spectra of all typeswhistler mode signals in the form of spreading in travel-time,varying from 5–10 to 40 ms (Sonwalkar et al., 2009). The spread-ing could have been due to propagation on multiple pathsthrough irregular region with transverse B0 scale size �1–10 kmor by forward and backward scattering from irregularities withscale size �10–100 m. The IMAGE RPI has added to previousstudies by providing nearly instantaneous measurements of thetrue field-aligned electron densities, two dimensional profiles ofdensity across 20 min of orbital passes and repeated transits ofplasmasphere. The data are very useful in developing moreaccurate and time-dependent models.

Whistlers mode signals frequently observed in the plasma-sphere are chorus emissions in a frequency range from a fewhundred Hz to several kHz. These waves have been observed bythe ground-based equipments as well as onboard spacecraft(Storey, 1953; Sazhin and Hayakawa, 1992; Singh et al., 2000).The generation mechanism of chorus emission is not yet wellunderstood. The widely accepted mechanism is a non-linearprocess involving the electron cyclotron resonance of whistlermode waves with energetic electrons (Nunn et al., 1997;Trakhtengerts et al., 2004; Singh and Patel, 2004). Whistler modechorus emissions have received increased attention in connectionwith the acceleration of energetic electrons in the radiation belt(Horne et al., 2005).

CLUSTER spacecraft have produced important new resultsabout the position and size of the chorus source region andpropagation of chorus from the source region. The source lies nearL¼4–4.4 in the equatorial region. The extent of the source parallelto the field line is �3000–5000 km (Santolık et al., 2004, 2005)and perpendicular to the field is �60–200 km (Santolık et al.,2004). The central position of the chorus source fluctuates at timescales of minutes within few thousands of km of the magneticequator with a typical speed of 100 km s�1 (Santolık et al., 2004).Results show that chorus can propagate to low attitudestowards the Earth if it is generated with earthward inclined wavevectors (Santolik et al., 2006; Bortnik et al. 2007). Chorus waves

propagate with slightly equatorward inclined downward wavevectors at lower MLAT and slightly poleward inclined at higherlatitudes (Santolik et al., 2006). Bortnik et al. (2008) have studiedthe propagation of chorus using numerical ray tracing for a set ofrays initiated at the geomagnetic equator and consistent withobservations. Fig. 12 shows the ray paths of chorus waves injectedat L¼5 in the equatorial region with different wave normal angles(Bortnik et al., 2008). Bortnik et al. (2008) have discussed theevolution of hiss from chorus waves propagating into the plasma-sphere from tens of thousands of kilometers away. Detailedmeasurements of CLUSTER spacecraft gave the time–frequencystructure and frequency of chorus along the reverse ray paths ofELF hiss, which are consistent with the hypothesis that ELF hiss isa low-altitude manifestation of whistler mode chorus. Goldenet al. (2011) based on IMAGE EUV measurements suggested thatchorus observed on the ground at mid-latitude stations propa-gates predominantly in the non-ducted mode and the plasma-pause extent critically affect the chorus propagation to lowlatitudes and the ground.

Plasmaspheric hiss is also whistler mode emissions confined tothe plasmasphere. It occurs at all local times but is more intenseon the dayside and further intensifies with geomagnetic activity(Hayakawa and Sazhin, 1992). The main source of plasmaspherichiss is considered to be lightning discharges (Draganov et al.,1992; Bortnik et al., 2003). Lightning generated whistlers whilepropagating along the field lines can finally merge into a broad-band signal that becomes plasmaspheric hiss. Even chorus emis-sions while propagating through the plasmasphere merge intobroadband signal that becomes plasmaspheric hiss (Bortnik et al.,2008). The analysis of DE-I and IMAGE (Green et al., 2005) dataand CRRES data (Meredith et al., 2006) showed that the geo-graphic distribution of hiss over a frequency range 1.0–5.0 kHz issimilar to the geographic distribution of lightning strikes. How-ever, the waves at lower frequency range (0.1–1.0 kHz) areindependent of lightning activity (Meredith et al., 2006). Theother mechanism is based on the in-situ growth and amplificationof background electromagnetic turbulence in space via cyclotronresonant instability (Thorne et al., 1973; Church and Thorne,1983).

The whistler mode waves, more particularly hiss emissionsgain energy from a gyro-resonance interaction with radiation beltrelativistic electrons near the magnetic equator. The interactioncauses the electrons to change pitch angle and precipitate (Kenneland Petschek, 1966). Plasmaspheric hiss was also shown to be thedominant emission responsible for the scattering of electrons inthe slot region (Thorne et al., 1973; Abel and Thorne, 1998).During magnetically disturbed periods, plasmaspheric hiss wasalso found to be an important loss mechanism inside plasma-spheric plumes (Summers et al., 2008), the outer radiation belt(Meredith et al., 2007) and the upper part of the inner belt(Tsurutani et al., 1975).

The mass density of the inner plasmasphere is estimated usingultra-low frequency (ULF) waves which are magnetic pulsationspropagating along magnetic field lines and excited by waves (Webbet al., 1977; Takahashi and McPherron, 1982). The measured eigen-frequency of oscillation is representative of the equatorial massdensity because of the slow equatorial speed. The driver wavefrequency is usually present in the frequency spectrum of the fieldline oscillations and hence it becomes difficult to identify theeigenfrequency of oscillations. Hence, the ability to uniquely identifythe flux tubes eigenfrequency depends on having a solar wind ormagnetospheric driving force. On the dayside of the Earth, ULF wavesare almost continuously present due to upstream wave impinging onthe magnetopause (Yumoto, 1986). Therefore, ULF waves are usuallyused to measure the mass density in the plasmasphere duringthe daytime only. Typical uncertainties in determining the ULF

Fig. 12. Figure shows the evolution of chorus into plasmaspheric hiss, (a) Top panel shows a schematic of the near-Earth space environment, with the gray regions

indicating the typical location of the dense plasmasphere. A set of 91 rays is launched from the geomagnetic equator (that is, l¼01) at L¼5, with f¼0.1 fce (704 Hz), in the

wave normal range �701 to þ201, at every 11. Each ray is traced until its power decreases to 1% of its initial value, at which point the ray is terminated. Chorus originating

with a different frequency and starting location will have a similar set of ray paths, which will merge together into an incoherent emission to fill the outer plasmasphere

with hiss. (b) A 10 s long spectrogram showing the intensity of a typical series of rising chorus elements, as a function of frequency and time, (c) A similar 10 s long

spectrogram showing plasmaspheric hiss intensity, on the nightside inside the plasmasphere. There is a sharp lower frequency cut-off at �200 Hz, and a more gradual

upper frequency cut-off at �1 kHz, and no apparent structure in the spectrum.

(credit—Bortnik et al., 2008).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 823

eigenfrequency and hence mass density are about 725% (Berubeet al., 2003, 2005).

Craven et al. (1997) using data from DE-1 reported Heþ to Hþ

ratios in the range �0.03–0.3 in the plasmasphere. This implies thatHeþ abundances are �3–23% by number. This is based on theassumption that no other heavy ions are present. Horwitz et al.(1984) found that in the aftermath of storm, Oþ density couldbecome comparable to Hþ density in the plasmasphere. IMAGE RPIelectron density data base was used to examine the ion massvariability under different geomagnetic activity levels (Berube et al.,2005). It is found that an average ion mass is largest under the mostdisturbed conditions. This shows that heavy ion concentrations(percent by number) are enhanced during large geomagnetic dis-turbances. Also the average ion mass was found to increase withincreasing L-value (below 3.2) indicating the presence of a heavy iontorus during disturbed periods. This supports the view that heavyions play significant role in storm-time plasmaspheric dynamics.

Fast magnetosonic waves (equatorial noise) at frequenciesbetween fce (electron gyrofrequency) and flh (lower hybrid

frequency) and at radial distances R¼2–7RE are observed closeto the magnetic equator (within �731). These electromagneticwaves propagate with wave vector nearly perpendicular to themagnetic field (Horne et al., 2007) and magnetic field fluctuationslinearly polarized in the direction of B0. Electric field fluctuationsare elliptically polarized with a low ellipticity (from 0.20 to 0.11),major polarization axis being oriented along the wave vector(Santolık et al., 2002). Using ray tracing analysis, Horne et al.(2000) showed that magnetosonic waves can be generated justoutside the plasmapause and propagate well inside the plasma-pause without substantial absorption. Instability analysis showsthat peak growth of magnetosonic waves occur for large wavenormal angles (�891).

Horne et al. (2007) demonstrated that the fast magnetosonicwaves can accelerate electrons in the outer Van Allen radiation beltbetween �10 keV and a few MeV. The acceleration occurs via theLandau resonance. Pitch angle and energy diffusion rates are compar-able to those obtained for whistler mode chorus. The magnetosonicwaves are very important for local electron acceleration and could

Fig. 13. Variation of electron density with L-values deduced from ground-based

whistler observations.

(credit—Singh et al., 1998a).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834824

play an important role in the process of energy transfer from the ringcurrent (where ion ring distributions are formed during magneticstorms as a result due to slow ion drift) to Van Allen radiation belts.

Whistler mode Proton cyclotron echoes and new resonances areoften observed in the plasmasphere by RPI instrument at frequenciesfrom �10% to 20% above electron gyrofrequency (Carpenter et al.,2007). They are observed in the plasmasphere at MLAT between�601 and þ601. These echoes are driven by a variety of mechanisms(Carpenter et al., 2007). Most of the echoes were observed whenIMAGE moved at low angles to B0 and was within a distance of�300 m transverse to the field line of original excitation of plasma.

The other wave modes observed in the plasmasphere arekilometric continuum, non-thermal continuum and Z-modes.Low frequency non-thermal kilometric continuum has beenobserved in the frequency range �15–300 kHz (Carpenter et al.,2000). Kilometric continuum (KC) has a narrow band structureover a number of discrete frequencies with time while auroralkilometric radiation (AKR) is a broader band emission extendingover a larger frequency band sporadically. The source region ofkilometric continuum lies in the plasmaspheric cavities (notches)deep within the plasmasphere (Carpenter et al., 2000). Kilometriccontinuum observations from IMAGE showed that plasmasphericnotches are typically as deep as �2RE or more. The average beamwidth has been found to be �441. The confinement of the KCemission cone due to the steep densities of the walls of the notchstructure indicates that the average size of the plasmasphericnotches must also be �441 in longitude.

Non-thermal continuum (NTC) commonly observed in the lowfrequency range of WHISPER (Decreau et al., 2004) are classifiedaccording to four main types: (a) equatorial spots, (b) narrow bandelements, (c) continuum enhancements, and (d) wide bandedemissions. The emission equatorial spots are limited in time(�30 min) and frequency (�10–30 kHz) (El-Lemdani Mazouzet al., 2009). Narrow band (about 1 kHz or less) emissions appearafter in series of waves at frequencies separated by a few kHz fromeach other during time intervals of long duration (up to severalhours). CLUSTER observation in the night sector indicates thatcontinuous enhancement emission has a large dimension sourceregion (Decreau et al., 2004). It develops after the start of anelectron injection event and its spectral shape evolves over aduration of several hours. The wide banded emission has beenobserved for the first time with CLUSTER (Grimald et al., 2008). Itconsists of one or several banded emissions with a frequencyseparation �5–10 kHz.

Z-mode echo activity can be classified into (a) ducted wavesthat are presumably constrained by field-aligned irregularities(FAI) to follow the direction of the magnetic field, (b) non-ductedechoes that follow ray paths extending generally Earthwarddirections, and (c) scattered echoes that are believed to returnto the spacecraft (Carpenter et al., 2003). Ducted wave modeshave been used to derive plasma density profiles (Carpenter et al.,2003, Huang et al., 2004) and plasma composition (Carpenteret al., 2003) along magnetic field lines. Sonwalkar et al. (2004)found that the observed echo delays of scattered Z-mode echoescould be explained by irregularities located within �20–3000 kmfrom IMAGE.

5. Plasma density distribution in the plasmasphere

5.1. Plasma density observations

The thermal plasma within the plasmasphere is confined bygravitational field of the Earth and decreases exponentially withaltitudes in the equatorial region with a characteristic scaleheight dependent on a constant temperature and gravitational

force. Fig. 13 shows the variation of equatorial electron density asa function of L-values derived from whistler observations madeat Varanasi (L¼1.07, Geomag. Lat. 14.921N, Geomag. Long.153.921E), India, Siple (L¼4, Geomag. Lat. 61.351S, Geomag. Long.3.751E), Antarctica and Tihany (L¼1.8, Geomag. Lat. 41.811N,Geomag. Long. 92.301E), Hungary. Electron density variesbetween 5�104 and 103 electrons cm�3 as the L-value changesfrom 1.4 to 3.5. The Varanasi data belongs to moderate activity(KP¼3–4). Tihany data are divided into March to August (sum-mer) and October to February (winter) periods. For L42.4, thereis a lower density in summer than in the winter. As a consequenceof difference between the magnetic and rotational axes of theEarth, the phenomenon depends not only on the magneticlatitude but also on the longitude (Tarcsai et al., 1988; Singhand Singh, 1997). In the absence of data it is not possible topresent longitude-dependent results.

The monotonic decrease of plasma density is usually inter-rupted by a sharp decrease known as plasma ‘‘knee’’ at a radialdistance between 3 and 6 Earth radii as KP-values varies between6 and 1 (Carpenter, 1967). The equatorial electron density at theknee decreases sometimes very abruptly by about two ordersof magnitude over a very small radial distance (�0.15RE). Theplasma knee derived from ground-based whistler observationshas been confirmed from in-situ satellite measurements(Gringauz et al., 1960; Chappell et al., 1970a). Carpenter (1966)investigated the effect of magnetic storms and local time depen-dence of the plasmapause surface from the study of whistler dataand showed a clear dawn–dusk asymmetry of the plasmasphereby exhibiting a bulge near 1800 local time. Further, with eachenhancement of geomagnetic activity a new density gradient isformed closer to the Earth in the post-midnight sector (Lemaire,1987).

Apart from the ground-based measurements, plasma density(ion and electron) was also measured in the wave and plasmaexperiments onboard satellites (Gurnett et al., 1979; Gringauz andBassolo, 1990; Singh et al., 1998b; Kotova et al., 2002). Measure-ments from the number of other satellites, such as INTERCOSMOS-24, MAGION-2, Hinotori and Akeebono have further refined ourknowledge of plasma density distribution within the plasmasphere(Kimura et al., 1995, 1997; Fatkullin et al., 1995; Su et al., 1995).The important results derived from satellite observations are thepresence of both large scale and small scale plasma densitystructures in the plasmasphere (Chappell et al., 1970b) and nearthe plasmapause (LeDocq et al., 1994; Moldwin et al., 1995;

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 825

Carpenter and Lemaire, 1997). Moldwin et al. (1995) reported finescale density structures in the dusk sector of outer plasmaspherecorrelated with substorms. They proposed that fine scale structuresmay have been caused by the penetrating electric fields associatedwith substorms. Complex density structures at medium scaleshave also been reported from in-situ observations (Horwitz et al.,1990; Carpenter et al., 2000). Formation of small scale densitystructures are explained either by the drift wave instability(Hasegawa, 1971) or by the pressure gradient instability mechan-ism (Richmond, 1973). The large scale density structures observedclosed to the plasmapause and to the plasmapause boundary layerusually extend outward and are connected to the main body of theplasmasphere (Carpenter and Lemaire, 2004). These structures arecalled as plasmaspheric tails or detached plasma elements orplasmaspheric plumes. In fact plumes were detected by ground-based instruments (Carpenter et al., 1993; Su et al., 2001) as well asby several satellites measurements such as OGO-4 (Taylor et al.,1971), OGO-5 (Chappell et al., 1970a), ISEE-I (Carpenter andAnderson, 1992), CRRES (Moldwin et al., 1995, 2002; Summerset al., 2008) and at several geosynchronous orbit (Moldwin et al.,1995; Borovsky et al., 1998).

Theoretical modelings of plasmaspheric processes also predictthe existence of the density structures in the plasmasphere andplasmapause. Modeling of plasmaspheric refilling produced den-sity irregularities in the equatorial region (Singh and Horwitz,1992). Plasma interchange motion (Lemaire, 1974, 2001), andturnings/changes of strengths of interplanetary magnetic field(Goldstein et al., 2002) could also create density irregularities.

During the last decade, the EUV instrument on the IMAGEspacecraft has provided an opportunity for remotely observingthe azimuthal distribution of plasma in the plasmasphere. Itprovides two dimensional views of the plasmasphere in a largedomain of local time and geocentric distance. Density structuresare clearly mapped. RPI instrument has resolution better than1 min in time, 0.1RE in range, and 10% intensity and henceprovides new results on thermal plasma structure.

Four satellites in CLUSTER provide a meridian view of plasma-spheric density by measuring it at four places simultaneously.Electron density is derived from low frequency cut-off of naturalplasma emissions, observed by WHISPER in the frequency range2–80 kHz with a frequency resolution of 163 Hz. This correspondsto 16% for low densities and 0.4% for high densities. Cluster IonSpectrometry (CIS) experiment on CLUSTER provides ion compo-sition and 3-dimension ion distribution function. The systematicestimation of plasma density spatial gradients becomes possiblefrom the CLUSTER data (Keyser et al., 2007). Field-aligned densitystructures, having dimension larger than spacecraft separationalong the magnetic field line and 10 to a few 100 km transverse tothe magnetic field have been observed in the outer plasmasphere.Sizes in the third dimension (longitude) are small, �20 km(Decreau et al., 2004). CLUSTER and IMAGE mission providedmuch insight in the plasmaspheric density structures and clearlyestablished that these structures partially or fully corotate aroundthe Earth (Darrouzet et al., 2004). In the equatorial region, thedensity gradients in the irregularities are parallel to the magneticfield, showing that these irregularities are field aligned.

5.2. Models of plasmaspheric density

Models of plasma density provide convenient ways to repre-sent plasma environment around the Earth and they play sig-nificant role in the studies of plasma waves, energetic particledynamics and space weather prediction. The plasmasphere isfilled with the ionospheric plasma during daytime and in turnit maintains ionosphere during nighttime. The processes ofexchange of charged particles between the ionosphere and the

overlying plasmasphere play decisive role in the dynamics ofplasmasphere. Therefore, while constructing a model for theplasmasphere, either parameters of neutrals in the atmosphereand ionosphere are taken as input parameters or a general modelfor ionosphere–plasmasphere is constructed.

The plasma density modeling in the plasmasphere has devel-oped along two different paths, with both ab-initio (first-princi-ple-based) and empirical approaches. The first-principle-basedmodel is known as global dynamics and structure model. Itdiscusses the dynamic state of the plasmasphere including densitydistribution, troughs and structures present in the plasma. Usingfluid description, dynamic global core plasma model (DGCPM) wasdeveloped to study the formation of density trough in the outerplasmasphere (Ober et al., 1997). The model uses magnetosphericmagnetic field model (Tsyganenko, 1989) and the ionosphericconvection electric field model (Sojka et al., 1986) and cold plasmarefilling rates of Carpenter and Anderson (1992). It is used tounderstand the role of subauroral ion drifts on the formation ofdensity troughs during the period of high magnetic activity. Themodel could explain the presence of fine structures in the plasmadensity distribution in the presence of steep gradients in theelectrical potential.

Reynolds et al. (1999) developed the multi-species kineticmodel to investigate the convection of magnetically trappedthermal particles. The model could define the plasmapause andthe mechanisms active at plasmapause could be deduced throughthe comparisons of model output with the observations. Lemaire(1999) developed a hydrostatic equilibrium model to investigatethe convective stability of the plasmasphere. Both hydrodynamics(Tu et al., 2003; Webb and Essex, 2004) and quasi-kinetic(Reynolds et al., 2001; Pierrard and Lemaire, 2004) approacheswith refinements have been used to study plasma densitydistribution. Limiting cases such as diffusive equilibrium modeland exospheric model for plasma density distribution in astationary plasmasphere have been proposed and discussed(Pierrard and Lemaire, 2001). Diffusive equilibrium model couldnot reproduce observed density distribution by ISEE satelliteseven for a prolonged quiet period. In exospheric model contribu-tion of trapped particles which mirror back and forth between thetwo hemispheres is absent. Exospheric model containing theparticle distribution function with contribution of trapped parti-cles which depends on L, could not explain the observed steadystate profiles of ions in the equatorial plasmasphere (Pierrard andLemaire, 2001). However, in the complete absence of trappedparticles, an exospheric model describes the minimal densityprofile of particles filling the plasmasphere. Theoretical modelsare being refined with more and more inputs and at the sametime becoming complex. Hence, empirical models are beingdeveloped and used frequently.

After the pioneering work of Storey (1953), huge data werecollected using ground based and satellites measurements and effortswere made to develop empirical models for plasmapause position(Moldwin et al., 2003; Carpenter and Lemaire, 1997) and plasma-spheric density (Carpenter and Anderson, 1992; Gallagher et al., 1988,2000; Sheeley et al., 2001). Johnson et al. (2003) showed the effects ofthe solar zenith angle on the plasma density at altitudes up to 4.5RE.The electron density distribution in the geomagnetic equatorial planeis reasonably described by ZeqpL�4 in the L-shell range of 1.6oLo7(Tu et al., 2006), whereas Denton et al. (2004) proposed asheqpR�3:45

max , where Rmax¼LRE for a dipole field. Carpenter andAnderson (1992) used ZeqpL�4.5 model, which is steeper than thatZeqp10�0.3145L proposed by Denton et al. (2004). Tu et al. (2006)indicated that the functional form used by them might be used fordeveloping a global plasmasphere and trough model that maydescribe both the equatorial plasma distribution and the field-aligneddistribution in the plasmasphere.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834826

Reinisch et al. (2009) have reviewed the improvementsachieved in the empirical models of plasmaspheric density andelectric fields using the data from IMAGE and CLUSTER satellites.Denton et al. (2006) used EUV and RPI data and obtained densitydistribution for the inner and outer plasmasphere as a function ofL-shell and magnetic local time (MLT). They have also obtainedfunctions for the plasmapause and for the plasmaspheric plume.The equatorial electron density, Zeq, averaged for all RPI derivedvalues independent of geomagnetic activity is given by (Funget al., 2001; Reinisch et al., 2009)

ZeqðLÞp10ð�0:66Lþ4:89Þð2Þ

The electron density in non-equatorial region is determinedeither from the upper hybrid noise band (Benson et al., 2004) orthe lower edge of the continuum radiation (LeDocq et al., 1994),apart from direct measurements. Data from POLAR spacecraft(Denton et al., 2002a, b, 2004), show a power law dependence offield-aligned density distribution

Ze ¼ ZeoðLRE=RÞp ð3Þ

where R (L, l) is the geocentric radius, Zeo is the equatorialelectron density, p is the power law coefficient. Similar expres-sions were also given by Reinisch et al. (2004) and Huang et al.(2004) to model results from RPI experiments. For a dipolegeometry, Eq. (3) becomes

Ze ¼ ZeoðcoslÞ�2pð4Þ

where l is the magnetic latitude. Recent data show that inthe plasmasphere p¼1 can be safely considered. The data fromEUV/IMAGE (Pierrard, 2006), CIS/IMAGE (Dandouras et al., 2005)and cluster observations (Schafer et al., 2007, 2008) compare wellthe results of different time-dependent simulations. RPI/IMAGEprovide nearly instantaneous measurements of the true field-aligned electron densities in the plasmasphere, which readilyhelps in developing and ‘‘refining a new’’ plasmaspheric densitymodel including radial, latitudinal, longitudinal and time depen-dent. These models are quite useful in simulation work and inunderstanding microphysical phenomena in the plasmasphereand plasmapause.

Fig. 14. Schematic diagram of trapped radiation belts around the Earth.

(Source: http://parts.jpl.nasa.gov/docs/Radcrs_Final.pdfS, NASA Jet Propulsion Laborato

6. Radiation belts and plasmasphere

The plasmasphere contains two radiation belts, one belowL¼2 known as inner belt (proton belt) and the other above L¼3,known as outer belt (electron belt). They are also called as VanAllen radiation belt (Van Allen and Frank, 1959). These radiationbelts are regions of high energy protons and electrons encirclingthe Earth and trapped by geomagnetic fields and have theirlargest fluxes at L�1.5 and near L¼4. Fig. 14 is a schematicdiagram of radiation belt (Crosby et al., 2006). Higher energycharged particles are trapped at lower L-values. Due to a weak-ness in the magnetic field in the South Atlantic (South AtlanticAnomaly) trapped energetic electrons (energy Z10 MeV) andproton (energy�100 MeV) penetrate deeper into the atmospherein this region. These high energy charged particles may pose aparticular hazard to satellites in LEO at that location (Singh et al.,2010).

The energy of proton in the proton belt may exceed 100 MeV.The cosmic ray albedo neutron decay (CRAND) process is respon-sible for energies 4100 MeV (White, 1973). In this process afraction of the incident cosmic ray flux on the atmosphere is backscattered as neutrons which subsequently decay into protons andelectrons. The second source is the direct entry and trapping ofsolar protons during solar proton events and geomagnetic storms(Kress et al., 2005). The third source could be transport of protonstowards the Earth by radial diffusion across geomagnetic field(Albert et al., 1998). The protons may be removed from theradiation belt due to Coulomb collisions with electrons chargeexchange with Hydrogen atoms and atmospheric absorptions.

The relativistic electrons in the inner belt have lifetimespanning from months to years and is quite stable. In fact, theinner part of the proton belt (Lo1.7) is quite stable. The flux mayvary only during the most intense geomagnetic disturbances. Thesecular changes in the geomagnetic field may gradually increasethe proton intensity due to contracting drifts shells (Selesnicket al., 2007). The proton flux displays solar cycle variation and isin anticorrelation with the solar flux (Miyoshi et al., 2000). Highsolar activity causes heating and expansion of the upper atmo-sphere which results into enhanced collision rate and protonlosses at a fixed altitude. The sources and losses of protons are

ry (Crosby et al., 2006).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 827

used in developing dynamics models of the proton radiation belt(Selesnick et al., 2007) which could reproduce the variability ofthe outer region (L41.7) of the proton radiation belt.

The outer electron belt encircles the inner belt and its state isvariable and is controlled by geomagnetic activity in terms of sourceand loss processes (Baker et al., 1994; Shprits et al., 2008a, b).It contains relativistic electrons (41 MeV) and various ions(�10 keV–10 MeV). The sources of electrons are acceleration andradial transport across the geomagnetic field. The electron accelera-tion occurs within the magnetosphere (Li et al., 1997) and severalacceleration mechanisms have been proposed (Li and Temerin,2001; Horne, 2002). Earlier it was considered that electrons areaccelerated during radial transport across the magnetic field throughbetatron and Fermi acceleration mechanism (Horne, 2002; Friedelet al., 2002). However, recent observations of phase space densitypeaks in the inner region near L¼4.5 suggest local accelerationmechanism (Horne, 2007; Chen et al., 2007).

In the local acceleration mechanism wave–particles interac-tion is the leading candidate (Horne and Thorne, 1998). Amongvarious wave modes that could contribute to local acceleration,whistler mode chorus waves are the most promising candidate(Horne and Thorne, 1998; Horne et al., 2006; Vainio et al., 2009).In this mechanism energy is transferred from a large number oflow energy electrons at small pitch angles to accelerate a fractionof the population at large pitch angles to higher energies viaDoppler-shifted cyclotron resonances. The source of low energyelectron (�10 keV) is plasmasheet electron injected into lowerL-shells by convective (and inductive) electric fields during sub-storms. As a result of injection into higher magnetic field regionand anisotropic distribution develops, peaked perpendicular tothe magnetic field, which excites whistler mode waves viaDoppler-shifted cyclotron resonance (Shprits et al., 2006). Thewaves grow by scattering low energy electrons at smaller pitchangles, but at the same time, they resonate with higher energyelectrons having large pitch angles and trapped by magneticfields. These high energy electrons are accelerated. Fig. 15 showsa schematic diagram indicating the regions where different typesof waves can scatter high energy electrons through resonantinteractions (Shprits et al., 2006).

Some wave modes such as plasmaspheric hiss, electromag-netic ion cyclotron waves, lightning generated whistlers and

Fig. 15. Schematic diagram showing the regions in MLT where different types of

waves can scatter relativistic electrons through resonant interactions. The light

and dark gray lines show the convection of injected ring current electrons and

ions. The gray shaded regions show different types of waves which scatter

electrons via Doppler-shifted cyclotron resonance and the wavy lines refers to

ULF waves.

(credit—Shprits et al., 2006).

whistler mode waves from ground-based transmitters causeelectron loss to the atmosphere (Abel and Thorne, 1998). Hissdominates the loss rates in the outer plasmasphere (Meredithet al., 2006). Lightning becomes more important at lower L-shells.The effect of transmitter dominates at L�2. Outward radialdiffusion onto open drift orbits as a result of changes in the outermagnetopause boundary may also cause to electron loss.

The scattering and loss of energetic electrons from the region2oLo3 creates a ‘‘slot’’ region characterized by the absence ofenergetic electrons separates the two radiation belts. The ener-getic electrons in the ‘‘slot’’ region are scattered by the broadbandwhistler mode hiss emissions and are lost from the plasmasphere(Meredith et al., 2007). Observations reveal the presence of hissemissions throughout the plasmasphere (Meredith et al., 2006,Bortnik et al., 2008). If pitch-angle scattering during wave (hiss)–particle (electron) interaction is the cause of slot region forma-tion, then the outer extent of the plasmasphere should predict theinner extent of outer belt electrons. This hypothesis has beenproved during intense geomagnetic storm (Baker et al., 1994;Goldstein et al., 2005). During intense storm, severe erosions ofplasmasphere causes plasmapause to penetrate into the slotregion and in the absence of hiss emissions, slot region maycontain newly energized electrons, this signature has beenobserved (Goldstein et al., 2005). Summers et al. (2008) showedthat plasmaspheric hiss are also an important loss mechanisminside plasmaspheric plums. The location of Van Allen radiationbelts depend on the solar conditions. The solar wind-inducedmagnetospheric dawn-to-dusk electric field enhances duringintense solar storm condition and erodes the outer regions ofthe plasmasphere causing compression of plasmasphere includ-ing radiation belts. Fig. 16a, b show schematically the threedimensional view of the Earth’s outer radiation belt and itsrelationship to the plasmasphere before and during the ‘‘Hallow-e’en storm’’ of 2003 (Baker et al., 2004). Fig. 16(a) shows a normalplasmasphere and outer radiation belt location under typicalconditions. The outer radiation belt extends far away fromthe Earth with its maximum intensity at �4–5RE geocentricdistances. The plasmasphere extends outward to 3–4RE.Fig. 16(b) shows the highly contracted plasmasphere and dis-torted radiation belt location during Hallowe’en storm period. Theplasmasphere was greatly contracted and remained in a reducedstate for many days (Baker et al., 2004). The Van Allen radiationbelt was also pushed inwards and the highest electron intensitieswere seen at 2–3RE.

7. Thermal structure of plasmasphere

The sets of data on the ion and the electron temperature in theplasmasphere are much less than the sets of density data may bedue to the fact that whistler measurements have rarely been usedto estimate plasma temperature and its variation. Data ontemperature were obtained only during the direct satelliteexperiments. The ion temperature (Ti) in the plasmasphere basedon Luna-2 rocket data was first estimated by Gringauz et al.(1960) to be �104 K. For complete picture of the plasmasphere,the temperature and heat flow are critical components and needmuch detail studies.

In the 1970s, using Prognoz data plasmasphere was dividedinto two zones, an inner zone (at Lo3), having ion temperaturelower than 8�103 K, and the warm outer zone (first calledhot zone), where the temperature rather rapidly changes withincreasing L-values and can reach �105 K. The electron tempera-ture also rises with increasing L-value (Decreau et al., 1982).However, a warm zone has also been observed under prolongedgeomagnetically quiet conditions when temperature remains

Fig. 16. Diagrams of the three dimensional view of the Earth’s outer belt and its relationships to the plasmasphere (a) a schematic diagram showing the Earth, the outer

radiation belt and the normal plasmaspheric locations. (b) similar to (a) but showing the highly contracted plasmasphere and greatly distorted radiation belt location

during the Hallowe’en storm period. This cut away diagrams show the highest electron fluxes as red and the lowest fluxes as blue. The radiation belt is a torus shaped

entity. The Earth is at the center. Plasmasphere is shown as a translucent green torus towards the center in each part of the figure (for interpretation of the references to

color in this figure legend, the reader is referred to the web version of this article).

(credit—Baker et al., 2004).

Fig. 17. Example of proton temperature (top) and density (bottom) variations during small magnetic storm development on five successive (every �6 h) nighttime

plasmasphere passes by INTERBALL-2. In the lowest panel KP and Dst variations during the time are shown. Columns on the time axis is the lowest correspond to the

moments of plasmasphere profile measurements marked by respective numbers.

(after—Kotova et al., 2008).

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834828

lower than 104 K in the entire plasmasphere. The GEO-I, ISEE-Iand DE-I data confirm the existence of two thermal zones in theplasmasphere (Gringauz and Bassolo, 1990). Ion temperature inthe plasmasphere was thoroughly studied based on the data ofthe RIMS/DE-I ion mass spectrometer (Comfort, 1986). Detailanalysis showed that thermal structure in the plasmasphere isclosely tied to the density structure and has a significant bearingon the composition of the plasma (Comfort, 1986). For example,the region of strong temperature gradients co-locates with theregion of strong density gradient. Further, the observed ions ofhigh temperature in the plasmasphere could not always beexplained through the process of energization by photoelectronsas is believed. Even the transfer of heat from hot ions to cold ionscould not always be explained by the coulomb collisions andwave–particle interactions. Wave–particle interactions could beimportant in explaining some of these observations (Khazanovet al., 1996, 1997). Akebono data have been systematically used tostudy thermal structure and temperature distribution in theplasmasphere (Oyama and Abe, 1995; Oyama et al., 1996; Abeet al., 1997). Model for the ion and electron temperature has alsobeen developed (Titheridge, 1998). Webb and Essex (2003)modified Titheridge’s model which is appropriate only for the

inner plasmasphere where it gives larger temperature increasethan have been observed especially on the nightside.

The plasmaspheric ion temperature derived from above mea-surements show that it is higher than the typical value of theF-region ionosphere and it raises with increasing L-value (Moffettet al., 1992; Balan et al., 1996). The ion temperature on averagerises along the magnetic field at a rate of �0.05–1.0 km�1, butin the dawn sector the field-aligned temperature gradient isobserved only in the outer region (L43) (Comfort, 1986). Theion temperature in the outer plasmasphere is less stable, inde-pendent of the local time and depends on the distance fromthe plasmapause. Further, the temperature of Hþ , Heþ and Oþ

are close to each other (Kotova, 2008).The dynamics of the proton temperature in the nightside

plasmasphere during moderate magnetic disturbances was con-sidered based on the INTERBALL-2 data. Fig. 17 shows the protontemperature variation during the development of a small mag-netic storm (Kotova et al., 2008). Measurements were made everysix hour. Simultaneous density measurements were also carriedout which is shown in the bottom of the panel. KP and Dst

variations are also shown. The first two distributions correspondto newly quiet condition. The third was measured during the

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 829

main phase of the storm, which shows a drop in temperature andcoincides with the minimum value of Dst. Number four wasmeasured during the recovery phase which shows even highertemperature than before the storm. Number five corresponds toquiet conditions under (Kotova et al., 2008). Here it may be notedthat in every pass the spacecraft scanned different region of theplasmasphere due to the rotation of the Earth. It was not the sameplasma that was first cooled and then heated. The enhancedproton temperature during recovery phase of magnetic stormcould be explained by considering interaction of cold plasma withthe energetic ring current particles via direct Coulomb or waveinteractions. During magnetic storms, plumes are produced thatextend sunward from the main plasmasphere in the noon-to-dusklocal time sector (Goldstein and Sandel, 2005). During therecovery phase this plume begins to rotate eastward. Also duringthe main phase of the storm, injected plasmasheet plasma createsa partial ring current on the nightside (Brandt et al., 2002). Thepeak of ring current migrates westward providing a possibleoverlap of plasmasphere and ring current.

During the main phase of the storm the overlap between thering current and the plasmasphere is minimal because ringcurrent contraction and plasmasphere erosion occurs simulta-neously. Thus plasma heating during main phase seems to belacking. Even photoelectrons at L43 cannot heat the nightsideduring storm (Khazanov et al., 1998). The observed lower tem-perature during the main phase may have resulted as a conse-quence of some cooling mechanism. Low temperatures may alsobe connected to reverse fluxes from the ionosphere to plasma-sphere at late dusk or night (Kotova et al., 2008). However, thedifficulty is that during moderate storms, depletion of plasma-sphere flux tubes at Lo2–6 was not observed.

8. Conclusions

We have presented review of the some of the processes takingplace inside the plasmasphere and their control on the location ofplasmapause. Results derived from the IMAGE and CLUSTERspacecrafts have made the future of the plasmasphere researchmore exciting and attractive. Recent observations of densitystructures (small scale, medium scale and large scale) in theplasmasphere as well as at the plasmapause show that the effortsmade for their explanations are insufficient. For example, mediumscale density structures such as channels and fingers are not fullyexplained. The distributions of field-aligned density structures inspace and time throughout the plasmasphere neither well docu-mented nor explained. Even the mechanisms that create smallscale density structures are not well understood. Much moreefforts in theoretical modeling and simulation are required.

Early hypothesis of plasmasphere erosion by enhanced con-vection have been partially confirmed by global mapping, butnew observation of plume formation and its evolution have posedserious physics-based challenges before the scientific community.IMAGE observations suggest erosion to be one aspect of thecoupled response of the entire ionosphere–magnetosphere sys-tem. A common opinion on the processes responsible for theplasmapause formation, location of plasmapause during magneticstorm and substorms, plasmasphere filling/erosion and thermalbalance is absent. What parts of the plasma are discharged fromthe ionosphere and escape into the outer magnetosphere duringmagnetic storms is still unclear. Most of the refilling models areone-dimensional in which plasma flows along a fixed L-value. Thediurnal effects, corotation of the flux tubes and associatedexpansions and contractions of their volumes and loss of plasmadue to convection and supply of the plasma from the ionosphereneed to be included in a dynamic refilling model.

Dynamics of the plasmasphere is influenced by the waveswhich influence transport properties of plasma. Whistler modewaves have been used as diagnostic tools. The IMAGE andCLUSTER spacecrafts in addition to ground-based measurementshave helped in collecting the data base of plasmaspheric densityprofiles. The density data together with ground-based ULF wavediagnostics has been used in determining the average ion mass asa function of L under different levels of geomagnetic activity. Fastmagnetosonic waves can accelerate electrons to relativisticenergy and hence could explain the required local accelerationmechanism.

Kilometric continuum originates from the plasmaspheric den-sity cavities and therefore observation and analysis of thesewaves could provide back information on plasmaspheric notchesand its dynamics. Density irregularities are linked with non-thermal continuum radiation. Many new class of radio sourceshave been revealed and identified by the IMAGE and CLUSTERmissions.

Recent studies have shown that plasmaspheric hiss is gener-ated from the merging of chorus emissions while propagatingthrough the plasmasphere. Hiss emissions frequently scatterenergetic electrons into the atmosphere from the radiation beltcreating a slot region. Thus waves control the stability of radiationbelt particles.

Wave–particle interaction plays significant role in creating andmaintaining slot region and in the observations of new types ofwaves and their explanation. Interaction is also responsible forcreating density and thermal structures both inside and outsidethe plasmasphere. Some aspect of the phenomena is known, butmuch effort is required in extending wave–particle studies inthe light of new data collected/analyzed from the IMAGE andCLUSTER spacecrafts. The thermal structures of the plasmasphereshow peculiar behavior where temperature suddenly rises in the12–20 MLT sector. The heating is more at high L-values suggest-ing a possible source of heating at high L-value that becomesstrongest in the noon-dusk MLT sector. Further, during moderatemagnetic storm main phase ion temperature drops as comparedto the pre-storm value and then increases above the undisturbedvalue during the recovery phase. The ion temperature relaxes tonormal value during quiet conditions. All these behavior are notconsistent with theoretical studies are needed to understand theobserved thermal structures.

Acknowledgments

A.K. Singh would like to thank the Department of Science andTechnology (DST), Government of India for providing financialsupport as a research project (File no. SR/S4/AS: 261/06). AKS, DSare also thankful to Indian Space Research Organization (ISRO) forpartial financial support under CAWSESS program. RPS is thankfulto the Head, Department of Physics, B.H.U. Varanasi for providingnecessary facilities. Authors are also thankful to learned refereesfor their critical remarks and suggestions.

References

Abe, T., Balan, N., Oyama, K.-I., Bailey, G.J., 1997. Plasmasphere electrontemperature—observations and theory. Adv. Space Res. 20, 401–405.

Abe, T., Kawano, H., Goldstein, J., Ohtani, S., Solovyev, S.I., Baishev, D.G., Yumoto, K.,2006. Simultaneous identification of a plasmaspheric plume by a groundmagnetometer pair and IMAGE Extreme Ultraviolet Imager. J. Geophys. Res.111, A11202.

Abel, B., Thorne, R.M., 1998. Electron scattering loss in Earth’s inner magneto-sphere, 1: dominant physical processes. J. Geophys. Res. 103, 2385–2396.doi:10.1029/97JA02919.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834830

Adrian, M.L., Gallagher, D.L., Avanov, L.A., 2004. IMAGE EUV observation of radiallybifurcated plasmaspheric features: first observations of a possible standingULF waveform in the inner magnetosphere. J. Geophys. Res. 109, A01203.

Albert, J.M., Ginet, G.P., Gussenhoven, M.S., 1998. CRRES observations of radiationbelt protons, 1: data overview and steady state radial diffusion. J. Geophys.Res. 103, 9261.

Alcayde, D., Caudal, G., Fontanari, J., 1986. Convection electric fields and electrostaticpotential over 611oDo721 invariant latitude observed with the Europeanincoherent scatter facility, 1: initial results. J. Geophys. Res. 91, 233–247.

Anderson, P.C., Hanson, W.B., Heelis, R.A., Craven, J.D., Baker, D.N., Frank, L.A., 1993. Aproposed production model of rapid subauroal ion drift and their relationship tosubstorm evolution. J. Geophys. Res. 108, 6069.

Baker, D.N., Callis, J.B., Cummings, L.B., Hovestadt, J.R., et al., 1994. Signatures ofthe substorm recovery phase at high altitude spacecraft. Geophys. Res. Lett.21, 409.

Baker, D.N., Kanekal, S.G., Li, X., Monk, S.P., Goldstein, J., Burch, J.L., 2004. Anextreme distortion of the Van Allen belt arising from the ‘Hallowe’en’ solarstorm in 2003. Nature 432.

Balan, N., Oyama, K.I., Bailey, G.J., Abe, T., 1996. Plasmasphere electron tempera-ture studies using satellite observations and a theoretical model. J. Geophys.Res. 101, 15,323–15,330. doi:10.1029/96JA00823.

Bankov, L., Gousheva, M., Lefterov, A., Binev, G., Vassileva, A., Potanin, Yu., Dubinin,E., 1993. Critical Ionization Velocity (CIV) experiment XANI onboard theINTERCOSMOS-24 ‘‘ACTIVE’’ satellite. Adv. Space Res. 13, 69–80.

Banks, P.M., Nagy, A.F., Axford, W.I., 1971. Dynamical behavior of thermal protonsin the mid-latitude ionosphere and magnetosphere. Planet. Space Sci. 19,1053.

Benson, R.F., Webb, P.A., Green, J.L., Garcia, L., Reinisch, B.W., 2004. Magneto-spheric electron densities inferred from upper-hybrid band emissions. Geo-phys. Res. Lett. 31, L20803.

Berube, D., Moldwin, M.B., Weygand, J.M., 2003. An automated method for thedetection of field line resonance frequencies using ground magnetometertechniques. J. Geophys. Res. 108, 1348.

Berube, D., Moldwin, M.B., Fung, S.F., Green, J.L., 2005. A plasmaspheric massdensity model and constraints on its heavy ion concentration. J. Geophys. Res.110, A04212.

Borovsky, J.E., Thomsen, M.F., McComas, D.J., Cayton, T.E., Knipp, D.J., 1998. Magneto-spheric dynamics and mass flow during the November 1993 storm. J. Geophys. Res.103, 26373–26394.

Bortnik, J., Inan, U.S., Bell, T.F., 2003. Frequency–time spectra of magnetospheri-cally reflecting whistlers in the plasmasphere. J. Geophys. Res. 108, 1030.doi:10.1029/2002JA009387.

Bortnik, J., Thorne, R.M., Meredith, N.P., 2007. Modeling the propagation char-acteristics of chorus using CRRES suprathermal electron fluxes. J. Geophys. Res.112, A08204. doi:10.1029/2006JA012237.

Bortnik, J., Thorne, R.M., Meredith, N.P., 2008. The unexpected origin of plasma-spheric hiss from discrete chorus emissions. Nature 452.

Boskova, J., Jirıcek, F., S&milauer, J., Trıska, P., 1993. VLF plasmaspheric emis-sions—‘wave’ signatures of inner magnetosphere boundaries. J. Atmos. Terr.Phys. 55, 1789–1795.

Brandt, P.C., Mitchell, D.G., Ebihara, Y., Sandel, B.R., Roelof, E.C., Burch, J.L.,Demajistre, R., 2002. Global IMAGE/HENA observations of the ring current:examples of rapid response to IMF and ring current plasmasphere interaction.J. Geophys. Res. 107, 1359.

Brice, N.M., 1967. Bulk motion of the magnetosphere. J. Geophys. Res. 72,5193–5211.

Burch, J.L., Mitchell, M.S.B., Moore, D.G., Pollock, T.E., 2001. Views of Earth’smagnetosphere with the IMAGE satellite. Science 291, 619–624.

Burch, J.L., Goldstein, J., Sandel, B.R., 2004. Cause of plasmasphere corotation lag.Geophys. Res. Lett. 31, L05802.

Calvert, W., Benson, R.F., Carpenter, D.L., Fung, S.F., Gallagher, D.L., Green, J.L.,Haines, D.M., Reiff, P.H., Reinisch, B.W., Smith, M.F., Taylor, W.W.L., 1995. Thefeasibility of radio sounding in the magnetosphere. Radio Sci. 30, 1577–1595.

Carpenter, D.L., 1962. Electron density variations in the magnetosphere deducedfrom whistler data. J. Geophys. Res. 67, 3345–3360.

Carpenter, D.L., 1963. Whistler evidence of a ‘knee’ in the magnetosphericionization density profile. J. Geophys. Res. 68, 1675–1682.

Carpenter, D.L., 1965. Whistler measurement of the equatorial profile of magneto-spheric electron density. In: Brown, G.M. (Ed.), Radio Science 1960–1963, vol.III, The Ionosphere. Elsevier, pp. 76–91.

Carpenter, D.L., 1966. Whistler studies of the plasmapause in the magnetosphere,1: temporal variation in the position of the knee and some evidence on plasmamotions near the knee. J. Geophys. Res. 71, 693–709.

Carpenter, D.L., 1967. Whistler evidence of the dynamic behavior of the dusksidebulge in the plasmasphere. J. Geophys. Res. 72, 2969.

Carpenter, D.L., 1970. Whistler evidence of the dynamic behavior of the dusksidebulge in the plasmasphere. J. Geophys. Res. 75 (19), 3837–3847.

Carpenter, D.L., 1983. Some aspects of plasmapause probing by whistlers. RadioSci. 18, 917–925.

Carpenter, D.L., 2004. Remote sensing the Earth’s plasmasphere. Radio Sci. Bull.308, 13–29.

Carpenter, D.L., Park, C.G., 1973. On what ionosphere workers should know aboutthe plasmapause–plasmasphere. Rev. Geophys. Space Phys. 11, 133–154.

Carpenter, D.L., Anderson, R.R., 1992. An ISEE/whistler model of equatorialelectron density in the magnetosphere. J. Geophys. Res. 97, 1097–1108.

Carpenter, D.L., Lemaire, J., 1997. Erosion and recovery of the plasmasphere in theplasmapause region. Space Sci. Rev. 80, 153–179.

Carpenter, D.L., Lemaire, J., 2004. The plasmasphere boundary layer. Ann. Geophys.22 (12), 4291–4298.

Carpenter, D.L., Giles, B.L., Chappell, C.R., Decreau, P.M.E., Anderson, R.R., Persoon, A.M.,Smith, A.J., Corcuff, Y., Canu, P., 1993. Plasmasphere dynamics in the dusksidebulge region: a new look at an old topic. J. Geophys. Res. 98, 19243–19271.

Carpenter, D.L., Anderson, R.R., Calvert, W., Moldwin, M.B., 2000. CRRES observationsof density cavities inside the plasmasphere. J. Geophys. Res. 105, 23323–23338.

Carpenter, D.L., Spasojevic, M.A., Bell, T.F., Inan, U.S., Reinisch, B.W., Galkin, I.A.,Benson, R.F., Green, J.L., Fung, S.F., Boardsen, S.A., 2002. Small-scale field-aligned plasmaspheric density structures inferred from the Radio PlasmaImager on IMAGE. J. Geophys. Res. 107, 1258.

Carpenter, D.L., Bell, T.F., Inan, U.S., Benson, R.F., Sonwalkar, V.S., Reinisch, B.W.,Gallagher, D.L., 2003. Z-mode sounding within propagation ‘‘cavities’’ andother inner magnetospheric regions by the RPI instrument on the IMAGEsatellite. J. Geophys. Res. 108, 1421.

Carpenter, D.L., Bell, T.F., Chen, D., Ng, D., Baran, C., Reinisch, B.W., Galkin, I., 2007.Proton cyclotron echoes and a new resonance observed by the Radio PlasmaImager instrument on the IMAGE satellite. J. Geophys. Res. 112, A08208.doi:10.1029/2006JA012139.

Chandler, M.O., Chappel, C.R., 1986. Observatioons of the flow of Hþ and Heþ

along magnetic field lines in the plasmasphere. J. Geophys. Res. 91, 8847.Chappell, C.R., 1974. Detached plasma regions in the magnetosphere. J. Geophys.

Res. 79, 1861.Chappell, C.R., Harris, K.K., Sharp, G.W., 1970a. A study of the influence of magnetic

activity on the location of the plasmapause as measured by OGO 5. J. Geophys.Res. 75, 50–56.

Chappell, C.R., Harris, K.K., Sharp, G.W., 1970b. The morphology of the bulge regionof the plasmasphere. J. Geophys. Res. 75 (19), 3848–3861.

Chen, A.J., Wolf, R.A., 1972. Effects on the plasmasphere of a time-varyingconvection electric field. Planet. Space Sci. 20, 483–509.

Chen, A.J., Grebowsky, J.M., 1974. Plasma tail interpretations of pronounceddetached plasma regions measured by OGO 5. J. Geophys. Res. 79, 3851.

Chen, Y., Reeves, G.D., Friedel, R.H.W., 2007. The energization of relativisticelectrons in the outer Van Allen radiation belt. Nat. Phys. 3, 614–617.

Chiu, Y.T., Schulz, M., 1978. Self-consistent particle and parallel electrostatic fielddistributions in the magnetospheric–ionospheric auroral region. J. Geophys.Res. 83, 629.

Church, S.R., Thorne, R.M., 1983. On the origin of plasmaspheric hiss: ray pathintegrated amplification. J. Geophys. Res. 88, 7941–7957.

Clilverd, M.A., Smith, A.J., Thomson, N.R., 1991. The annual variation in quiet timeplasmaspheric electron density, determined from whistler mode group delays.Planet. Space Sci. 39, 1059–1069.

Comfort, R.H., Horwitz, J.L., 1981. Low-energy ion pitch angle distributions observed onthe dayside at geosynchronous altitudes. J. Geophys. Res. 86, 1621.

Comfort, R.H., 1986. Plasmasphere thermal structure as measured by ISEE-1 andDE-1. Adv. Space Res. 6, 31–40.

Comfort, R.H., 1988. The magnetic mirror force in plasma fluid models. In: Moore,T.E., Waite Jr., J.H. (Eds.), Modeling Magnetospheric Plasma, GeophysicalMonograph Series, vol. 44. American Geophysical Union, Washington, DC, p. 51.

Craven, P.D., Gallagher, D.L., Comfort, R.H., 1997. Relative concentration of He+ inthe inner magnetosphere as observed by the DE1 retarding ion mass spectro-meter. J. Geophys. Res. 102, 2279.

Crosby, N.B., Rycroft, M.J., Tulunay, Y., 2006. Overview of a graduate coursedelivered in Turkey, emphasizing solar–terrestrial physics and space weather.Surv. Geophys. 27, 319–364.

Curtis, S.A., 1985. Equatorial trapped plasmasphere ion distributions and trans-verse stochastic acceleration. J. Geophys. Res. 90, 1765.

Dandouras, I., Pierrard, V., Goldstein, J., Vallat, C., Parks, G.K., R�eme, H., Gouillart, C.,Sevestre, F., McCarthy, M., Kistler, L.M., Klecker, B., Korth, A., Bavassano-Cattaneo,M.B., Escoubet, P., Masson, A., 2005. Multipoint observations of ionic structuresin the plasmasphere by CLUSTER-CIS and comparisons with IMAGE-EUVobservations and with model simulations. In: Burch, J., Schulz, M., Spence, H.(Eds.), Inner Magnetosphere Interactions: New Perspectives from Imaging,Geophysical Monograph Series, vol. 159. American Geophysical Union, Washington,pp. 23–53.

Darrouzet, F., Decreau, P.M.E., Keyser, J.D., Masson, A., Gallagher, D.L., Santolık, O.,Sandel, B.R., Trotignon, J.G., Rauch, J.L., Le Guirriec, E., Canu, P., Sedgemore, F.,Andre, M., Lemaire, J.F., 2004. Density structures inside the plasmasphere:cluster observations. Ann. Geophys. 22, 2577–2585.

Darrouzet, F., Keyser, J.D., Decreau, P.M.E., El Lemdani-Mazouz, F., Valli�eres, X.,2008. Statistical analysis of plasmaspheric plumes with CLUSTER/WHISPERobservations. Ann. Geophys. 26, 2403–2417.

Darrouzet, F., Gallagher, D.L., Andre, N., Carpenter, D.L., Pierrette, I.D., Decreau,M.E., Keyser, J.D., Denton, R.E., Foster, J.C., Goldstein, J., Moldwin, M.B.,Reinisch, B.W., Sandel, B.R., Tu, J., 2009. Plasmaspheric density structuresand dynamics: properties observed by the CLUSTER and IMAGE Missions.Space Sci. Rev. 145, 55–106. doi:10.1007/s11214-008-9438-9.

Decreau, P.M.E., Beghin, C., Parrot, M., 1982. Global characteristics of the coldplasma in the equatorial plasmapause region as deduced from the GEOS-1,mutual impedance probe. J. Geophys. Res. 87, 695–712.

Decreau, P.M.E., Carpenter, D., Chappell, C.R., Comfort, R.H., Green, J.L., Olsen, R.C.,Waite Jr., J.H., 1986. Latitudinal plasma distribution in the dusk plasmasphericbulge: refilling phase and quasi-equilibrium state. J. Geophys. Res. 91, 6929.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 831

Decreau, P.M.E., Ducoin, C., Le Rouzic, G., Randriamboarison, O., Rauch, J.L.,Trotignon, X., Valli�eres, J.G., Canu, P., Darrouzet, F., Gough, M.P., Buckley, A.M.,Carozzi, T.D., 2004. Observation of continuum radiations from the CLUSTERfleet: first results from direction finding. Ann. Geophys. 22, 2607–2624.

Decreau, P.M.E., Le Guirriec, E., Rauch, J.L., Trotignon, J.G., Canu, P., Darrouzet, F.,Lemaire, J., Masson, A., Sedgemore, F., Andre, M., 2005. Density irregularities inthe plasmasphere boundary player: cluster observations in the dusk sector.Adv. Space Res. 36, 1964–1969.

Dent, Z.C., Mann, L.R., Menk, F.W., Goldstein, J., 2003. A coordinated ground-basedand IMAGE satellite study of quiet-time plasmaspheric density profiles.Geophys. Res. Lett. 30, 2-1.

Dent, Z.C., Mann, I.R., Goldstein, J., Menk, F.W., Ozeke, L.G., 2006. Plasmasphericdepletion, refilling and plasmapause dynamics: a coordinated ground basedand IMAGE satellite study. J. Geophys. Res. 111, A03205. doi:10.1029/2005JA011046.

Denton, R.E., Goldstein, J., Menietti, J.D., 2002a. Field line dependence of magneto-spheric electron density. Geophys. Res. Lett. 29, 2205.

Denton, R.E., Goldstein, J., Menietti, J.D., Young, S.L., 2002b. Magnetosphericelectron density model inferred from Polar plasma wave data. J. Geophys.Res. 107, 1386.

Denton, R.E., Menietti, J.D., Goldstein, J., Young, S.L., Anderson, R.R., 2004. Electrondensity in the magnetosphere. J. Geophys. Res. 109, A09215. doi:10.1029/2003JA010245.

Denton, R.E., Takahashi, K., Galkin, I.A., Nsumei, P.A., Huang, X., Reinisch, B.W.,Anderson, R.R., Sleeper, M.K., Hughes, W.J., 2006. Distribution of density alongmagnetospheric field lines. J. Geophys. Res. 111, A04213.

Draganov, A.B., Inan, U.S., Sonwalkar, V.S., Bell, T.F., 1992. Magnetosphericallyreflected whistlers as a source of plasmaspheric hiss. J. Geophys. Res. 19,233–236.

Ebihara, Y., Ejiri, M., 2002. Numerical simulation of the ring current: review. SpaceSci. Rev. 105, 377–452.

El-Lemdani Mazouz, F., Grimald, S., Rauch, J.L., Decreau, P.M.E., Bozan, G., Le Rouzic, G.,Suraud, X., Valli�eres, X., Trotignon, J.G., Canu, P., Darrouzet, F., Boardsen, S., 2006.Electrostatic and electromagnetic emissions near the plasmasphere. A case event:27 May 2003. In: Proceedings of the Cluster and Double Star Symposium, FifthAnniversary of Cluster in Space ESA SP-598, 2006.

El-Lemdani Mazouz, F., Rauch, J.L., Decreau, P.M.E., Trotignon, J.G., Valli�eres, X.,Darrouzet, F., Canu, P., Suraud, X., 2009. Wave emissions at half electrongyroharmonics in the equatorial plasmasphere region: CLUSTER observations andstatistics. Adv. Space Res. 43, 253–264.

Escoubet, C.P., Russell, C.T., Schmidt, R. (Eds.), 1997. The Cluster and PhoenixMissions. Kluwer, Dordrecht.

Farrugia, C.J., Young, D.T., Geiss, J., Balsiger, H., 1989. The composition temperatureand density structure of cold ions in the quiet terrestrial plasmasphere: GOES1 results. J. Geophys. Res. 94 (11), 865.

Fatkullin, M.N., Gasilov, N.A., Forster, M., Schwarz, U., Markwardt, M., 1995.Medium-scale irregularities of electron density at the bottom of the plasma-sphere. Cosmic Res. 33, 64–71.

Feygin, F.Z., Pokhotelov, O.A., Pokhotelov, D.O., BrAaysy, T., Kangas, J., Mursula, K.,1997. Exo-plasmaspheric refilling due to ponderomotive forces induced bygeomagnetic pulsations. J. Geophys. Res. 122, 4841–4845.

Foster, J.C., Ericson, P.J., Coster, A.J., Goldstein, J., 2002. Ionospheric signatures ofplasma tails. Geophys. Res. Lett. 29. doi:10.1029/2002GL015067.

Foster, J.C., Coster, A.J., Erickson, P.J., Rideout, W., Rich, F.J., Immel, T.J., Sandel, B.R., 2005.Redistribution of the stormtime ionosphere and the formation of the plasma-spheric bulge. In: Burch, J.L., Schulz, M., Spence, H. (Eds.), Inner MagnetosphereInteractions: New Perspectives from Imaging. Geophysical Monograph Series, vol.159. American Geophysical Union, Washington, pp. 277–289.

Friedel, R., Reeves, R., Obara, T., 2002. Relativistic electron dynamics in the innermagnetosphere—a review. J. Atmos. Sol.–Terr. Phys. 64, 265–282.

Fung, F., Garcia, L.N., Green, J.L., Gallagher, D.L., Carpenter, D.L., Reinisch, B.W.,Galkin, I.A., Khmyrov, G., Sandel, B.R., 2001. Plasmaspheric electron densitydistributions sampled by Radio Plasma Imager on the IMAGE satellite. EosTrans. AGU 82 SM11A-0771.

Gallagher, D.L., Craven, P.D., Comfort, R.H., 1988. An empirical model of the Earth’splasmasphere. Adv. Space Res. 8, 15.

Gallagher, D.L., Craven, P.D., Comfort, R.H., 2000. Global core plasma model.J. Geophys. Res. 105, 18819–18883.

Gallagher, D.L., Adrian, M.L., Liemohn, M.W., 2005. Origin and evolution ofdeep plasmaspheric notches. J. Geophys. Res. 110, A09201. doi:10.1029/2004JA010906.

Gallagher, D.L., Adrian, M.L., 2007. Two-dimensional drift velocities from theIMAGE EUV plasmaspheric imager. J. Atmos. Sol.–Terr. Phys. 69 (3), 341–350.

Galvan, D.A., Moldwin, M.B., Sandel, B.R., 2008. Diurnal variation in plasmasphericHeþ inferred from extreme ultraviolet images. J. Geophys. Res. 113, A09216.

Ganguli, G., Reynolds, M.A., Liemohn, M.W., 2000. The plasmasphere and advancesin plasmaspheric research. J. Atmos. Sol.–Terr. Phys. 62, 1647–1657.

Golden, D.I., Spasojevic, M., Inan, U.S., 2011. Determination of solar cycle varia-tions of mid-latitude ELF/VLF chorus and hiss via automated signal detection.J. Geophys. Res.. doi:10.1029/2010JA016193.

Goldstein, J., 2006. Plasmasphere response: tutorial and review of recent imagingresults. Space Sci. Rev. 124, 203–216.

Goldstein, J., Sandel, B.R., 2005. The global pattern of evolution of plasmasphericdrainage plumes. In: Burch, J.L. Schulz, M., Spence, H. (Eds.), Inner Magneto-sphere Interactions: New Perspectives from Imaging. Geophysical MonographSeries, vol. 159. American Geophysical Union, Washington, pp. 1–22.

Goldstein, J., Spiro, R.W., Reiff, P.H., Wolf, R.A., Sandel, B.R., Freeman, J.W.,Lambour, R.L., 2002. IMF-driven overshielding electric field and theorigin of the plasmaspheric shoulder of May 24, 2000. Geophys. Res. Lett.29, 1819.

Goldstein, J., Sandel, B.R., Reiff, P.H., Hairston, M.R., 2003. Control of plasmasphericdynamics by both convection and sub-auroral polarization stream. Geophys.Res. Lett. 30, 2243.

Goldstein, J., Sandel, B.R., Forrester, W.T., Thomsen, M.F., Hariston, M.R., 2005.Global plasmasphere evolution 22–23 April 2001. J. Geophys. Res.110, A12218. doi:10.1029/2005JA011282.

Goldstein, J., Sandel, B.R., Frey, H.U., Mende, S.B., 2007. Multiple plasmapauseundulations observed by the IMAGE satellite on 20 March 2001. J. Atmos.Sol.–Terr. Phys. 69, 322–333.

Grebowsky, J.M., 1970. Model study of plasmapause motion. J. Geophys. Res.75, 4329–4333.

Grebowsky, J.M., 1972. Model development of supersonic trough wind withshocks. Planet. Space Sci. 20, 1923.

Green, J.L., Benson, R.F., Fung, S.F., Taylor, W.W.L., Boardsen, S.A., Reinisch, B.W.,Haines, D.M., Bibl, K., Cheney, G., Galkin, I.A., Huang, X., Myers, S.H., Sales, G.S.,Bougeret, J.-L., Manning, R., Meyer-Vernet, N., Moncuquet, M., Carpenter, D.L.,Gallagher, D.L., Reiff, P.H., 2000. Radio Plasma Imager simulations andmeasurements. Space Sci. Rev. 91, 361–389.

Green, J.L., Boardsen, S., Fung, S.F., Matsumoto, H., Hashimoto, K., Anderson, R.R.,Sandel, B.R., Reinisch, B.W., 2004. Association of kilometric continuum radia-tion with plasmaspheric structures. J. Geophys. Res. 109, A03203.

Green, J.L., et al., 2005. On the origin of whistler mode radiation in the plasma-sphere. J. Geophys. Res. 110, A03201. doi:10.1029/2004JA010495.

Grimald, S., Decreau, P.M.E., Canu, P., Rochel, A., Valli�eres, X., 2008. Medium-latitude sources of plasmaspheric nonthermal continuum radiations observedclose to harmonics of the electron gyrofrequency. J. Geophys. Res. 113,A11216.

Gringauz, K.I., 1963. The structure of the ionized gas envelope of Earth from directmeasurements in the USSR of local charged particle concentrations. Planet. SpaceSci. 11, 281–296.

Gringauz, K.I., Bassolo, V.S., 1990. Structure and properties of the Earth’s plasma-sphere (a review). Geomagn. Aeronom. 30, 1–17.

Gringauz, K.I., Bezrukikh, V.V., Ozerovand, V.D., Rybchinskii, R.E., 1960. Studyingthe interplanetary ionized gas, energetic electrons, and solar corpuscularradiation using three electrode traps of charged particles on the second Sovietrocket. Dokl. Akad. Nauk. SSSR 131, 1302–1304.

Guiter, S.M., Gombosi, T.I., 1990. The role of high-speed plasma flows in plasma-spheric refilling. J. Geophys. Res. 95 (10), 427.

Guiter, S.M., Gombosi, T.I., Rasmussen, C.E., 1991. Diurnal variations on plasma-spheric flux tubes: light ion flows and F region temperature enhancements.Geophys. Res. Lett. 18, 813.

Guglielmi, A., Pokhotelov, O.A., Feygin, F.Z., Kurchashov, Yu.P., McKenzie, J.F.,Shukla, P.K., Stenflo, L., Potapov, A.S., 1995. Ponderomotive forces in long-itudinal MHD waveguides. J. Geophys. Res. 100, 7997–8002.

Gurnett, D.A., Anderson, R.R., Scarf, F.L., et al., 1979. Initial results from ISEE-1 and2, plasma wave investigation. Space Sci. Rev. 23, 103–122.

Hasegawa, A., 1971. Drift-wave instability at the plasmapause. J. Geophys. Res. 76,5361–5364.

Hayakawa, M., Sazhin, S.S., 1992. Mid-latitude and plasmaspheric hiss: a review.Planet. Space Sci. 40, 1325–1338.

Helliwell, R.A., Crary, J.H., Pope, J.H., Smith, R.L., 1956. The ‘nose’ whistler—a newhigh latitude phenomena. J. Geophys. Res. 61, 139–142.

Hoffman, J.H., Dodson, W.H., 1980. Light ion concentrations and fluxes duringmagnetically quiet times. J. Geophys. Res. 85, 626.

Hoogeveen, G.W., Jacobson, A.R., 1997. Improved analysis of plasmasphere motionusing the VLA radio interferometer. Ann. Geophys. 15, 236–245.

Horne, R.B., 2002. The contribution of wave–particle interactions to electron loss andacceleration in the Earth’s radiation belts during geomagnetic storms, Review ofRadio Science 1999–2002. Wiley, New York, pp. 801–828.

Horne, R.B., 2007. Plasma astrophysics: acceleration of killer electrons. Nat. Phys.3, 590.

Horne, R.B., Thorne, R.M., 1998. Potential waves for relativistic electron scatteringand stochastic acceleration during magnetic storms. Geophys. Res. Lett.25, 3011–3014.

Horne, R.B., Wheeler, G.V., Alleyne, H.S.C.K., 2000. Proton and electron heating byradially propagating fast magnetosonic waves. J. Geophys. Res. 105,27597–27610.

Horne, R.B., Thorne, R.M., Shprits, Y.Y., Meredith, N.P., Glauert, S.A., Smith, A.J.,Kanekal, S.G., Baker, D.N., Engebretson, M.J., Posch, J.L., Spasojevic, M., Inan, U.S.,Pickett, J.S., Decreau, P.M.E., 2005. Wave acceleration of electrons in the Van Allenradiation belts. Nature 437, 227–230.

Horne, R.B., Meredith, N.P., Glauert, S.A., Varotsou, A., Boscher, D., Thorne, R.M.,Shprits, Y.Y., Anderson, R.R., 2006. Recurrent magnetic storms: corotating solarwind streams. In: Tsurutani, B.T., McPherron, R.L., Gonzalez, W.D., Lu, G.,Sobral, J.H.A., Gopalswamy, N. (Eds.), Geophysical Monograph Series, vol. 167.American Geophysical Union, Washington, p. 151.

Horne, R.B., Thorne, R.M., Glauert, S.A., Meredith, N.P., Pokhotelov, D., Santolık, O.,2007. Electron acceleration in the Van Allen radiation belts by fast magneto-sonic waves. Geophys. Res. Lett. 34, L17107.

Horwitz, J.L., Chappell, C.R., 1979. Observations of warm plasma in the daysideplasma trough at geosynchronous orbit. J. Geophys. Res. 84, 7075.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834832

Horwitz, J.L., Comfort, R.H., Chappell, C.R., 1984. Thermal ion compositionmeasurements of the formation of the new outer plasmasphere and doubleplasmapause during storm recovery phase. Geophys. Res. Lett. 11, 701.

Horwitz, J.L., Brace, L.H., Comfort, R.H., Chappell, C.R., 1986. Dual-spacecraft measure-ments of plasmasphere-ionosphere coupling. J. Geophys. Res. 91 (11), 203.

Horwitz, J.L., Comfort, R.H., Chappel, C.R., 1990. A statistical characterization ofplasmasphere density structure and boundary locations. J. Geophys. Res. 95,7937–7947.

Huang, X., Reinisch, B.W., Song, P., Green, J.L., Gallagher, D.L., 2004. Developing anempirical density model of the plasmasphere using IMAGE/RPI observations.Adv. Space Res. 33, 829–832.

Jacobson, A.R., Hoogeveen, G.W., Carlos, R.C., Wu, G., Fejer, B.G., Kelley, M.C., 1996.Observations of inner-plasmasphere irregularities with a satellite-beaconradio-interferometer array. J. Geophys. Res. 101, 19665–19682.

Johnson, M.T., Wygant, J.R., Cattell, C.A., Mozer, F.S., 2003. Seasonal variationsalong auroral field lines: measurements from the polar spacecraft. Geophys.Res. Lett. 30, 77-1. doi:10.1029/2002GL015866.

Jordanova, V.K., Thorne, R.M., Farrugia, C.J., Dotan, Y., Fennell, J.F., Thomsen, M.F.,Reeves, G.D., McComas, D.J., 2004. Ring current dynamics during the13–18 July 2000 storm period. Sol. Phys. 24, 361–375. doi:10.1023/A:1014241527043.

Kennel, C.F., Petschek, H.E., 1966. Limit on stably trapped particle fluxes.J. Geophys. Res. 71, 1–28.

Keyser, J., De., F., Darrouzet, M.W., Dunlop, Decreau, P.M.E., 2007. Least-squaresgradient calculation from multi-point observations of scalar and vector fields:methodology and applications with cluster in the plasmasphere. Ann. Geo-phys. 25, 971–987.

Keyser, J., De, D.L., Carpenter, F., Darrouzet, D.L., Gallagher, J., 2009. Tu, CLUSTERand IMAGE: new ways to study the Earth’s plasmasphere. Space Sci. Rev. 145,7–53. doi:10.1007/s11214-008-9464-7.

Khazanov, G.V., Kuen, M.A., Konikov, Yu.V., Sidorov, I.M., 1984. Simulation ofionosphere-plasmasphere coupling taking into account ion inertia and tem-perature anisotropy. Planet. Space Sci. 32, 585.

Khazanov, G.V., Moore, T.E., Krivorutsky, E.N., 1996. Lower hybrid turbulence andponderomotive force effects in space plasmas subjected to large-amplitudelow-frequency waves. Geophys. Res. Lett. 23, 797–800.

Khazanov, G.V., Liemohn, M.V., Kozyara, J.V., Moore, T.E., 1997. Inner magneto-spheric super thermal electron transport: photoelectron and plasmasheetelectron sources. J. Geophys. Res. 103, 23485–23501.

Khazanov, G.V., Liemohn, M.W., Kozyra, J.U., Moore, T.E., 1998. Inner magneto-spheric superthermal electron transport: photoelectron and plasma sheetelectron sources. J. Geophys. Res. 103 (23), 485–23,501.

Khazanov, G.V., Liemohn, M.W., Fok, M.-C., Newman, T.S., Ridley, A.J., 2004.Stormtime particle energization with high temporal resolution AMIE poten-tials. J. Geophys. Res. 109, A05209. doi:10.1029/2003JA010186.

Kim, K.H., Goldstein, J., Berube, D., 2007. Plasmaspheric drainage plume observedby the Polar satellite in the prenoon sector and the IMAGE satellite during themagnetic storm of 11 April 2001. J. Geophys. Res. 112, A06237.

Kimura, I., Hikuma, A., Kasahara, Y., Sawada, A., Kikuchi, M., Oya, H., 1995.Determination of electron density distributions in the plasmasphere by usingwave data observed by Akebono satellite. Adv. Space Res. 15, 103–107.

Kimura, I., Tsunehara, K., Hikuma, A., Su, Y.Z., Kasahara, Y., Oya, H., 1997. Globalelectron density distribution in the plasmasphere deduced from Akebonowave data and the IRI model. J. Atmos. Sol.–Terr. Phys. 59, 1569–1586.

Kivelson, M.G., Russell, C.T. (Eds.), 1995. Introduction to Space Physics. CambridgeUniversity Press, New York.

Kotova, G.A., 2008. The Earth’s plasmasphere: state of studies (a review).Geomagn. Aeronom. 47, 409–422.

Kotova, G.A., Bezrukikh, V.V., Verigin, M.I., et al., 2002. INTERBALL1/Alpha 3 coldplasma measurements in the evening plasmasphere: quiet and disturbedmagnetic conditions. Adv. Space Res. 30.

Kotova, G.A., Bezrukikh, V.V., Verigin, M.I., Smilauer, J., 2008. New aspects inplasmaspheric ion temperature variations from INTERBALL 2 and MAGION5 measurements. J. Atmos. Sol.–Terr. Phys. 70, 399–406.

Krall, J., Huba, J.D., Fedder, J.A., 2008. Simulation of field-aligned Hþ and Heþ

dynamics during late-stage plasmasphere refilling. Ann. Geophys. 26,1507–1516.

Kress, B.T., Hudson, M.K., Slocum, P.L., 2005. Impulsive solar energetic ion trappingin the magnetosphere during geomagnetic storms. Geophys. Res. Lett. 32,L06108.

Larsen, B.A., Klumpar, D.M., Gurgiolo, C., 2007. Correlation between plasmapauseposition and solar wind parameters. J. Atmos. Sol.–Terr. Phys. 69, 334–340.

LeDocq, M.J., Gurnett, D.A., Anderson, R.R., 1994. Electron number densityfluctuations near the plasmapause observed by the CRRES spacecraft.J. Geophys. Res. 99, 23661–23671.

Lemaire, J.F., 1974. The ‘‘Roche-limit’’ of ionospheric plasma and the formation ofthe plasmapause. Planet. Space Sci. 22 (5), 757–766.

Lemaire, J.F., 1985. Frontiers of the plasmasphere. Aeronom. Acta 298 (EditionsCabay, Louvain–la–Neuve), ISBN:2-87077-310-2.

Lemaire, J., 1987. The plasmapause formation. Phys. Scr. T18, 111–178.Lemaire, J.F., 1989. Plasma distribution models in a rotating magnetic dipole and

refilling plasmaspheric flux tubes. Phys. Fluids B 1, 1519–1525.Lemaire, J., 1990. Plasma distribution models in a rotating magnetic dipole and

refilling of plasmaspheric flux tubes. In: Paper Presented at the HuntsvilleWorkshop on Plasmasphere Refilling, University of Alabama in Huntsville,October 1990.

Lemaire, J.F., 1999. Hydrostatic equilibrium and convective stability in theplasmasphere. J. Atmos. Sol.–Terr. Phys. 61, 867–878.

Lemaire, J.F., 2001. The formation of the light-ion-trough and peeling off theplasmasphere. J. Atmos. Sol.–Terr. Phys. 63, 1285–1291.

Lemaire, J.F., Gringauz, K.I., 1998. With Contribution from D.L. Carpenter and V.S.Bassolo, The Earth’s Plasmasphere. Cambridge University Press, Cambridge.

Lemaire, J., Storey, L.R.O., 2001. The plasmasphere revisited: a tribute to DonaldCarpenter. J. Atmos. Sol.–Terr. Phys. 63, 1105–1106.

Li, X., Temerin, M., 2001. The electron radiation belt. Space Sci. Rev. 95,569–580.

Li, X., Baker, D.N., Temerin, M., Larson, D., Lin, R.P., Reeves, G.D., Looper, M.D.,Kanekal, S.G., Mewaldt, R.A., 1997. Are energetic electrons in the solar windthe source of the outer radiation belt? Geophys. Res. Lett. 24, 923.

Lin, J., Horwitz, J.L., Wilson, G.R., Ho, C.W., Brown, D.G., 1992. A semi kinetic modelfor early stage plasmasphere refilling, 2: effects of wave–particle interactions.J. Geophys. Res. 97, 1121–1134.

Marasigan, V.S.J., 1958. Height-gradient of electron-loss in the F-region. J. Atmos.Terr. Phys. 13, 107–112.

Masson, A., Santolık, O., Carpenter, D.L., Darrouzet, F., Decreau, P.M.E., Green, J.L.,Grimald, S., El-Lemdani Mazouz, F., Moldwin, M.B., Nemec, F., 2009. Advancesin plasmaspheric wave research with CLUSTER and IMAGE observations. SpaceSci. Rev. 145, 137–191.

Matsui, H., Puhl-Quinn, P.A., Jordanova, V.K., Khotyaintsev, Y., Lindqvist, P.A.,Torbert, R.B., 2008. Derivation of inner magnetospheric electric field (UNH-IMEF) model using Cluster data set. Ann. Geophys. 26, 2887–2898.

Maynard, N.C., Chen, A.J., 1975. Isolated cold plasma regions: observations andtheir relation to possible production mechanisms. J. Geophys. Res. 80,1009–1013.

McIlwain, C.E., 1986. A KP dependent equatorial electric field model. Adv. SpaceRes. 6, 187–197.

Meier, R.R., Nicholas, A.C., Picone, J.M., Melendez-Alvira, D.J., Ganguli, G.I.,Reynolds, M.A., Roelof, E.C., 1998. Inversion of plasmaspheric EUV remotesensing data from the STP 72-1 satellite. J. Geophys. Res. 103 (17), 505.

Menietti, J.H., Burch, J.L., Gallagher, D.L., 1988. Statistical study of ion flows in thedayside and nightside plasmasphere. Planet. Space Sci. 36, 693.

Meredith, N.P., Horne, R.B., Glauert, S.A., Thorne, R.M., Summers, D., Albert, J.M.,Anderson, R.R., 2006. Energetic outer zone electron loss timescales during lowgeomagnetic activity. J. Geophys. Res. 111, A05212. doi:10.1029/2005JA011516.

Meredith, N.P., Horne, R.B., Glauert, S., Anderson, R.R., 2007. Slot region electronloss timescales due to plasmaspheric hiss and lightning generated whistlers.J. Geophys. Res. 112, A08214. doi:10.1029/2007JA012413.

Miyoshi, Y., Morioka, A., Misawa, H., 2000. Long term modulation of low altitudeproton radiation belt by the Earth’s atmosphere. Geophys. Res. Lett. 27, 2169.

Moffett, R.J., Bailey, G.J., Jenkins, B., 1992. Effects of greatly increased Oþ loss in theionospheric F-region. Planet. Space Sci. 40, 631–1637.

Moldwin, M.B., Thomsen, M.F., Bame, S.J., McComas, D., Reeves, G.D., 1995. The fine-scale structure of the outer plasmasphere. J. Geophys. Res. 100, 8021–8029.

Moldwin, M.B., Downward, L., Rassoul, H.K., Amin, R., Anderson, R.R., 2002. A newmodel of the location of the plasmapause: CRRES results. J. Geophys. Res. 107,1339.

Moldwin, M.B., Sandel, B.R., Thompson, M.F., Elphic, R.C., 2003. Quantifying globalplasmaspheric image with in situ observations. Space Sci. Rev. 109, 47–61.

Morgan, M.G., Allcock, G.M., 1956. Observations of whistling atmospherics atgeomagnetically conjugate points. Nature 177, 30–31.

Nagai, T., Horwitz, J.L., Anderson, R.R., Chappell, C.R., 1985. Structure of theplasmapause from ISEE 1 low-energy ion and plasma wave observations.J. Geophys. Res. 90, 6622.

Nakamura, M., Yoshikawa, I., Yamazaki, A., Shiomi, K., Takizawa, Y., Hirahara, M.,Yamashita, K., Saito, Y., Miyake, W., 2000. Terrestrial plasmaspheric imagingby an extreme ultraviolet scanner on planet-B. Geophys. Res. Lett. 27,141–144.

Nishida, A., 1966. Formation of plasmapause, or magnetospheric plasma knee, bythe combined action of magnetospheric convention and plasma escape fromthe tail. J. Geophys. Res. 71, 5669–5679.

Nunn, D., Omura, Y., Matsumoto, H., Nagano, I., Yagitani, S., 1997. The numericalsimulation of VLF chorus and discrete emissions observed on the Geotailsatellite using a Vlasov code. J. Geophys. Res. 102, 27083–27097.

Ober, D.M., Horwitz, J.L., Gallagher, D.L., 1997. Formation of density troughembedded in the outer plasmasphere by subauroal ion drifts (SAID).J. Geophys. Res. 102, 14595.

O’Brien, T.P., Moldwin, M.B., 2002. Empirical plasmapause models from magneticindices. Geophys. Res. Lett. 30, 1152. doi:10.1029/2002GL016007.

Olsen, R.C., Chappell, C.R., Gallagher, D.L., Green, J.L., Gurnett, D.A., 1985. Thehidden ion population: revisited. J. Geophys. Res. 90 (12), 121.

Olsen, R.C., Chappell, C.R., Gallagher, D.L., Green, J.L., Chappell, C.R., Anderson, R.R.,1987. Plasma observations at the magnetic equator. J. Geophys. Res. 92, 2385.

Oyama, K.-I., Abe, T., 1995. Morphology of electron temperature in the highlatitude plasmasphere. Adv. Space Res. 16, 85–93.

Oyama, K.-I., Abe, T., Sakaide, Y., Koutiev, I., Okuzawa, T., Choi, T., Choi, Y., 1996.Electron temperature distribution in the inner plasmasphere I (mid- and lowlatitudes). Adv. Space Res. 17, 185–188.

Park, C.G., 1972. Technical Report: Methods of Determining Electron Concentra-tion in the Magnetosphere from Nose Whistlers. Stanford University,California.

Park, C.G., 1974. A morphological study of sustorm-associated disturbances in theionosphere. J. Geophys. Res. 79, 2621–2827.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834 833

Park, C.G., Banks, P.M., 1974. Influence of thermal plasma flow on the mid-latitudenight-time F2 layer; effects of electric fields and neutral winds inside theplasmasphere. J. Geophys. Res. 79, 4661–4668.

Park, C.G., Carpenter, D.L., Wiggin, D.B., 1978. Electron density in the plasma-sphere: whistler data on solar cycle, annual and diurnal variations. J. Geophys.Res. 83, 3137–3144.

Parks, G.K., 1991. Physics of Space Plasmas. Addison-Wesley Publishing Company,p. 511.

Pierrard, V., 2006. The dynamics of the plasmasphere. In: Maravell, N.S. (Ed.),Space Science: New Research. Nova Science, New York, pp. 83–96.

Pierrard, V., Cabrera, J., 2005. Comparisons between EUV/IMAGE observations andnumerical simulations of the plasmapause formation. Ann. Geophys. 23,2635–2646.

Pierrard, V., Cabrera, J., 2006. Dynamical simulations of plasmapause deforma-tions. Space Sci. Rev. 122, 119–126.

Pierrard, V., Lemaire, J., 2001. Exospheric model of the plasmasphere. J. Atmos.Sol.–Terr. Phys. 63, 1261–1265.

Pierrard, V., Lemaire, J., 2004. Development of shoulders and plumes in the frameof the interchange instability mechanism for plasmapause formation. Geo-phys. Res. Lett. 31, L05809.

Pierrard, V., Stegen, K., 2008. A three-dimensional dynamic kinetic model of theplasmasphere. J. Geophys. Res. 113, A10209.

Pierrard, V., Khazanov, G.V., Cabrera, J., Lemaire, J., 2008A. Influence of theconvection electric field models on predicted plasmapause positions duringmagnetic storms. J. Geophys. Res. 113, A08212. doi:10.1029/2007JA012612.

Pierrard, V., Goldstein, J., Andre, N., Jordanova, V.K., Kotova, G.A., Lemaire, J.F.,Liemohn, M.W., Matsui, H., 2009. Physics-based models of the plasmasphere.Space Sci. Rev. 145, 193–229.

Pope, J.H., 1961. An estimate of electron densities in the exosphere by means ofnose whistlers. J. Geophys. Res. 66, 67–75.

Rasmussen, C.E., Schunk, R.W., 1988. Multistream hydrodynamic modeling ofinter-hemispheric plasma flow. J. Geophys. Res. 93 (14), 557.

Reinisch, B.W., Haines, D.M., Bibl, K., Cheney, G., Galkin, I.A., Huang, X., Myers, S.H.,Sales, G.S., Benson, R.F., Fung, S.F., Green, J.L., Boardsen, S., Taylor, W.W.L.,Bougeret, J.L., Manning, R., Meyer-Vernet, N., Moncuquet, M., Carpenter, D.L.,Gallagher, D.L., Reiff, P., 2000. The Radio Plasma Imager investigation on theIMAGE Spacecraft. Space Sci. Rev. 91, 319–359.

Reinisch, B.W., Huang, X., Song, P., Green, J.L., Fung, S.F., Vasyliunas, V.M.,Gallagher, D.L., Sandel, B.R., 2004. Plasmaspheric mass loss and refilling as aresult of a magnetic storm. J. Geophys. Res. 109, A01202.

Reinisch, B.W., Moldwin, M.B., Denton, R.E., Gallagher, D.L., Matsui, H., Pierrard, V.,Tu, J., 2009. Augmented empirical models of plasmaspheric density andelectric field using IMAGE and CLUSTER data. Space Sci. Rev. 145, 231–261.

Reynolds, M.A., Ganguli, G., Fedder, J.A., Lemaire, J., Meier, R.R., Melendez-Alvira, D.J.,1999. Thermal plasmaspheric morphology: effect of geomagnetic and solaractivity. J. Geophys. Res. 104, 10,285–10,294.

Reynolds, M.A., Melender-Alvira, D.J., Ganguli, G., 2001. Equatorial couplingbetween the plasmasphere and the topside ionosphere. J. Atmos. Sol.–Terr.Phys. 63, 1267–1273.

Reynolds, M.A., Ganguli, G., Su, Y.J., Thomsen, M.F., 2003. The local-time variationof the quiet plasmasphere: geosynchronous observations and kinetic theory.Ann. Geophys. 21, 2147–2154.

Richmond, A.D., 1973. Self-induced motions of thermal plasma in the magneto-sphere and the stability of the plasmapause. Radio Sci. 8, 1019–1027.

Sagawa, E., Yau, A.W., Whalen, B.A., Peterson, W.K., 1987. Pitch angle distributionsof low-energy ions in the near-Earth magnetosphere. J. Geophys. Res. 92 (12),341.

Sandel, B.R., Denton, M.H., 2007. Global view of refilling of the plasmasphere.Geophys. Res. Lett. 34, L17102.

Sandel, B.R., et al., 2000. The extreme ultraviolet imager investigation for theIMAGE mission. Space Sci. Rev. 91, 197–242.

Sandel, B.R., King, R.A., Forrester, W.T., et al., 2001. Initial results from the IMAGEExtreme Ultraviolet Imager. Geophys. Res. Lett. 28, 1439–1442.

Sandel, B.R., Goldstein, J., Gallagher, D.L., Spasojevic, M., 2003. Extreme ultravioletimager observations of the structure and dynamics of the plasmasphere. SpaceSci. Rev. 109, 25–46.

Santolık, O., Pickett, J.S., Gurnett, D.A., Maksimovic, M., Cornilleau-Wehrlin, N.,2002. Spatiotemporal variability and propagation of equatorial noise observedby Cluster. J. Geophys. Res. 107, 1495.

Santolık, O., Gurnett, D.A., Pickett, J.S., Parrot, M., Cornilleau-Wehrlin, N., 2004. Amicroscopic and nanoscopic view of storm-time chorus on 31 March 2001.Geophys. Res. Lett. 31, L02801.

Santolık, O., Gurnett, D.A., Pickett, J.S., Parrot, M., Cornilleau-Wehrlin, N., 2005.Central position of the source region of storm-time chorus. Planet. Space Sci.53, 299–305.

Santolik, O., et al., 2006. Propagation of whistler mode chorus to low altitudes:spacecraft observations of structured ELF hiss. J. Geophys. Res. 111, A10208.doi:10.1029/2005JA011462.

Saxton, J.M., Smith, A.J., 1989. Quiet time plasmaspheric electric fields andplasmasphere–ionosphere coupling fluxes. Planet. Space Sci. 37, 283.

Sazhin, S.S., Hayakawa, M., 1992. Magnetospheric chorus emissions: a review.Planet. Space Sci. 40, 681–697.

Schafer, S., Glassmeier, K.H., Eriksson, P.T.I., Pierrard, V., Fornac-on, K.H., Blomberg,L.G., 2007. Spatial and temporal characteristics of poloidal waves inthe terrestrial plasmasphere: a CLUSTER case study. Ann. Geophys. 25,1011–1024.

Schafer, S., Glassmeier, K.H., Eriksson, P.T.I., Mager, P.N., Pierrard, V., Fornac-on,K.H., Blomberg, L.G., 2008. Spatio-temporal structure of a poloidal Alfven wavedetected by Cluster adjacent to the dayside plasmapause. Ann. Geophys. 26,1805–1817.

Schulz, M., Koons, H.C., 1972. Thermalization of colliding ion streams beyond theplasmapause. J. Geophys. Res. 77, 248.

Selesnick, R.S., Looper, M.D., Mewaldt, R.A., 2007. A theoretical model of the innerproton radiation belt. Space Weather 5, S04003. doi:10.1029/2006SW000275.

Sheeley, B.W., Moldwin, M.B., Rassoul, H.K., Anderson, R.R., 2001. An empiricalplasmasphere and trough density model: CRRES observations. Denton, R.E.,J. Goldstein, D.H. Lee, R.A. King, Z.C. Dent, D.L. Gallagher, D. Berube, K.Takahashi, M. Nose, D. Milling, F. Honary. Realistic magnetospheric densitymodel for 29 August 2000. J. Geophys. Res. 106, 25,631–25,642.

Shprits, Y.Y., Thorne, R.M., Horne, R.B., Glauert, S.A., Cartwright, M., Russell, C.T.,Baker, D.N., Kanekal, S.G., 2006. Acceleration mechanism responsible for theformation of the new radiation belt during the 2003 Halloween solar storm.Geophys. Res. Lett. 33, L05104. doi:10.1029/2005GL024256.

Shprits, Y.Y., Elkington, S.R., Meredith, N.P., Subbotin, D.A., 2008a. Review ofmodeling of losses and sources of relativistic electrons in the outer radiationbelt: 1. Radial transport. J. Atmos. Sol.–Terr. Phys. doi:10.1016/j.jastp.2008.06.008.

Shprits, Y.Y., Subbotin, D.A., Meredith, N.P., Elkington, S.R., 2008b. Review ofmodeling of losses and sources of relativistic electrons in the outer radiationbelts: II. Local acceleration and loss. Atmos. Sol.–Terr. Phys. 70 (14),1694–1713. doi:10.1016/j.jastp.2008.06.014.

Singh, A.K., 1995. The Study of Inner Magnetosphere by VLF Waves. Ph.D. Thesis.Banaras Hindu University, India.

Singh, A.K., 2010. Remote sensing of Earth’s plasmasphere. Adv. Geosci. 21,415–427.

Singh, A.K., Singh, R.P., 1999. Duct lifetimes at mid-latitudes. Acta Tech. Acad.(Hungary) 43, 43–52.

Singh, A.K., Singh, R.P., 2005. Ionosphere–magnetosphere dynamics and plasma-sphere. In: Proceedings of the XXVIIIth General Assembly of URSI, No. 2(0154).

Singh, A.K., Singh, Abhay K., Singh, D.K., Singh, R.P., 1998b. Thermal effect on thedispersion characteristics of proton whistlers. J. Atmos. Sol.-Terr. Phys. 60,551–561.

Singh, A.K., Siingh, D., Singh, R.P., 2010. Space weather: physics, effects andpredictability. Surv. Geophys. 31, 581–638. doi:10.1007/s10712-010-9103-1.

Singh, N., 1988. Refilling of a plasmaspheric flux tube: microscopic plasmaprocesses. In: Moore, T.E., Waite Jr., J.H. (Ed.), Modeling MagnetosphericPlasma, Geophysical Monograph Series, vol. 44. American Geophysical Union,Washington, DC, p. 87.

Singh, N., 1990. Comment on ‘‘Multistream hydrodynamic modeling of interhemi-spheric plasma flow’’ by C.E. Rasmussen and R.W. Schunk. J. Geophys. Res. 95,17,273.

Singh, N., 1991. Role of ion temperature anisotropy in multistage refilling of theouter plasmasphere. J. Geophys. Res. Lett. 18, 817.

Singh, N., Hwang, K.S., 1987. Perpendicular on heating effects on the refilling of theouter plasmasphere. J. Geophys. Res. 92 (13), 531.

Singh, N., Torr, D.O., 1990. Effects of in temperature anisotropy on the interhemisphericplasma transport during plasmaspheric refilling. Geophys. Res. Lett. 17, 925.

Singh, N., Horwitz, J.L., 1992. Plasmaspheric refilling: recent observations andmodeling. J. Geophys. Res. 97 (A2), 1049–1079.

Singh, N., Schunk, R.W., Thiemann, H., 1986. Temporal features of the refilling of aplasmaspheric flux tube. J. Geophys. Res. 91 (13), 433.

Singh, R.P., Patel, R.P., 2004. Hiss triggered chorus emissions at Indian stations.J. Atmos. Sol.–Terr. Phys. 66, 1027–1033.

Singh, R.P., Singh, A.K., Singh, D.K., 1998a. Plasmaspheric parameters as derivedfrom whistler spectrograms: a review. J. Atmos. Sol.–Terr. Phys. 60, 495–508.

Singh, R., Patel, R.P., Singh, R.P., Lalmani, 2000. An experimental study of hiss-triggered chorus emissions observed at low latitude. Earth Planet Space 52,37–40.

Singh, U.P., Singh, R.P., 1997. Study of plasmasphere–ionosphere coupling fluxes.J. Atmos. Sol.–Terr. Phys. 59, 1321–1327.

Smith, R.L., 1961. Properties of the outer ionosphere deduced from nose whistlers.J. Geophys. Res. 66, 3709–3716.

Smith, R.L., Helliwell, R.A., 1960. Electron densities to 5 Earth radii deduced fromnose whistlers. J. Geophys. Res. 65, 2583.

Sojka, J.J., Wrenn, O.L., 1985. Refilling of geosynchronous flux tubes as observed atthe equator by GEOS 2. J. Geophys. Res. 90, 6379.

Sojka, J.J., Schunk, R.W., Johnson, J.F.E., Waite Jr., J.H., Chappell, C.R., 1983.Characteristics of thermal and suprathermal ions associated with the daysideplasma trough as measured by the Dynamics Explorer retarding ion massspectrometer. J. Geophys. Res. 88, 7895.

Sojka, J.J., Rasmussen, C.E., Schunk, R.W., 1986. An interplanetary magnetic fielddependent model of the ionospheric convection electric field. J. Geophys. Res.91, 11281–11290.

Sonwalkar, V.S., Carpenter, D.L., Bell, T.F., et al., 2004. Diagnostic of magneto-spheric electron density and irregularities at altitudes o5000 km usingwhistler and z mode echoes from radio sounding of the IMAGE satellite. J.Geophys. Res. 109, A11212.

Sonwalkar, V.S., Hazra, S., Mayank, K., Reddy, A., Carpenter, D.L., Reinisch, B.W.,2009. Whistler Mode Echoes Observed Below 2000 km Altitude by RadioPlasma Imager (RPI) on IMAGE: Radio Sounding of Electron Density and IonComposition (Hþ , Heþ , Oþ). American Geophysical Union, Fall Meeting, 2009.

A.K. Singh et al. / Planetary and Space Science 59 (2011) 810–834834

Spasojevic, M., Goldstein, J., Carpenter, D.L., Inan, U.S., Sandel, B.R., Moldwin, M.B.,Reinisch, B.W., 2003. Global response of the plasmasphere to a geomagneticdisturbance. J. Geophys. Res. 108, 1340.

Spasojevic, M., Frey, H.U., Thomsen, M.F., Fuselier, S.A., Gary, S.P., Sandel, B.R., Inan,U.S., 2004. The link between a detached subauroral proton arc and a plasma-spheric plume. Geophys. Res. Lett. 31, L04803.

Stern, D.P., 1974. Models of the Earth’s electric field. NASA/GSFC X Doc, 602-74-159.Storey, L.R.O., 1953. An investigation of whistling atmospheric. Philos. Trans. R.

Soc. (London) 246, 113–141.Su, Y.Z., Oyama, K.I., Bailey, G.J., Takahashi, T., Watanabe, S., 1995. Comparison of

the satellite electron density and temperature measurements withplasmasphere–ionosphere model. In: Presented to the Workshop on Lowand Equatorial Latitudes in the IRI, New Delhi, 9–13 January 1995, will alsoappear in Adv. Space Res. in 1995.

Su, Y.Z., Thomsen, M.F., Borovsky, J.E., Foster, J.C., 2001. A linkage between polarpatches and plasmaspheric drainage plumes. Geophys. Res. Lett. 28, 111–113.

Summers, D., Ni, B., Meredith, N.P., Horne, R.B., Thorne, R.M., Moldwin, M.B.,Anderson, R.R., 2008. Electron scattering by whistler-mode ELF hiss in plasma-spheric plumes. J. Geophys. Res. 113, A04219.

Takahashi, K., McPherron, R.L., 1982. Harmonic structure of Pc 3-4 pulsations.J. Geophys. Res. 87, 1504–1516.

Tanaka, Y., 1983. Introduction to Hinotori. Sol. Phys. 86, 3–6.Tarcsai, G., Szemeredy, P., Hegymegi, L., 1988. Average electron density profiles in

the plasmasphere between L¼1.4 and 3.2 deduced from whistlers. J. Atmos.Terr. Phys. 50, 607–611.

Taylor Jr., H.A., Grebowsky, J.M., Walsh, W.J., 1971. Structured variations of theplasmapause: evidence of a corotating plasma tail. J. Geophys. Res. 76, 6806–6814.

Thorne, R.M., Smith, E.J., Burton, R.K., Holzer, R.E., 1973. Plasmaspheric hiss.J. Geophys. Res. 78, 1581.

Titheridge, J.E., 1998. Temperatures in the upper ionosphere and plasmasphere.J. Geophys. Res. 103, 2261–2277.

Trakhtengerts, V.Y., Demekhov, A.G., Titova, E.E., Kozelov, B.V., Santolik, O.,Gurnett, D., Parrot, M., 2004. Interpretation of Cluster data on chorus emis-sions using the backward wave oscillator model. Phys. Plasma 11, 1345–1351.

Tsurutani, B.T., Smith, E.J., Thorney, R.M., 1975. Electromagnetic hiss and relati-vistic electron losses in the inner zone. J. Geophys. Res. 80, 600–607.

Tsyganenko, N.A., 1989. Quantitative models of the magnetospheric magneticfield—methods and results. Space Sci. Rev. 54, 75–186.

Tsyganenko, N.A., Stern, D.P., 1996. Modeling of the global magnetic field of thelarge scale Birkland current system. J. Geophys. Res. 101, 27187–27198.

Tu, J.N., Horwitz, J.L., Song, P., et al., 2003. Simulating plasmaspheric field-aligneddensity profiles measured with IMAGE/RPI: effects of plasmasphere refillingand ion heating. J. Geophys. Res. 108, 1017.

Tu, J.N., Song, P., Reinisch, B.W., Green, J.L., Huang, X., 2006. Empirical specificationof field-aligned plasma density profiles for plasmasphere refilling. J. Geophys.Res. 111, A06216.

Tu, J.N., Song, P., Reinisch, B.W., Green, J.L., 2007. Smooth electron densitytransition from plasmasphere to the subauroral region. J. Geophys. Res. 112,A05227. doi:10.1029/2007JA012298.

Vainio, R., Desorgher, L., Heynderickx, D., Storini, M., Flu€ckiger, E., Horne, R.B.,Kovaltsov, G.A., Kudela, K., Laurenza, M., McKenna-Lawlor, S., Rothkaehl, H.,Usoskin, I.G., 2009. Dynamics of the Earth’s particle radiation environment.Space Sci. Rev. 147, 187–231. doi:10.1007/s11214-009-9496-7.

Van Allen, J.A., Frank, L.A., 1959. Radiation around the Earth to a radial distance of107400 kilometers. Nature 183, 430–434.

Volland, H., 1973. A semi empirical model of large-scale magnetospheric electricfields. J. Geophys. Res. 78, 171–180.

Webb, P.A., Essex, E.A., 2003. Modifications to the Titheridge upper ionosphere andplasmasphere temperature model. J. Geophys. Res. 108, 1359. doi:10.1029/2002JA009754.

Webb, P.A., Essex, E.A., 2004. A dynamical global model of the plasmasphere.J. Atmos. Sol.–Terr. Phys. 108, 1057–1073.

Webb, D.C., Lanzerotti, L.J., Park, C.G., 1977. A comparison of ULF and VLFmeasurements of magnetospheric cold plasma densities. J. Geophys. Res. 82,5063–5072.

Weimer, D.R., 1996. A flexible, IMF dependent model of high-latitude electricpotentials having ‘‘space weather’’ applications. Geophys. Res. Lett. 23,2549–2552.

Wilson, G.R., Horwitz, J.L., Lin, J., 1992. A semi-kinetic model for early stageplasmasphere refilling, 1, effects of Coulomb collisions. J. Geophys. Res. 97,1109–1119.

White, R.S., 1973. High-energy proton radiation belt. Rev. Geophys. 11, 595–632.Yizengaw, E., Dewar, J., MacNeil, J., Moldwin, M.B., Galvan, D., Sanny, J., Berube, D.,

Sandel, B., 2008. The occurrence of ionospheric signatures of plasmasphericplumes over different longitudinal sectors. J. Geophys. Res. 113, A08318.

Yizengaw, E., Moldwin, M.B., Galvan, D.A., 2006. Ionospheric signatures of aplasmaspheric plume over Europe. Geophys. Res. Lett. 33, L17103.

Yoshikawa, I., Yamazaki, A., Yamashita, K., Takizawa, Y., Nakamura, M., 2003.Which is a significant contributor for outside of the plasmapause, an iono-spheric filling or a leakage of plasmaspheric materials? Comparison of He II(304 A) images. J. Geophys. Res. 108, 1080. doi:10.1029/2002JA009578.

Yumoto, K., 1986. Generation and propagation mechanisms of low-latitudemagnetic pulsations—a review. J. Geophys. 60, 79–105.

Glossary

Magnetic indices are used to characterize the disturbances of a magnetizedplanet. The three most commonly used magnetic indices for Earth are KP, Dst and AE.KP a planetary index, is tied to the disturbance level of the solar wind. Dst and AE

measure respectively, the disturbance levels of magnetic storms and auroralelectrojets.

KP index:: KP is intended to be a ‘‘quantitative’’ measure of the planetarydisturbance level. KP ranges from 0 to 9 with plus and minus designationsgiven between two intervals. Higher KP values mean more intense disturbancelevels and the scale is logarithmic. The construction of KP values involves Kindices from a number of stations which are collected and regional peculia-rities are removed. The average of the ‘standardized’ values represents thegeomagnetic planetary KP index (Parks, 1991);

Dst index:: The Dst index provides information on magnetic storms and isconstructed from world-wide mid-latitude and equatorial magnetograms.The number of stations that recorded the onset of a storm (generallydetermined by the Sudden Commencement) is noted, and then values of themagnetic field from each station are written out in rows, commencing withthe storm. The values are then averaged from all the stations. The averageeffect of a storm time variation is a reduction of the horizontal component ofthe geomagnetic field. Dst is negative and a larger negative Dst means a moreintense storm (Parks, 1991);

AE index:: This index provides information on the auroral activity and the index isconstructed from magnetic records obtained by magnetometers locatedthroughout the auroral zone. The magnetic records, arranged in UniversalTime, are superimposed and two traces are then drawn, creating an envelopeof positive and negative variations. The AE represents the value of themagnetic field between the upper and the lower envelopes. In constructingthe AE index, it is important to use as many magnetograms from as many localtime regions as possible because auroal currents have strong local timedependence (Parks, 1991).