Indian Journal of Radio & Space Physics Vol. 28. October 1999. pp. 203-210

An investigation of two types of interplanetary transient disturbances and their effects on cosmic ray intensity in relation with solar wind plasma parameters

Subhash C Kaushik

Department of Physics, Govt. Autonomous (PG) College,sidhi (MP) 486 661

and

Pankaj K Shrivastava

Department of Physics, Govt. New Science College, Rewa (MP) 486 00 I

Received 20 November 1998; revised 2 August 1999; accepted 24 August 1999

Two types of interplanetary disturbances, namely, magnetic clouds events (MCEs) and bidirectional events (BDEs) are taken to study the short-term changes in solar wind plasma components as well as in cosmic ray intensity. These two types of disturbances are again separated into two categories: (i) coronal holes associated events for both MCEs and BDEs, and (ii) without coronal holes associated events for both MCEs and BDEs. Analysis of thi s study indicates distinctly different effects on solar wind plasma velocity and cosmic ray intensity. The BDEs associated with coronal holes are found to be significantly responsible for enhanced plasma velocity. Both the disturbances in either category produce short-term decrease in cosmic ray intensity, but lesser decrease and fast recoveries are evident with the association of coronal holes.

1 Introduction

Interplanetary disturbances with a shock wave at AU, generally, have distinct plasma and field signatures by which they can be distinguished from ordinary so lar wind l. Magnetic cloud events (MCEs) present a descriptive image of a coronal mass ejection (CME)-caused interplanetary disturbance. The cloud carries an intrinsic field , often with the characteristics of a flux rope2

,3. It interacts with other solar wind structures and with the ambient medium4

-6

. Magnetic c louds have been identified with geomagnetic effects at earth 7,8. Other measurable characteristics of potentially geoeffective parcels of solar wind include bidirectional electron heat flux (BEHF) events, which are termed as bidirectional events (BDEs). These are interpreted as population of electrons travelling along interplanetary magnetic field lines which either are rooted at both ends in the slln or else are on closed loops entirely disconnected from the sun. Now, it has been fOllnd that the plasma and magnetic field characteristics of these two types of transient disturbances are distinguished markedly. Theories have predicted that if magnetic cloud and BEHF are truly the fast magnetized plasmoids movin g away from the sun into interplanetary space, then the interplanetary magnetic field (IMF) must be draping ,> rnund them9

. This field line draping around BEHF is

treated as a likely source of the out-of-the ecliptic magnetic fields at I AU (Ref. 10). The IMF draping was thought of as being the possible cause of the characteristic eastward deflection of B EH F at earth's orbit. This provides insight into the physical nature of these two interplanetary transients. In this paper a systematic study has been performed to draw the relationship of magnetic field characteristics of MCE and BDE with various interplanetary / geomagnetic phenomena as well as with cosmic rays intensity.

2 Data and method of analysis

The data of MCE and BDE events with an accuracy of I hour for the period 1978-1982 are taken from IMP 8 and ISEE 3 satellite observations ll . These events are listed in Table I. The shock wave in the solar wind is, generally, identified, in high time resolution plots of solar wind plasma and magnetic fie ld data, as abrupt and simultaneous increase in bulk flow speed V, particle density Np, proton temperature Tp and magnetic field B (Ref. 12). The values of solar wind components are taken from interplanetary medium data bookl '. For cosmic rays, the daily mean temperature and pressure corrected values recorded by super neutron monitor at Calgary (lat.51.05°N, long. 114.08°W. cut-off rigidity 1.09 GY) have been taken . In the present investigation, superposed epoch

204 INDIAN J RADIO & SPACE PHYS, OCTOBER 1999

Table I-List of the events analyzed MCE

Event ordinal Date of crossi ng Time Date of recording Time Type of so lar number hrs hrs wind

I 04 Jan. 1978 12 05 Jan. 1978 23

2 03 Apr. 1978 22 04 Apr. 1978 09

3 04 June 1978 24 05 June 1978 17

4 27 Aug. 1978 18 28 Aug. 1978 \3

5 29 Sep. 1978 10 30 Sep. 1978 03

6 29 Oct. 1978 23 30 Oct. 1978 22 CH

7 07 Jan. 1979 02 08 Jan . 1979 07

8 03 Apr. 1979 04 04 Apr. 1979 20

9 18 Sep. 1979 08 19 Sep. 1979 07

\0 16 Feb. 1980 04 17 Feb. 1980 04

II 19 Mar. 1980 16 21 Mar. 1980 19

12 II Dec. 1980 23 13 Dec. 1980 07 CH 13 19 Dec. 1980 14 20 Dec. 1980 13

14 06 Feb, 1981 19 07 Feb. 198 1 19 CH

15 05 Mar. 1981 13 06 Mar. 198 1 07

16 17July1981 22 18 July 1981 15

17 19 Sep. 1981 07 19 Sep. 198 1 20

18 12 Feb. 1982 04 13 Feb. 1982 2 1

BDE

12 Nov. 1978 15 13 Nov.1978 12

2 25 Dec. 1978 18 25 Dec. 1978 20

3 25 Jan. 1979 07 25 Jan. 1979 13

4 04 Feb. 1979 16 05 Feb. 1979 24 CH

5 18 Feb. 1979 13 19 Feb. 1979 04

6 21 Feb. 1979 16 21 Feb. 1979 18 CH

7 09 Mar. 1979 24 10 Mar. 1979 12 CH

8 22 Mar. 1979 17 22 Mar. 1979 18

9 29 Mar. 1979 09 29 Mar. 1979 16 CI-I

\0 05 Apr. 1979 13 06 Apr. 1979 09

II 25 Apr. 1979 07 25 Apr. 1979 13 CI-I

12 29 May 1979 21 31 May 1979 09 CI-I

13 20 Aug. 1979 14 21 Aug.1979 06

14 29 Aug. 1979 19 30 Aug.1979 17 CI-I

15 07 Oct. 1979 02 02 Oct. 1979 09

16 12 Nov. 1979 02 13 Nov. 1979 19

17 30 Nov. 1979 15 01 Dec. 1979 02

18 06 Apr. 1980 24 07 Apr. 1980 04

19 19 July 1980 01 19 July 1980 09

20 29 Dec.1981 16 13 Dec.1981 09

Column 2: Date of spacecraft crossing the forward boundary of magnetic cloud or of a SW region with bidirectional electron heat flux.

Column 3: Time of spacecraft crossing the forward boundary of MC or BDE.

Column 4: Date of recording of the rear boundary of MC or BDE.

Column 5: Time of recording of the rear boundary ofMC or BDE.

Column 6: Characteristic of the type of solar wind flow.

KAUSHIK & SHRIVASTAVA: INTERPLANETARY DISTURBANCES & THEIR EFFECTS ON COSMIC RAYS 205

aualysis has been applied to find short-term effects. By this statistical technique one can detect the periodic or recurrent, and non-periodic variation. In this analysis, data event position having special

features are taken as zero position or zero epoch day. The values are written down with reference to zero epoch day in series of column of Table I for positive and negative days, the succeeding values to this zero epoch consecutively written in column + 1,+2,+ 3,-----, etc. and preceding values are entered in column -I, -2, -3,--------, etc. and then average value for each column is calculated and their devi<;ttions from average values of relevant day (zero epoch day) are also calculated. The value of deviation for each day is plotted against the column number of both sides of the zero epoch time and a curve is obtained. The curve depicts the expected variations in the values of a particular physical quantity with respect to time.

3 Results and discussion

It is well established fact that magnetic cloud

..... ~-! > • ·102 .q;z a:Q

~ ~ 100 ~> IIlW 00 U

0.

'" 10

BOE

NUTRON CALGARY

events (MCEs) and bidirectional events (BDEs) consist of different plasma and magnetic fi eld characteristics. Such disturbances in interplanetary medium certainly produce influences on energetic particles of galactic cosmic rays as well as disturbances in the earth ' s magnetic field. ~o observe the average behaviour of solar and geomagnetic disturbances during the period of BDEs and MCEs, the Chree analysis, as shown in Fig. I, has been done in this study. Lower and middle panels of Fig. I show the behaviour of Rz and A p index for both the events. A significant decrease in sunspot numbers after zero day (onset day of event) is seen up to seven days for MCEs. On the other hand, sunspot numbers are found almost high for BDEs. Results depicted in the middle panel show significant increase in Ap index for both types of events. Upper panel of Fig. I reveals average behaviour of cosmic ray intensity. It is seen that both types of events (BDEs and MCEs) produce decrease in cosmic ray intensity on short-term basis. In earlier studies it has been found that the magnetic cloud 111

MCE

NUTRON CALGARY

Or-----------4-------------------~ 160

150 ,. a: 140

130

-4 -2 o 2 4

DAYS 6 8 10 -4 - 2 o 2

DAY5 6 6 10

Fig. I-Superposed epoch analys is plots of sunspot numbers Rz. Ap index and cosmic ray intensity (percent devi ation) associated with both transient disturbances BOE (left) and MCE (right).

206 INDI AN J RADIO & SPACE PHYS, OCTOBER 1999

association remarkable decrease in fi eld l4

.

with shock waves are one of the factors , which produce short-term cosmic rays as well as geomagnetic

It is a well known fact that solar wind velocity plays an important role to produce short-term as well as long-term modulation in cosmic rays. Earlier investigators have reported that the solar-flare generated high-speed solar wind streams are responsible to produce short-term transient decrease in cosmic ray intensity and, on the other hand, larger increase in geomagnetic field 8

, ls. Thus the energy propagating in interplanetary medium through these plasma streams produces influence on the low and hi gh cosmic ray particles as well as on geomagnetic field of earth.

In fig . 2, cross-corre lation have been drawn between the average va lues of solar winds velocity and cosm ic rays for both the periods of bidirectional events and magnetic clouds events . Left and right panels of Fig. 2 show the scattered diagram for RDEs and MCEs, respcctively. We obtained better corre lation coeffic ient (R=0.64±0.17) between solar wind and cosmic rays for magnetic clouds events (MCEs), which was expected due to the influence of magnet ic c louds and coronal mass ejections during MCEs events. The presence of such configurations in the structure of solar wind generates a perturbation in the interplanetary magnetic field s that modulate the ga lactic cosm ic ray particles in interplanetary medium. Larger distribution for MCEs is ev ident as compared to BDEs.

Skylab's x-ray images of the sun , together with interplanetary plasma data from other spacecraft,

BDE 600

f-R=0.42~ 0.11

f-500

~. • • .: ~. ~ ~ • 400 • • •

a: j 300~~I ~I ~I~~' ~I~I~I-y~~~~~~~ o C/) 2480 2500 2520 2540 2560

COSM IC RAY (ACTUAL COUNT)

allowed rapid progress in relating coronal holes with high-speed low-density solar wind l6

. Various investigations have proved that coronal holes are useful as a diagnostic factor in geomagnetic forecasting, but like disappearing filaments they are not especially reliable 17

. We fU11her. need to know whether the terrestrial effects are primari Iy the result of high-speed solar winds associated with coronal holes or they are the consequences of 'the interaction between differing solar winds flow. Thus, for further analysis, coronal holes are taken to observe their effect on interplanetary medium in relation with interplanetary / geomagnetic di sturbances. We know that the coronal holes are the regi ons on the outer surface of the sun , which propagate high-speed solar wind stream into interplanetary medium. Both MCE and BDE have been d ivided into h¥o classes-Dne which is associated with coronal holes, and the other which is not associated with coronal ho les and then their influences have been explicitly analysed .

Figure 3 shows the results of Chree analysis for BDE and MCE events with so lar w ind components, such as bulk flow speed, ion density and proton temperature on left and right panels, respectively. These two events are associated with coronal ho les zero day on the time sca le corresponding to the begi nning of the event. Coronal holes associated BDEs show significant increase in so lar wind velocity and' proton temperature. Both show transient enhancements lasting six to seven days after the zero day . Ion density decreases immediately after the occurrence of the event. However, on the other hand, results of Chree analysis of coronal holes associated MCEs show different effects on solar wind velocity as

MCE 600~------------------------,

500 - • • 400 ••

R=O.64 "±: 0.17

. .. .. ••••

300~~-y~~~~~yl~~~~, ~~

2500 2520 2540 2560 2580 2600

COSMIC RAY (ACTUAL CO U T)

Fig. 2--Cross-corrclat ion plot between the daily values of Calgary neutrons and solar wind veloci ty for hnth types of even ts TI1DE (\eft)

and MCE (r ight) panels] .

KA USH IK & SHRIVASTAVA: INTERPLANETARY DISTURBANCES & THEIR EFFECTS ON COSMIC RAYS 207

BDE MCE

CII- ASSOCIATED [VENTS CII - ASSOCI.\TU' [\T~'TS

130

x: t-/V ~ M 90 S? >< n 50

0-

10

17

,., 13 I

E v

ci 9 z

5

510

-;-III 450 f .x

>" 390

330 ' ,

" , 0 2 " 6 8 10 " 2 0 2 " 8 10

DAYS DAYS

Fig. 3-Superposed epoch analysis plots of the plasma parameters, i.e. solar wind velocity V, ion densi ty Np and proton temperature, Tp in transient di sturbances of two types of events [SDE (left) and MCE (righ t) associated with coronal holes (CH)] .

shown in Fig. 3 (right panel). An enhanced bulk solar wind velocity and ion density on the onset of event day are evident. Proton temperature does not provide any meaningful result except a sharp decrease on .the first day after the event day. Hence, it is concluded that the bidirectional events (BOEs) are more effective to produce increase in solar wind velocity as well as for transient fluctuations in ion temperature than the events of magnetic clouds (MCEs).

Further, the Chree analysis has been performed to observe the effects of BOEs and MCEs which are not associated with coronal holes. Results are shown in Fig. 4. The BOEs show a maximum solar wind speed on the event day which starts increasing before I day of the event, goes to maximum and then decreases sharply taking 5 days on the time scale. Similarly, the ion density and temperature start increasing before one day and after having the maximum values on the event day decrease continuously for the following (next) day . Similar results for solar wind velocity are observed for MCEs without coronal hole-associated events. However, density values are found minimum up to four days after the zero epoch day. Fluctuation are seen for the proton temperature. Here, we can conclude that only coronal hole-associated magnetic

cloud events are found responsible to prod uce significant decrease in solar wind velocity on short­term basis.

Figure 5 shows the effect of these two tran sient variations on cosmic ray intensity on short.-term basis. Lower panel of Fig. 5 shows the coronal-hole associated events and the upper panel represents the events without coronal holes. Slight decreases are evident in the Chree plots for both the categories of BOEs and MCEs. From critical examination it can be said that the bidirectional events with or without coronal hOoles produce larger decrease in cosmic ray intensity than the magnetic cloud events. Hence, it can be inferred that coronal holes are not a prominent factor to produce decrease in cosmic ray intensity.

On comparing averaged relationsh ip of plasma parameters for MCE and BOE from Figs 3 and 4, it is found that the characteristics of these two types of transient disturbances . in association with coronal holes (CH) or without coronal holes differ appreciably in plasma velocity and temperature. Increase in the solar wind plasma velocity in the BOE with CH case is about 50% larger than that for MCE with CH case. The MCE and BOE without CH only show slight differences in proton temperature where BOE without

208

<;"

E v

. a. Z

"", E 450

oX

-> 390

IND IAN J RADIO & SPACE PHYS, OCTOBER 1999

BDE

CII-NOI' -ASSOCIATI:D I:V[,. .. 5

-4 - 2 0 2 DAYS

MCE

CII _r.: ON _ASSOCIATl: O r.V[""T$

o 2 DAYS

~ I

4

fig. 4--Superposed epoeh analysis plots of the plasma parameters, i.e. so lar wind velocity I', ion density Np• proton temperature Tp. in

transient disturbances of two types of events I BDE (Ieli) and MCE (ri ght) without association with coronal ho les «(' 11 )1 ·

>­.....

2550

2590

;:n 2530 z w ..... z 2470

~ 0:: 2550 u

MCE BDE

CH : NON _AssorUl [I) EVE'"~ S

ClI =NUN _,\SSOC IAT ED Ev rN I 5

ClI - ASSOCIAT ED EV [~-rS

C it =- ASSOC IATED EVENTS

~ 2590 o

u :::: ~~~~Ull~~~ullWll~~WWilli~ilL~~~~~~~~~~~~'~ - L. - 7 0 2 8 -t. - 'I (I ? /. 5 a 1C'

DAYS DA'C,

Fig. 5--Superposed epoch analysis plots of cosmic ray intensity associated with transient disturball(;es (HDE and 1\1C L) ILd't panel shows BDE associated with C H (bottom) and without CII (top). Right panel rep resents MC E wi th CH (hottol'1) and \\ ithout C! I C\(:ilt

c

(top)· 1

..

KAUSHIK & SHRIV ASTA V A: INTERPLANETARY DISTURBANCES & THEIR EFFECTS ON COSMIC RAYS 209

CH produce 40% large decrease in comparison to MCE without CH events. However, other investigators9 have reported different magnetic field characteristics in these two types of transient disturbances (BDE and MCE) . The magnitude of the magnetic field in the sheath region in MCE increases as one approaches the MC boundary, while in BDE, immediately infront of BEHF boundary, the magnetic field decreases. Larger and varying magnetic field during the period of MCEs certainly produces decrease in solar wind plasma velocity as well as in the intensity of cosmic rays. Therefore, increase in plasma velocity is associated with bidirectional events accompanied with coronal holes. Observational results indicate physical influence of coronal holes on solar wind as well as on cosmic rays. It is known that the area and position of coronal holes may serve as a certain quantity in the distribution of an open magnetic flux on the sun. A coronal hole consists of a region of open magnetic field lines from which plasma can apparently easily expand and contribute significantly to the high speed solar wind . A close correspondence has been demonstrated between fast solar wind streams, the magnetic polarity of coronal holes and open field lines in the corona l8

. Thus, high speed plasma streams from shock / contact surface along neutral sheet, as it interacts with low speed ones coming from other part of the sun . When the galactic cosmic rays coming from deep surface interact with this shock / contact surface generated by the interaction of high speed streams with low speed solar wind streams, they suffer modulation effects. The propagation of these coronal hole-associated streams into the interplanetary medium significantly produces modulation in high energy particles as well as in magnetic field9

. The modulation of galactic cosmic rays at neutron monitor energies var ies year-to-year showing their dependence on solar sunspot cycles. It is expected to be so due to the variation of coronal holes area during sunspot maxima and mi nima. During sunspot maxi ma conditions, the coronal holes are small in area and concentrated near the poles. During solar minima conditions. the polar coronal holes ale much lal ger in area dom inating a larger fraction of the sf'lar disk and greatly influenc ing the hclio~}jh('nc CO;-:ditiGr.;o ill tlIe e~ li tic p\""e .

4 Cuudu"ions

It is con·:;\udcd from the (Jrcscnt analysis that: (i) Bidirectional events associated \'. ilh coronal

holes are found responsible for ploducing

increase in solar wind plasma velocity on short­tenn basis.

(ii) The magnetic cloud events do not produce any significant change in plasma components .

(iii) Both types of transient disturbances are fou nd respon sible in producing decrease in cosmic ray intensity on short-term basis, while solar wind velocity shows better correlation with cosmic rays for MCEs than for BDEs.

(iv) Some differences are found in cosmic ray intensity for BDEs and MCEs. Slight decrease and gradual recoveries are found in cosmic rays for either types of events but for without coronal hole association .

Acknowledgements

The authors thank the World Data Center A, USA, for providing interplanetary medium data. One of the authors (PKS) is grateful to the organizers of 23rd International Cosmic Ray Conference, Ca lgary, Canada, for providing the Calgary neutron monitor data to its partic ipants. One of the authors (SCK) thanks the University Grants Commission, CRO, Bhopal, for financia: support.

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210 INDIAN J RADIO & SPACE PHYS, OCTOBER 1999

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