asen 5335 aerospace environment -- the magnetosphere 1 aerospace environment asen-5335 instructor:...

ASEN 5335 Aerospace Environment -- The Magnetosphere 1

Aerospace EnvironmentASEN-5335

• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)

• Contact info: e-mail: [email protected] (preferred)

phone: 2-3514, or 5-0523, fax: 2-6444,

website: http://lasp.colorado.edu/~lix

• Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before and after class Tue and Thu.

• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166

• Read Chapter 4&5 and class notes

• HW4 due 3/13, Thursday

• Mid-Term, 3/20, close book.

mailto:[email protected]

http://lasp.colorado.edu/~lix


The Terrestrial Magnetosphere

• The terrestrial magnetosphere comprises the region of space where the properties of naturally occurring ionized gases are controlled by the presence of Earth’s magnetic field.

• This broad definition means that the terrestrial magnetosphere extends from the bottom of the ionosphere to more than ten RE in the sunward direction and to several hundred RE in the anti-sunward direction.

The magnetosphere is formed as a result of the interaction of the solar wind with the intrinsic magnetic field. Other planets’ magnetospheres are formed in the same way.


As the magnetized solar wind flows past the Earth, the plasma interacts with

Earth’s magnetic field and confines the field to a cavity, the magnetosphere.


Planets in Solar System


Planet orbits in Solar System


Properties of Some Solar System Objects


The Bow Shock and the Magnetopause The Earth’s magnetic field can be envisioned as a stationary object placed into the high-speed magnetized plasma flow: the solar wind, which is supersonic and superalfvenic.

We can expect the development of a standing shock wave ahead of the Earth.

Such a shock is necessary to slow down the solar wind so that the flow can diverted around the dipole.

Gas dynamics was used to model the bow shock in early times. For the simplest case, perfect gas, one-dimentional and steady-state case, we have the following:

d(nu)/dx=0

mnu du/dx + dp/dx =0

(3/2)u dp/dx + (5/2)p du/dx =0

Where m is the particle mass and n(x), u(x), and p(x) represent the particle number density, flow velocity , and pressure, respectively.


Earth’s Bow Shock

Since the solar wind is much faster than the Alfvenic speed, the intermediate shock solution is not relavant, and we need to look at compressive fast mode shock solution.

In the solar wind both the sonic and Alfvenic Mach numbers are large. Therefore the shock jump conditions can be greatly simplified. Such as the effects of the upstream thermal pressure and magnetic field can be neglected altogether. The following shows the jumping conditions:

These jumping conditions do not give its location. The location of the bow shock (BS) is basically determined by the shape and size of the magnetospheric “obstacle”, the magnetopause (MP). The region between the BS and MP is called the magnetosheath. What determines the location of the MP, BS (or the thickness of the magnetosheath)?


Example of Satellite Observations of the Bow Shock


Location of MP and BS

The location of the bow shock is basically determined by the shape and size of the magnetospheric “obstacle”, the MP and also the Mach number. The actual nonlinear processes involved are still a on going research.

The location of the subsolar point of MP is determined by the pressure balance, basically between solar wind dynamic pressure and Earth’s magnetic pressure. For average solar wind conditions, one gets RMP ~ 10 RE .

The ratio (RBS – RMP)/RMP has been found

empirically to be 1.1 (SW/2), such that RBS

~ 1.275 RMP ~ 13 RE.


The neutral points are the only points that connect the earth's surface to the magnetopause.

The neutral points are regions of interest since this is where solar wind particles (from the magnetosheath) can enter the magnetosphere without having to cross field lines. There is experimental evidence that this does happen. Particles with energies typical of the sheath are observed over some 5° of latitude around 77°, and over 8 hours of local time around noon.

These regions, being more extended than an idealized point, are called the polar cusps or polar clefts.


The plasma contained between the lobes, mostly on closed field lines, is called the plasma sheet.

The main distinguishing feature of the plasma sheet is that it consists of hot (keV) particles.

Plasma sheet is a mixture of particles originating in the solar wind (H+) and ionosphere (O+).

The Magnetotail & Plasma Sheet

Field lines emerging from the polar regions are swept back, away from the Sun; some of these would have connected on the dayside in a dipole field. These field lines constitute the magnetotail.

Magnetosheath

MagnetopauseRadiation Belts

BowShock

PolarCusp


The boundary of the plasma sheet is determined by a balance between the magnetic pressure of the tail lobes and the kinetic pressure of the plasma sheet plasma:

where BT = tail field intensity outside the plasma sheet.

The tail region is highly dynamic, and is only reasonably represented by the above description during quiescent periods.

nkT ~BT

2

2o


In the correct geometry, magnetic field lines can interconnect.

In general, the field lines must be anti-parallel and there must bean electric field as shown.

The electric field causes plasma and field lines to drift into the merging region (black arrows pointing along Z)

After merging, the B field lines have a component directed along Z, causing a drift perpendicular to the new B direction and to the E field. The new flow is along X as shown.

Magnetic connection occurs in the magnetotail and at the daysidemagnetopause.

Magnetic Connection


Historically, the current system (as inferred from ground magnetometers) led to the first speculations about circulation patterns in the magnetosphere, and theories about solar wind - magnetosphere interactions.

Magnetospheric Circulation

SqP

Given that e- are bound to field lines in the E-region (where ionospheric current flows) and ions are stationary by comparison, the following motions of field lines (electrons, opposite to conventional current flow) are inferred from the current patterns.

Motion of“feet” of field lines

SqP


The generally accepted explanation involves magnetic connection or merging (between the terrestrial magnetic field and the IMF) on the dayside and reconnection on the night side.

Magnetospheric Circulation


Recall from earlier notes that in a plasma an applied E-field results in the plasma drift

E and Vp are equivalent in highly conducting plasmas (frozen-in) such as the magnetosphere and solar wind.

VP

E

B

B2

B

E

Vp


Convection Electric Field

The ultimate driver of magnetospheric circulation is the momentum of the solar wind.

Applying the frozen-in-flux concept, an observer on Earth detects an electric field of E = -VP x B.

If the solar wind cannot penetrate the magnetopause, the convection electric field is also excluded from the magnetosphere.

The convection electric field in the solar wind is about 2 mV/m (assuming u=400km/s and B=5 nT), corresponding to about a 640 kV potential drop across the geomagnetic tail (assuming a tail diameter of 50 RE). The observed electric potential drop across the region of open field lines in the ionosphere is about 10 times smaller. This implies that about 10% of the interplanetary magnetic flux that impacts the geometric cross section of the magnetosphere reconnects with the geomagnetic field.


Dusk

E

E

+ + + + + +

- - - - - -

Magnetospheric Circulation - Equatorial Plane View

Dawn

The cross-cap potential is often used as a measure of intensity of solar-wind/magnetosphere interaction, and can reach values of order 200 kV during intense magnetic disturbances.


Polar Cap Electric Potential - Southward IMF


Polar Cap Electric Potential - Northward IMF


recon

Some of the sunward-convecting particles precipitate into the upper atmosphere and produce the aurora.



• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred) phone: 2-3514, or 5-0523, fax: 2-6444, website: http://lasp.colorado.edu/~lix• Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before

and after class Tue and Thu.• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166

• Read Chapter 5 and class notes• HW4 due today• Quiz-5, 3/18 (Tue), close book.• Mid-Term, 3/20 (Thu), close book.




PARTICLES IN THE MAGNETOSPHERE

The main particle populations are:

-- plasmasphere -- ring current

-- radiation belts -- plasma sheet

-- boundary layers (magnetosheath, mantle)

We have discussed the radiation belts extensively, and the plasma sheet to some extent. We will return to the plasma sheet when discussing magnetic storms.

The plasmasphere represents the relatively cold ionospheric plasma (~ .3 eV or T ~ 2000 K) which is co-rotating with the earth (frictional coupling).


Principal Plasma Populations in Earth’s Magnetosphere


Main Zones of Magnetospheric Plasma Precipitation

Boundary betweenOpen field lines atHigh latitudes andClosed field lines atLow latitudes


The Plasmapause

However, the plasmapause boundary is very dynamic, and varies between about 3 to 6 RE, sometimes getting as low as 2 RE.

Although not depicted as such in the previous figure, note that the plasmasphere overlaps with a considerable part of the radiation belt region. However, these represent two entirely different particle (energy) populations.

The outer boundary of the plasmasphere, at about 4 RE, is where the plasma density undergoes a sudden drop. This is the plasmapause.

Ring Current


The co-rotating plasmasphere sets up a "co-rotation" electric field:

Outside the plasmapause the plasma is not co-rotating, and the circulation there is determined by the cross-tail potential.

Essentially, the plasmapause represents the boundary where these two electric fields are of the same order:

where BE = equatorial magnetic flux density at the surface, L = distance in RE, and RE = radius of earth.

E

R

B

ET ~BE

L3 LRE

Plasmapause Boundary


Putting in numbers,

mVm-1

~ 1 mVm-1 at 4 RE

Put another way, the plasmapause represents the boundary between co-rotating plasmas and plasmas strongly influenced by the solar wind interaction (see following figure):

Viewed this way, one expects intensification of the outer magnetospheric circulation to lead to a contraction of the plasmasphere (inward movement of the plasmapause). This indeed happens (see subsequent figures).

In fact, it is thought that the intensified outer circulation leads to a peeling off of outer layers of the plasmasphere, which are then lost as detached plasma chunks in the magnetotail and solar wind.

ET ~14.4

L2


Schematic view of the plasmapause in relation to plasma convection in the equatorial plane


Satellite observations of ion density, showing the plasmapause at several Kp levels

L


Flow patterns for cross-tail fields of 0.2 and 0.6 mV/m

For 0.6 mVm-1, the magnetospherecirculation “intrudes”upon the plasmasphere.


Detaching of plasma due to changing flow patterns during a magnetic storm


With the decay of magnetic activity, the magnetospheric circulation and electric fields return to their previous state but now the outer tubes of magnetic flux are devoid of plasma.

These gradually refill from the ionosphere over a period of days.

The rate of filling is determined by the diffusion speed of protons (formed in the upper ionosphere by charge exchange between hydrogen atoms and oxygen ions) coming up along the field, and by the volume of the flux tube which varies as L4(?). It therefore takes much longer to refill tubes originating at higher latitude.

Observations of the filling are shown in the following figure. Since active periods may occur every few days there will be times when the outer tubes are never full and the plasmasphere has some degree of depletion.

“Filling In” of Plasmasphere

O H H O


“Filling In” of Plasmasphere

Refilling of plasmasphere after a storm, 18-19 June 1965


BOUNDARY LAYERS AND PARTICLE TRANSFER TO PLASMA SHEET

Solar wind particles find their way from the magnetosheath into the cusp region. There is experimental evidence for this entry, in that particles with characteristic "magnetosheath energy" (i.e., less than 1 keV) have been observed over a limited region centered around 77° magnetic latitude and noon (see following figure).

Such particles on newly-merged field lines flow down towards the earth, mirror there, and then return to find themselves on a field line sweeping back towards the tail. These particles form a particle population known as the "plasma mantle" (see following figures).

At many (~100) RE, these particles are swept into the plasma sheet. Another closer (~ 50 RE) source of plasma sheet particles is the polar wind emanating from the ionosphere at high latitudes.


Details of the Cusp Region

Plasma MantleMagnetosheath

EntryLayer

Low-Latitude Boundary Layer


Note: since ~1028-1029 particles/s impact the dayside magnetopause, and ~ 1026 particles/s are estimated to enter the plasma sheet, only 1% efficiency of this process is required.


1. The AE-8 and AP-8 radiation belt models are used by satellite designers to estimate mission lifetime particle fluxes. These models can be run on-line at

http://nssdc.gsfc.nasa.gov/space/model/models/trap.html

The input parameters are energy range, L-value and B/Bo where Bo is the magnetic field strength at the magnetic equator (minimum value).

a. Estimate the flux of electrons in particles/cm2/MeV between energies of 1 and 2 MeV that would be experienced by a geostationary satellite over the period of one day during solar maximum conditions. Do this for two cases: B/Bo={1, 2}.

b. Assuming isotropy, estimate the flux of electrons in units of particles/cm2/s/sr/keV between energies of 200 and 300 keV that would be experienced by a satellite in geostationary orbit during solar maximum conditions.

Note: The differential flux gives the flux between the energy values given. Integral flux gives the total flux greater than the each energy value input.

ASEN-5335 Aerospace Environment Homework Assignment #5: Radiation BeltsPrintable Version







2. Daily plots of actual measured charged particle fluxes in geostationary orbit can be obtained from the Los Alamos National Laboratory at

http://leadbelly.lanl.gov/lanl_ep_data/lanl_ep.html

Under the selection "Browse the Summary Plot Database", one can select a date and request a plot of proton or electron flux from the Los Alamos satellites in geostationary orbit on a particular day.

a. Generate plots of the low-energy (30-300 KeV) electron fluxes (particles/cm2/s/sr/KeV) that were measured in geostationary orbit on March 22, 1979 (this is the same magnetically disturbed day considered in HW #3).

b. What LANL satellites were available to provide these data? (international satellite designations are of the form 1985-032).

c. What LANL instrument provided these data?

d. Comment on the behavior of the electron fluxes exhibited in the plots. What features are noteworthy?

e. How do the measurements of the 200-300 KeV fluxes compare with those which you estimated in question #1b?










http://lasp.colorado.edu/~lix/class/asen5335/hw5-p.html


























Particle Flow in the Merging - Reconnection - Convection Process

During the return flow, the particles are energized by the down-dusk electric field and their first adiabatic remains as an invariant,

= const As particles convect towards the earth, B increases, therefore the particle energies increase. The energy comes from the E-field.

1

2

mv2

B

“Dipolarization” of the B-field


The Phases of the Aurora

Quiet Growth Onset

Expansion Maximum Area Recovery


Early Auroral Studies


Producing the Aurora

Auroras are produced by electrons andprotons striking Earth’s atmosphere.When oxygen and nitrogen atoms arehit by these energetic particles, theybecome excited. As they relax to their original state, they emit light of a characteristic color

Green = OxygenRed = Oxygen (lower energy electrons)

Blue = NitrogenAlso emitted in UV and X-ray


Magnetospheric Current Systems

Currents are produced by moving electrically charged particles. In turn, current systems generate magnetic fields. In the magnetosphere there are five principal current systems that play an important role in the formation of the magnetosphere. (1) the magnetopause current, (2) the tail current , (3) the ring current, (4) field-aligned (Birkeland) currents and (5) ionospheric currents.

(1) The magnetopause current generates a magnetic field that “prevents” the terrestrial dipole field from penetrating into the solar wind (vice versa). An enhanced MP current gives rise to an increase of the Dst index.

(2) The magnetic tail is formed by the highly stretched (“open”) magnetic field lines originating from the polar ionosphere (called the polar cap). The tail magnetic field is naturally generated by a current system, based on the Ampere’s law: xB=0 j.


(Temerin and Li , 2002)

Prediction efficiency=91%

Linear correlation coeff.=0.95


MAGNETOSPHERIC CURRENTS

The combination of plasma and electric fields in the magnetosphere allows electric current to flow. Several current systems have been identified:

• magnetopause current (A) • tail current (B) • Birkeland (field-aligned) currents • ring current (C)

The strong deviations of the magnetosphere from a dipole shape are in fact due to the first three (A, B, C) of the above current systems. These are now discussed in turn.


MAGNETOPAUSE CURRENT

If we ignore any magnetic or electric field in the solar wind, the origin of the magnetopause current and the corresponding modifications of the magnetic field can be grossly understood as follows:

Consider a small section of the dayside magnetopause with the solar wind normal to it (see following figure). The ions are deflected one way and the electrons the other, causing a current to flow (consisting mostly of ions due to their greater penetration depth). The current flow at the magnetopause is such that its magnetic field cancels the geomagnetic field outside the boundary.

Similarly, earthward of the boundary the field strength is doubled -- this is essentially the "compression" of the dayside magnetosphere that we have alluded to before.


Currents and Fields at the Magnetopause Boundary


It turns out that the force produced by this current (the so-called Lorentz or J X B force) balances the momentum force of the solar wind, which is another way of stating the "dynamic pressure" vs. "magnetic pressure" balance we discussed before.

When the solar wind intensifies, the magnetopause current is increased:

-- This further "compresses" the dayside magnetosphere;

-- The ground magnetic signature of this sudden currentincrease associated with the compression is called a "sudden impulse" (SI), or if it is connected with the beginning of a storm, it is called a "sudden storm commencement" (SSC).


TAIL CURRENT

The down-wind extension of the magnetosphere into a tail indicates the presence of a current system as follows:

View from Earth


This is what keeps the dark-side magnetic field from assuming its dipolar form. The energy to do this comes from the solar wind. The magnetic flux in a current loop is

Since BT ~ 20 nT, then iT ~ .016 A/m. Assming something reasonable for the tail length, iTail ~ 108 A. For a cross-tail potential of 60 kV, the power extracted from the solar wind ~ 6,000 GW !!

BT oiT

The neutral sheet separates the northern lobe from the southern lobe.

"neutral sheet" here refers to the magnetic field, and the region where currents flow so that reconnection is inhibited, similar to the heliospheric current sheet.

• • • • • •

• Current out of page

j = 1

0

B

2

3

1

j

o

j1 =

B =

B2

x3

ˆ i 1


Energy is Imparted to the Plasma During Reconnection


Ring Current and Field-Aligned Current

The ring current is primarily composed of geomagnetically trapped or quasi-trapped 10 to 200 keV particles (H+ and O+) drifting around the Earth, due to the gradient-curvature drift in conjunction with ExB drift. The electric field is directed from dawn from dusk (as we have discussed earlier).

The gradient-curvature drift is ~L2 while the ExB drift is ~L3.

The ring current is the main contributor to the Dst index.


RING CURRENT

Under magnetic storm conditions the magnetic field of the earth at low latitudes may be depressed ~ 1-2% for a day or two (main phase). This is mainly due to a westward ring current which we have already discussed in relation to particle drift (gradient-curvature drift) on curved field lines with the magnitude of B increasing towards the earth.

Recall that the gradient drift depends on the particle "magnetic dipole moment"

-- hence gradient drift is not important for "cold"particles like those populating the ionosphereand plasmasphere; these particles co-rotate.

vg B B

eB2

1

2

mv2

B


However, it is also true that it is not the energetic Van Allen particles that are the main contributors to the ring current. The fluxes of these particles are too small. the main ring current particles are H+ and O+ of 20 - 100 keV (see following figure).

The ring current is located between 3 and 6 RE, close to the inner edge of the plasma sheet and and outer edge of the slot region.

Where do the ring current particles come from?

----- magnetospheric convection, after reconnection, charged particles are transported inward by the dawn to dusk electric field. As they gain energies, their

gradient-curvature B drift will start to compete with the ExB drift. Since E varies with time, some of the charged particle will be trapped.


Energy and Number of Protons in the Ring Current at L = 4 during a Magnetic Storm



• Instructor: Prof. Xinlin Li (pronounce: Shinlyn Lee)• Contact info: e-mail: [email protected] (preferred) phone: 2-3514, or 5-0523, fax: 2-6444, website: http://lasp.colorado.edu/~lix• Instructor’s office hours: 9:00-11:00 am Wed at ECOT 534; before

and after class Tue and Thu.• TA’s office hours: 3:15-5:15 pm Wed at ECAE 166

• Read Chapters 1-5 and class notes.• Quiz-5, today, close book.• Mid-Term, 3/20 (Thu), close book.• HW5 due 4/3 (Thu).




2. Daily plots of actual measured charged particle fluxes in geostationary orbit can be obtained from the Los Alamos National Laboratory at


Under the selection "Browse the Summary Plot Database", one can select a date and request a plot of proton or electron flux from the Los Alamos satellites in geostationary orbit on a particular day.

a. Generate plots of the low-energy (30-300 KeV) electron fluxes (particles/cm2/s/sr/KeV) that were measured in geostationary orbit on March 22, 1979 (this is the same magnetically disturbed day considered in HW #4).

b. What LANL satellites were available to provide these data? (international satellite designations are of the form 1985-032).

c. What LANL instrument provided these data?

d. Comment on the behavior of the electron fluxes exhibited in the plots. What features are noteworthy?

e. How do the measurements of the 200-300 KeV fluxes compare with those which you estimated in question #1b?

















BIRKELAND (FIELD-ALIGNED) CURRENTS

Magnetometers carried on board satellites have detected persistent perturbations of the earth's magnetic fields over the auroral zones which can only be interpreted as resulting from currents flowing into and out of the ionosphere.

The following figure shows the average locations of these currents for two levels of magnetic disturbance as determined from measurements on the TRIAD satellite.

The solar wind/magnetosphere interaction provides energy and momentum to the magnetosphere system; the magnetospheric circulation is determined by redistributing its plasma and fields in a way that allows for dissipation of this energy. This dissipation occurs in the form of:

• energizing particles which give up their • dispelling blobs of plasma out the energy to the neutral atmosphere; magnetotail;

• developing a current system capable of • transferring momentum dissipating energy through ohmic losses; to the neutral atmosphere.


R2

R1

R1

R2

Quiet Active

Region 1 and Region 2 Current Systems

R2

R2

Current flow is also consistent with the requirement for dissipation of the energydeposited into the magnetosphere by the solar wind; ohmic dissipation of currents inthe ionosphere is one way of doing this.


Region 1 Current System

Dusk Dawn

B

Sun

J E 0

The magnetic field lines are highly conducting, and so it is natural that the magnetosphere seeks some closure of current through an ionospheric route. In fact, the so-called Region 1 currents are necessary if we are to require the polar ionosphere to convect with the magnetic field lines:


The origin of the Region 2 currents is connected with a longitudinal asymmetry in the ring current, also referred to as partial ring current. The partial ring current must be closed, it must therefore discharge current (into the conducting ionosphere) on the dusk sector and draw current from the ionosphere in the dawn sector, as shown in the following figure.

A variety of plasma processes also occur that induce electric fields || to B; these fields accelerate energetic particles into and out of the auroral region.

Connection between Ionospheric and Magnetospheric Currents


Magnetic Storms and Substorms

Recall that coupling between the magnetosphere and solar wind is considerably more efficient during southward IMF (Bz < 0) than northward IMF (Bz > 0), simply because of the way that magnetic merging occurs in the dayside magnetosphere. A reminder of this process for Bz < 0 is shown on the following page.

During extended periods of northward IMF, the magnetosphere is usually in a quiescent state, unless discontinuities in the solar wind impulsively impact the magnetosphere. In this case there is sure to be an impulsive response of some significance; but the exact scenario through which this occurs is not very well established for Bz > 0. We will concentrate on those "storm" events for which Bz < 0.

At a southward turning of the IMF, merging on the dayside, day/night transfer of magnetic flux, and nightside reconnection are increased. The system is no longer in equilibrium. Energy in the form of magnetic flux builds up in the tail.


Summary of a Magnetic Substorm Scenario

A substorm is a time period of enhanced energy input from the solar wind into the magnetosphere and its subsequent dissipation in the magnetosphere-ionosphere system.


recon

Some of the sunward-convecting particles precipitate into the upper atmosphere and produce the aurora.


A thinning and pinching of the plasma sheet occurs at the beginning of a substorm

This neutral point in the tail now occurs much closer to earth. This buildup occurs over about 1/2 - 1 hour, and is called the initial or growth phase of the substorm.

The substorm onset phase occurs when the tail energy is released, and marks the beginning of the substorm expansion phase.


After magnetic merging near ~100 RE then another merging at 20-30 RE , a plasmoid is ejected out the magnetotail

Neutral line location under quiescent conditions ~1000 RE

Formation ofsecond neutralline andaccumulationof magneticflux.

Return to pre-substorm conditions

expel plasmoid


Particles are energized and injected into the ring current, as reflected in the Dst index

Region 1 currents flowbetween the plasma sheet

and the ionosphere

(100’s keV)

The flow of current through the ionospheric circuit is consistent with the collapse of the tail at the neutral point; the latter can only occur if the cross-tail current is substantially reduced, as it is when being diverted along the field lines into the ionosphere.


• Concurrent with the flow of Region-1 currents into the ionosphere at the onset of a substorm, currents and electric fields spread throughout the conducting ionosphere; even at equatorial latitudes the signatures of “penetration” electric fields are seen.

• After ~1 hour, the Region-2 currents close in the ionosphere and set up a counter-potential field, shielding the lower latitudes from electro-dynamic coupling from high latitudes.

• The magnetosphere can be viewed as a voltage generator imparting the same potential across N. and S. polar regions; the currents that flow depend upon the conductivity of the ionosphere.


Auroral Potential Structure


Electron Precipitation & the Aurora


Major Magnetic StormsA major magnetic storm generally follows an extended period of southward IMF.

However, although severe and nearly continuous substorms always accompany major magnetic storms, a magnetic storm is not just a collection of magnetic substorms.

The main distinguishing feature is the build up of an enhanced radiation environment in the inner magnetosphere.

Consequences of Major Magnetic Storms


COMPARATIVE MAGNETOSPHERES

The outer planets -- the "gas giants" (Jupiter, Saturn, Uranus, Neptune) provide interesting comparisons with the earth's magnetosphere.


The magnetospheres of the outer planets have been explored by the Pioneer, Voyager, and Voyager 2 missions.

V2V2


Some of the factors accounting for differences between the magnetospheres of the outer planets and that of earth include:

• Properties of the solar wind change as we move outward, affecting the coupling of energy flux from the solar wind to the magnetospheres.

• Magnetic fields of the outer planets are generally much larger than earth's.

• Rapid rotation of the outer planets provide centrifugal forces large compared to those of earth.

• Jupiter's moon Io represents a dominant source of plasma to the Jovian magnetosphere.

• The outer planets have rings capable of absorbing trapped radiation (especially in the case of Saturn).


VARIATION IN SOLAR WIND PROPERTIES

The solar wind number density and the radial component of the IMF decrease (inverse square) as distance from the sun increases. (electron and ion temperatures of the solar wind plasma also decrease with distance). The solar wind velocity, on the other hand, increases (slightly) with distance.

The increased Mach numbers consistent with the above imply stronger shock fronts at the

outer planets.

The weaker dynamic pressure suggests largermagnetospheres for comparable B-values.

Weaker IMF suggests merging and reconnectionnot so important.


MAGNETOSPHERIC SIZE

The size of a magnetosphere is determined by the relative importance of the planetary magnetic field strength and the solar wind dynamic pressure.

The following table provides data on the relative sizes of the magnetospheres (in terms of subsolar magnetopause distances) for earth and the four outer planets:


Relative magnetospheric sizes based on pressure balance


URANUSAxis tilted about 59 degrees and offset from center of planet by 30% of its radius, placing magnetic poles nearer the equator.

NEPTUNEField highly tilted: 47 degrees from rotational axis and offset at least 0.55 radius from physical center of planet.


Jupiter and Io


For Jupiter, the actual subsolar magnetopause is found to be over 100 RJ , as opposed to the value of 45 RJ expected from pressure balance arguments. This is due to the effects of the plasma spewing from its moon, Io.

Io representsa source ofparticles inthe innermagneto-sphereof Jupiter.


The largest entity in the Jovian system is the invisible force of the planet's magnetic field. Planetary magnetic fields are created by the motion of fluid interiors. Fifteen thousand miles deep within Jupiter's interior, hydrogen undergoes a dramatic change. At a pressure three million times that at Earth's surface, and at temperatures exceeding 19,000 degrees Fahrenheit, the hydrogen changes from molecular liquid to a state called liquid metallic hydrogen, an excellent electrical conductor.

The liquid metallic hydrogen and the planet's rapid rotation (9 hours 55 minutes) generate electric currents that create Jupiter's magnetic field, which is more than 10 times stronger than that of Earth.

Jupiter's ring and moons are embedded in an intense radiation belt of electrons and ions trapped in the magnetic field. The Jovian magnetosphere, which comprises these particles and fields, balloons two or three million miles towards the Sun and tapers into a wind sock-shaped tail extending at least 465 million miles behind Jupiter as far as Saturn's orbit.

The relationship between the magnetic field and Io is unique. As the magnetosphere rotates with Jupiter, it sweeps past Io, stripping away about a ton of matter per second and forming a torus—a doughnut-shaped ring around Jupiter predominantly composed of electrified oxygen and sulfur glowing in the ultraviolet.

As these heavy ions migrate outward, their pressure inflates the magnetosphere to more than twice its expected size. Some of the more energetic ions fall into the atmosphere along the magnetic field to create Jupiter's auroras.

As Io moves through Jupiter's magnetic field, it acts as an electrical generator, developing 400,000 volts across its diameter and generating an electrical current of three million amperes. The current flows along the magnetic field to Jupiter's ionosphere.

Jupiter and Io


Jupiter’s Io The reason for the larger value of Juipter’s subsolar magnetopause is that Io provides a significant source of mass to the Jovian magnetosphere, which in turn accelerates this mass to high velocities because of the rapid rotation of the planet. The resulting centrifugal force of the plasma pushes out against the solar wind leading to a more distant standoff distance.

The weak atmosphere of Io is maintained by continual volcanic eruptions. Radiation belt particles collide with atoms/molecules in Io’s atmosphere, knocking off atoms in a process called “sputtering”, so that atoms escape from Io and form a cloud in orbit around the planet. Neutral atoms in the cloud become ionized, and form a co-rotating torus about the planet


Saturn

The rings of Saturn are efficient absorbers of radiation-belt particles and limit the buildup of intense fluxes of energetic particles in theinner radiation zone of Saturn.

Saturn's moon Titan is also an important source of particles, but much of this mass is added near Saturn's magnetopause, and subsequently much of it is lost to the solar wind.


The moons of the gas giants also provide sinks for energetic (radiation belt) particles (absorption).


HST Images of Aurora


NASA and Prof. John Clarke (Boston University) HST-STIS

Jupiter Aurora


Mercury has a significant magnetic field, but no atmosphere; interaction of the solar wind with Mercury forms a magnetosphere that shares many gross characteristics with that of earth.

Mercury


Some ExistingConcepts


Schematic of Mercurian Magnetosphere and Anticipated Processes


VENUS AND MARS

Venus and Mars have very weak magnetic fields, and the interaction of the solar wind with these planets is governed by different processes.

The interaction of a solar wind with a weakly-magnetized planet is determinedby the planetary ionosphere.

without an IMF, the flow around the planet is similar to that of fluid flow around a sphere.


With an IMF that is not steady (as in the solar wind), the interaction of the changing IMF with the conducting ionosphere generates currentsthat keep the field from penetrating through the body by generating a canceling field (Faraday's law -- time changing magnetic field induces currents in a conductor).

A steady IMF would eventually diffuse into the body (time constant depends on ionosphercic conductivity)

A bow shock is expected, since the supersonic solar wind plasma is diverted around the body.


For Venus, pressure balance considerations similar to those for a magnetosphere apply, except with the ionospheric pressure replacing the magnetic pressure. An ionopause is formed when the solar wind pressure balances the ionospheric pressure.

The subsolar ionopause of Venus is quite low -- about 350 km, as compared with about 1,000 km at the terminator. The bow shock is at about 2,000 km.


Ionospheric ions can gyrate around the flowing IMF lines, and thus be "picked up" by the solar wind.

Integrated over the 4.5 billion year lifetime of the planet, this scavenging by the solar wind has probably had an impact on the evolution of the atmospheres of Mars and Venus.

Ionpickup


As shown below, this ion removal process becomes evident in comparisons of measured ionospheric profiles with those obtained from models which do not take into account this process.

asen 5335 aerospace environment -- the magnetosphere 1 aerospace environment asen-5335 instructor:...

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