Indian Journal of Radio & Space PhysicsVol. 19, August 1990, pp. 241-256

Magnetospheric phenomena

ACDas

Physical Research Laboratory, Ahmedabad 380 009

The solar wind-magnetosphere interaction processes and the couplingbetween ionosphere and magne­tosphere are reviewed.The role of interplanetary magneticfieldin the magnetic reconnection at the daysidemagnetosphere aswellas in the tail isdiscussed.Direct and indirect observations of reconnection at daysideand the ISEE satelliteobservations ofplasmoid in the tailare described to establish the reconnection modelof magnetosphere beyond any doubt. Significantprogress made in our understanding of many importantionospheric and magnetospheric phenomena including substorm through ionosphere-magnetospherecoupling has also been surveyed. Very low frequency (VLF) emissions and auroral kilometric radiations(AKRs) in the earth's magnetosphere have been discussed on the basis of wave-particle and wave-wavein­teractions, and powerful plasma processes in exchangingenergy between wavesand particles. Suggestionsfor future work on the solar wind interaction withmagnetosphere in the presence of northward componentof IMF and emission processes on the basis of wave-waveparticle interaction have been made.

1 Introduction

The topic of magnetospheric phenomena is verywide and includes many physical processes and quan­titative cause-effect relationships among variousphenomena. A large number of very interesting andpuzzling phenomena have been observed in the mag­netosphere by ground-based instruments as well asby rockets and satellite probes. Recently, attemptshave also been made to perform active experiments.The plasma processes that are responsible for theseobservations have been studied theoretically by usingboth fluid and kinetic approaches. However, therestill exist a number of puzzling phenomena which areyet to be understood fully. Recent satellite observ­ations, development of new theoretical models on thebasis of plasma instabilities and the results of activeexperiments have been able to answer a few import­ant questions but at the same time have generatedmany more such questions.

It will not be possible in this brief review to includeall the phenomena resulting from both the macros­copic and microscopic processes in the plasma envi­ronment in and around the magnetosphere. We willconsider here some central issues of magnetosphericphysics which can roughly be divided into three cate­gories: (1) Solar wind-magnetosphere interactionleading to reconnection processes and convection;(2) Magnetosphere-ionsophere coupling that givesrise to two-loop currents in high latitude ionosphere,Birkeland currents and substorm; and (3) Emissionprocesses, namely, VLF emission, auroral kilometricradiation (AKR) and broad-band electrostatic noise.

An attempt has been made in this paper to highlightthe recent developments in magnet()spheric physics,but before that a short historical account on each ofthese three topics will be in order.

2 Solar wind interaction with earth'smagnetosphereSolar wind in the form of a plasma cloud was first

postulated by Chapman and Ferraro in 1931 and thenit was proposed that sudden changes in the geomag­netic field measured at the ground could be attributedto the interaction of this cloud with earth's magneticfield. In the model put forward by them, an electriccurrent is induced in the cloud as it first encountersthe magnetic field of the earth. This is a problem ofboundary between collisionless unmagnetized plas­ma and a vacuum field. Solar plasma can confine themagnetic field. Essentially, equilibrium occurs whenthe plasma pressure balances the magnetic field pres­sure, Thus the earth's magnetic field protects theplanet from the impinging solar wind as well as fromhigh energy particles of both solar and galactic orig-..ms.

The Chapman-Ferraro model does not include theeffect of the interplanetary magnetic field which leadsto an interesting process of reconnection of magneticfield lines. The concept of magnetic field reconnec­tion has played a central role in providing a unifiedtheoretical framework for large scale dynamics ofmagnetosphere. Dungey! first shaped the reconnec­tion model for earth's magnetosphere by includingthe effect of interplanetary magneuc field. It has been

241

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

Fig. 1-Flow pattern and the magnetic field lines near the neutralpoint

,.

t

... (1)

... (2)II

I III i I Iii H

1E+-(vXB)=Oc

where E and B are the electric and magnetic fields re­spectively, vis the bulk flow velocity ofttJ,eplasma andcis the velocity oflight. Eq. (1) is valid in many regionsof cosmic plasma. The nature of plasma flow can vir­tually be obtained fro~ Eq. (1) and it implies that themagnetic flux through any element of plasma initimlycoincident with a magnetic flux tube continues toform a flux tube at all times. Hence a transport of mag­netic flux near the separatrix is always associated withthe plasma motion. However, finite conductivity be­comes essential for such transport across the separa­trix. This flow is for all practical purposes the essentialelement in the definition of reconnection concept.The magnitude of the plasma flow is a measure of thereconnection rate.

Eq. (1) is not valid near the neutral points and theflow has a special pattern around the point. Dungey3pointed out that it was only in such regions that largecurrent could be produced in a plasma without beingopposed by electromagnetic force, j x BIc.The equ­ations governing the motion are Maxwell's equation,the continuity equation, the hydrodynamic equationof motion, and the Ohm's law given by

difficult to obtain different components of the pres­sure tensor and the motion cannot be described in a

simple way. However, the pressure can be neglectedin some regions and the Ohm's law for infinite con­ductivity is given by

1E+-(vXB)=17jc

where 17 is the electrical resistivity and j is the currentdensity. The flow pattern and the magnetic fieldlines are shown in Fig. 2. The current direction isperpendicular to the plane of the paper. An oversim­plified microscopic picture of this current has beenpresented by Cowley4.

The main point about the large current is the exist­ence of finite resistivity in such a plasma. The resistiv-

I'" I" !f

able not only to explain many observations but also togenerate an extensive activi~yin plasma research bothfor space and laboratory plasmas.

The basic process of reconnection depends on (i)the topology of the magnetic field and (ii) the motionof plasma flow near the neutral point. A simple modelfor magnetic field line configurations in the earth'smagnetosphere has the topology shown in Fig. 1.There are three classes of field lines:

(i) Closed field lines that connect to the earth atboth ends.

(ii) Open field lines that connect to the earth at oneend and the other end is indistinguishable from the in­terplanetary magnetic field.

(iii) The interplanetary magnetic field lines that donot connect to the earth at all.

Each group is separated from the other by a linecalled separatrix. The seJmratrices intersect alongX-lines which encircle the earth. If now the magneticflux in one regime is to be increased or diminished, itrequires the motion offield lines across the separatrix.One visualizes how an individual field line is trans­ported across the separatrix in the following way. Afield line of cla~s(iii)and one of class (i)must be simul­taneously cut and reconnected to make two field linesof class (ii)if the open flux is increased, or reverse if theflux is diminished. So far as the magnetic field linealone is considered, the transfer of these fluxes doesnot produce remarkable effect. The importance ofthis process in space plasma arises,from the fact thatthe magnetic field line is closely coupled to the plasmamotion I.:?

There are many difficulties to describe the plasmamotion in the magnetosphere. The collisions are rareand the flow is quite complicated. However, this canbe outlined as follows. Boltzmann equation is valideven for negligible collisions and by taking momentsusual equation of motion can be obtained except thatpressure takes the form of asymmetrical tensor. It is

Fllg. )- Topology of the magnetic :"eld. There are three classes ofmagnetic field lines. each class is separated from the other hyaline

called separatrix.

242

DAS: MAGNETOSPHERIC PHENOMENA

ity has the classical value when it is entirely dpe to par­ticle scattering by Coulomb collisions. However, in acollisionless plasma, a higher anomalous resistivitycan occur from the scattering by many microscopicprocesses weich will be discussed later. It may be use­ful to reproduce the scenario of electric field in the re­gion discussed by Dungeyl. Near the neutral pointsome electric field is necessary to drive the currentwhich is given by E = 17j,where j is not very large. Faraway from the neutral sheet or line, Eq. (1) is valid andthe fluid flows towards it with velocity v.The electricfield determined by v x B has the same sign as that ofthe electric field near the neutral point. Thus the oc­currence of reconnection rate can be stated in two

waysZ, and there exists an electric field E parallel to

allin 'th .. E x B d' dneutr e WI pomtmg vector c-- lfecte so as4Jl'

to represent a flux of magnetic energy to the reversalregion from both sides. In other words plasma flowwith bulk velocities ± v from both sides transportingmagnetic energy in the field reversal region. The mag­nitude of E or v is then the measure of reconnection

rate. However, this process of transport fails in the re­gion where the magnetic field tends to zero and there­fore the process of diffusion must take place to trans­port the energy to the neutral line.

We have discussed above a very simple picture ofreconnection under the assumption that there existsfinite conductivity near the neutral line. It is also obvi­ous from Fig. 1 that reconnection occurs both in thefront side as well as in the magnetotail. However, thedynamics of the tail reconnection depends on the day­side reconnection. We shall now consider these two

cases separately.It is quite clear from Fig. 1 that the reconnection is

expected with a southward component of the inter­planetary magnetic field and this will be exploited inthe discussion of the indirect evidences for reconnec­tion.

2.1 Indirect evidences

One of the important predictions that the magneticdisturbance should be associated with southward in­terplanetary magnetic field (IMF) line has been con­firmed. Fairfield and CahiP showed from their analy­sis of bays that when the interplanetary magnetic fieldis southward, there tends to be an associated magneticdisturbance on the ground. Arnoldy6 showed strongcorrelation of IMF north-south component with theauroral electrojet (AE) index which is a measure ofauroral electrojet current intensity based on magneticrecords. The DP:, variations are shown to be closelyrelated to the southward turning of the interplanetarymagnetic field B/. Another important contribution is

the prediction of magnetic activity index DSI on the ba­sis of the southward component of interplanetarymagnetic field. DSI is a measure of the worldwide devi­ation of the horizontal component of the magneticfield Hfrom its quiet day value, an effect produced bymagnetic storm induced ring current.

2.2 Direct evidences

The configuration most appropriate for reconnec­tion at the subsolar magnetopause is shown in Fig. 3(Ref. 7). The solid lines with arrows represent themagnetic field lines. The magnetopause (MP) isshown as the region of current layers with finite thick­ness and BL is the boundary layer adjacent to it. Mag­net osheath is on to the left side, whereas the magne­tosphere is on to the right side of magnetopause. Themagnetosheath and the magnetospheric field linesare connected at the X-type neutral point from the ou­ter (Sl) and the inner (Sz) separatrices. Dashed lineswith thick arrows in between them represent the plas­ma motion around the neutral point or line. Thesefield lines or surfaces have topological resemblanceto those shown in Fig. 1and the plasma flows are simi­lar to those shown in Fig. 2 which are obtained on thebasis of a simple MHD model of reconnection 1.8. Thereconnection electric field Er is also seen to be parallelto the current I at the magnetopause so that Er . I> O.

In such a situation the electromagnetic energy gets

~-----

Fig. 3-Configuration for reconnection at the suhsolar magne­topause. Thc reconnectio!1 electric field E, is out of the plane of

paper near the neutral point.

243

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

converted into other forms and in this case it must be

plasma (kinetic) energy. In the model shown in Fig. 1,this energy is transported by the high speed plasma jetflowing away from the reconnection region.

The magnetic field and plasma data from ISEE-Iand ISEE-2 magnetopause crossings of 6 Sep. 1978are shown in Fig. 4. The satellite moved from the ou­ter magnetosphere into the magnetosheath (MS)through the boundary layer (BL) and magnetopause(MP). The crossing of the outer separatrix is denotedby S. From top to bottom the figure shows plasmadensity, bulk velocity, the north-south components ofthe magnetic field, the total magnetic field and pres­sure, etc. Multiple magnetopause crossings shown inthe figure is confirmed by the transition of the north­south component of the field. These data support thegeneral configuration ofthe reconnection on the day­side magnetosphere as depicted in Fig. I. The basicfeatures, namely, the open field line and the IMF con­trol could be produced, and it gives enough confid­ence that the reconnection must occur on the daysideof the magnetosphere.

2.3 Flux transfer e\ent

A transient fluxerosion represents an evidence fora non-stationary nature of the reconnection processin the dayside of the magnetosphere. The large spikesare seen in the magnetic data for an inbound crossingofthe magnetopause by HEOS-2 and they are inter­preted as the events of impulsive erosion of magneticflux tubeY• The solar wind can do work on the magne­tic field at a location where an interplanetary magne­tic field flux tube is connected with the terrestrial mag­netic field and can erode magnetic flux that formerlybelonged to the magnetosphere. When mechanicalwork is done against magnetic tension, the result willbe increase in the field strength. Consequently, onesees enhancement of the magnetic field strength in theregIOn.

The reconnection process for these flux tubes in thepolar cusp is transient because the events last only fora short duration. These events are seen inside themagnetosphere but similar events are also seen out­side the magnetosphere and are called flux transfereventsJO•

The bipolar nature of the magnetic field along thelocal magnetopause normal, observed by satellitesISEE-l and ISEE- 2 (Ref. 10), is also the characteris­tic of the flux transfer event. The qualitative picture ofthis flux transfer event is shown in Fig. 5. Magnetosh­eath field lines denoted by slanted arrows are con­ll(~cted with the magnetospheric (closed) field linesrepresented by vertical lines. The connected flux tubeis then carried away by the magnetosheath flow in thedirection shown by the big arrow. The magnetosheath

244

field lines which are not connected with the magne­tospheric field lines -are drapped over the connectedflux tube and pass through the spacecraft and thenshow the bipolar characteristics in the magnetopausenormal component of magnetic field.

2.4 Magnetospheric tail dynamics and substorm

Solar wind energy would be primarily deposited tothe nightside magnetosphere due to dayside recon­nection and flux transfer events. The tail current or

equivalently the lobe magnetic flux is enhanced andthe energy associated with this would eventually beconverted into other form through nights ide recon­nection. A part of this energy will be carried away byplasma jet down the tail and the rest would be ejectedtowards the earth consistent with the basic features of

plasma motion around the neutral point shown inFig.2. The plasma flow towards the earth then producesconvection and field-aligned potential that generateenergetic electrons in the auroral region.

If the reconnection at the nose of the magnetos­phere occurs, there must also be reconnection in thetail to balance rates of flux transport. The plasma andthe magnetic field distribution within the magnetotailbecome unstable against excitation of a number ofplasma collective modes. If the wavelength A. ~ L,where L is the characteristic length scale of plasmaconfiguration, then one excites microinstability.Among the microinstabilities that can be generatedin the tail, the tearing mode instabilityll,12 is the mostsuccessful one to produce reconnection and sub­storm phenomena which are two of the most import­ant phenomena in the magnetospheric physics. Theconcept of tail reconnection for subs to I'm is support­ed by various pieces of evidences. The main argumentin its favour is the fact that the long magnetospherictail, which has a diameter of the order of 30 earth radiiand extends by 60 RE, provides a huge reservoir forsubstorm. The magnetic field inside the magnetotailshows an increase of its strength prior to substormbreak-up. The diameter of the magneto tail also in­creases suggesting that the magnetic flux is trans­ferred from the dayside magneto pause into the tail.

Let us now discuss the tearing mode instability in aqualitative manner. The simplest magnetic field mod­el in the magnetotail is described in Fig. 6. Such a con­figuration is supported by the plane current sheetwhich could be represented in the form of elementarycurrent filaments uniformly distributed along thesheet. The attraction between a pair of filamentsgrows quickly as they draw together and the attractionof filaments from neighbouring pairs decreases asthey move away (Fig.6 ).This redistribution of currentchanges the magnetic field topology. Part of the openfield lines in the middle of the sheet current now re-

r

DAS: MAGNETOSPHERIC PHENOMENA

00:598.913

cx>:237.997

0.01 .-

350Vp 250

150

+300+100

"'2 -JOO-300

+30+10-10-)0

10

Np 1Np 0.1

6Z

pB

+30

BZ

+JO

-10-30 ~UT

00:2'3

RE

8.301

TIM~,s

Fig. 4-The magnetofield and the plasma data from ISEE-1 and ISEE- 2 magnetopause crossings of 6 Sep. 1978 [Np is the total plasmanumber density while is the density of the energetic ions. vp and Vz are respectively the magnitude and z-component of the bulk veloc­ity, and Pis the total pressure. Bz is the z-component ofthe magnetic field while B represents the total field strength. UT stands for univer-

sal time (in hrs), RE is the geometric radial distance in earth radii and RC is the ring current.]

245

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

Fig. 6- The simplest magnetic field model in the magnetotail.Plane current sheet is represented in the form of elementary

current filaments uniformly distributed along the sheet. Physicalprocess of tearing mode instability and island formation is

shown.

t

x••

Fig. 5-Qualitative picture of flux transfer event

connect and close around the pair of current fila­ments drawn together. It is seen that the developmentof tearing mode needs the dissipation process whichcould violate the frozen-in-field condition. In the mic­roscopic instability, this dissipation arises in a colli­sionless plasma due to wave-particle interaction. Inthis mode, the electromagnetic wave is created by theperiodic structure of current filaments in the narrowlayer in the neutral sheet.

Thus in the tearing mode process, the current sheetspontaneously breaks up into a series of current fila­ments and the magnetic field associated with theseforms a series of islands. A configuration consisting ofa series of magnetic islanc!sin equilibrium is subject toa coalescence instability in which the islands movetowards one another 11.1 ~. In the presence of resistivitythis instability leads to a single island which is calledplasmoid. Magnetic reconnection occurs during thismerging. There exists, therefore, two-step process: is­land formation followed by the rapid coalescence ofsmall islands into larger ones.

The expansion phase of a substorm occurs on atime scale of the order of 5-10 min I';. If the tail recon­nection caused by the tearing mode is responsible forthis, then a much higher resistivity in the tail would beneeded for this model to operate efficiently. This hasled to new mechanism by considering the effect ofbackground turbulence on the generation of tearingmode, which effectively gives rise to high resistivityand thereby makes it more efficientl';16.

Recently. many computer simulations have beencarried out for the development of magnetic islands

which are subject to coalescence. Wu et al.\7 investi­gated magnetohydrodynarnic and particle simul­ations in which tearing mode formed magnetic islandsin current sheet which subsequently coalesced. Theyalso found that coalescence growth rates were almostten times faster than the tearing mode growth. How­ever, two processes cannot take place simultaneouslybecause the energy sources are different, e.g. the is­lands have to be formed before coalescence takes

place. Using magnetohydrodynamic simulationmethods, Richard et al.lx have investigated the char­acteristics and relevance of the magnetic island coal­escence instability. Fig. 7 shows the development ofthe coalescence process. The full island nearest theearth merges with the closed field lines while the threeislands farther down the tail merge to form a largeplasmoid-like island.

2.5 Observation ofplasmoid

Baker el al.l,} have examined the plasma and fielddata in detail for a period during which ISEE-3 is inthe tail region between 80 and 140 RE. Several exam­ples of strong southward magnetic field followed by astrong northward field occur when ISEE-3 is in theplasma neutral sheet. The analysis of plasma and fielddata reveals significant changes in plasma propertiesand magnetic field topology during the onset of sub­storm and these have been schematically shown in amodel in Fig. 8. Prior to substorm, the neutral lines ap­pear to be located near X = - 100 RE. The plasmasheet is thin and flattened during this phase. When theISEE-3 was at 100-140 RI' the plasma and field data

246

III tdI '

DAS: MAGNETOSPHERIC PHENOMENA

Magnetic Field Lines Pressure Current (y) Velocity Vectors

'ii •·•••i

(a)

----,1 T= 42.71,;

(b)

T = 62.8 'Ta

------£' ------:::- ~~-- ..----~-- h-+r.- : ':~j~~Jm11 : ,--

(c)

T = 10 1~

Fig. 7- Development ofthe coalescence process in computer simulation work (after Wu et al. 17)

.•• EAR THW ARD TAl LWARD ••

_____ PLASMA FLOW

: ; -~-_ U ISEE-3

CURRENT ~ -- - -- --

SHEET -. m,,~..~:m,""'m=J=_ =-"'"=- :. - .:;:.~~_:-!:!~:-!:!~~-!!~•__ .---.-. ------..... 1 r ~ - - ..•.----~",-- ----,,,..,,- ~,//~//

=> /~/< < - ~-----" ~ ~--'--' .•... - :.,,-"""'-- ----­

.•....... ----

PRE-QNSET:

- QUASI-STEADYNEUTRAL LINE

- THIN PLASMASHEET

POST-QNSET:

- PLASMOID FORMATION

- HIGH SPEED, TAILWARDFLOW

- BULGED, THICKENEDPLASMA SHEET

Fig. 8-Plasma properties and magnetic field topologyduringthe onset of substorm (after Baker et al.19)

were consistent with an earthward neutral point (top

panel).A short time after the expansion phase onset, signi­

ficant changes in the configuration of tail at ~ 100 RE

occur (shown in bottom panel). High speed plasmabulk .flows begin within a few minutes after the suh­storm. The plasma sheet thickens and bulges up con­siderahly immediately after the substorm onset. Plas­maid is formed and the release of plasmoid pushes theplasmasheet ahead of it, steepening the field line incli­nation throughout the region.

One of the most important discoveries of recenttimes in the tail of the magnetosphere is that of plas­moid travelling tailward through the distant plas­masheet consequent to the occurrence of substorm atearth ell. The detailed characteristics of a plasmoid

were obtained by the satellite ISEE- 3 during its passes

t~rough the distant magnetotail. Plasmoids arc large­scale structures. The plasmoid observed by ISEE-.3 at

about a distance - 220 RE was about 75-150 R1longand it was travelling tailwards at 500-1000 km/s. Itappeared 27 min after a very strong intensification of

247

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

NEAR-EARTH NEUTRAL liNE

let lie,"", :::-15 R[)

\. I

I.I

OUTER I

EPIIUlATRlll'

(~L~ E I B DRIFT PlASMOID BOUNDARY

(INNER SEPARATRiXI

••••

ISEE - 3

let lit".. ; -217 A[ I

"

Fig. 9-Schematic diagram of a plasmoid observed by ISEE-3 [after Hones (Jr) etal.20]

and VI and V 1 are the velocity components paralleland perpendicular to B, m is the particle mass, q theparticle charge. EI the parallel component of the elec­tric field.;; the parallel component of the accelerationdue to gravity and b the unit vector in the direction of

the auroral electrojet was recorded at seven.!1groundstations. Thc schematic diagram of a plasmoid ob­served by ISEE-3 is shown in Fig. 9. The plasmoidwas covered by a separatrix layer within which ener­getic particle streamed freely tailward along the fieldlines. The plasmoid contained plasma of uniformlyhigh temperature and its boundary b:$ 1 RE thick.The discovery of these plasmoids has enhanced ourrecognition of the role of magnetic reconnection inmagnetotail processes.

... (5)

B. The last term of Eq. (3) is called centrifugal term.This can be understood by rewriting Eq. (3) into afield-aligned energization equation given by (consid­ering only relevant terms),

d] 2 _ ( 2 Rc) (2 R 1)

-(, mvlI)- mvll- 'vc+ mvc- VII

df Rc R,

where Rc and R, are described by Fig. lO(a).The first term is associated with a centrifugal force

experienced by the guiding centre motion of a particlealong the curved line with radius of curvature Re' En­ergization results when the centrifugal force that bal­ances magnetic force of the field in which a particlemoves, acts in the direction of the convection velocityvC' It can be understood qualitatively in the followingmanner. It appears that the radius of curvature is re­duced because of force acting along Vc and as a result vneeds to be increased to maintain the original force,which actually means the energization of the particlealong the magnetic field line. Mauk and Meng21 de­scribe this process of energization analogous to theenergization that results to a stone that is whirlingaround a circle on the end of a rope, when the length ofthe rope is shortened. The second term can be under­stood heuristically by the following manner. There isa centrifugal force associated with convection of par­ticle from one field line to another field line that ispointing in different direction with an effective mo­ment arm R, defined within an inertial frame of refer­ence. The energization takes place when centrifugalforce acts against v. This process seems to be analo­gous to the energization that results to a bead slidingloosely along a sraight length of a metal rod when therod is whirled overhead in a whip-like motion (seeMauk and Meng2! ).

The importance of this centrifugal acceleration tothe inner and middle magnetospheric regions hasbeen demonstrated with regard to the magnetic field­aligned kinetic acceleration of ions. In terms of kineticacceleration, the consequences of guiding centre cen­trifugal acceleration are virtually identical to the con­sequences of the ion guiding centre acceleration that

... (3)

... (4)ExB

v =c--C B2

where

2.6 Macroscopic particle acceleration

Mauk and Meng21 have made an extensive study ofkinetic energization throughout the planetary magne­totail environments. The process of energization isnot completely new. The standard guiding centre mo­tion, when fully utilized, can, in the presence of con­vection electric field, give rise to complex energiza­tion pattern.

Such energization processes inside the magneto­sphere are shown to be similar to non-guiding centreenergization in the tail region. These processes playan important role in enhancing magnetosphere-ion­osphere coupling in terms of field-aligned electricfield or auroral-like field-aligned discharges. Follow­ing Mauk and Meng2', let us first discuss the energiza­tion process briefly. The equation of motion of a parti­cle along the magnetic field with guiding centre ap­proximation can be written as

dVIl dlBI dllm-=mo +qE - /J--+mv'-

dt <">11 II r dt C dt

.;.mv2

!l = lEI = Constant

24H

'. III !;I

DAs: MAGNETOSPHERIC PHENOMENA

II,

I

Fig. lO(a)- The motion of the particle's guiding centre along acUIf1:d field line with radius of curvature Re. There is a centrifu­gal force associated with the convection of the particles fromone field line to another field line that is pointing in different di__rection. Rl is defined as an effective moment arm within an iner-

tial frame of reference (after Mauk and Meng21).

occurs in the vicinity of narrow neutral sheet region ofthe distant magneto tail. The similarities result fromthe existence of different but analogous first adiabaticinvariants in the two regions.

Lyons and Speiser22 have demonstrated the exist­ence of these kinds of simple acceleration processeswithin the deep magneto tail. It is, however, found thatthe motion of particles within a narrow neutral sheet iswell behaved and that within a thick neutral sheet hasthe appearance of disorder or chaos23• Based on thisprocess, an organizational scheme for the magneto­tail has been suggested by Mauk and Meng21 wherebytwo adiabatic regions, one in the near magneto tail andanother in the distant magnetotail, are separated by aregion of non-adiabatic behaviour24.25 [see Fig. 1O(b)].

It is important to make distinction between 'adia­batic' and 'non-adiabatic' behaviours of the particles.There are many consequences of adiabatic behaviourand one of them is the generation of magnetic field­aligned electric fields and perhaps through the ionos­pheric interaction, the formation of field-aligneddouble layers and corresponding auroral like dis­charge.

3 Ionosphere-magnetosphere couplingThe auroral ionosphere is strongly coupled with

the magnetosphere mainly due to large scale motionof plasma inside the magnetosphere known as magne­tospheric convection. There are two principal me­chanisms for this motion. One is the reconnection

model discussed in detail in the previous section andthe.other is the viscous interaction described by Ax­ford and Hines26 and Axford27. The electric field as­sociated with the plasma motion in the equatorialplane in the magnetosphere can be brought down tothe ionosphere at high latitude by assuming that themagnetic field lines are equi-potentiallines. Study ofdirect, Pedersen and Hall conductivities representedby 00' 01 and 02 respectively in the ionosphere further

shows that the field will extend down to E-regionwithout much reduction and the integrated conduc­tivity may be appropriate.

It is now well established that the convection elec­tric field is strongly dependent on the solar wind andthe direction of the interplanetary magnetic field(Bx' By, Bz). When the IMF is southward, plasma con­vection at high latitude exhibits a two-cell pattern withanti-sunward flow over the polar cap and the returnflow equatorward of the auroral oval. The potentialdrop across the polar cap determines the convectionspeed. This, of course, varies with magnetic activity.In the absence of By and co-rotational electric field,the two-cell convection patterns are symmetric bothfor quiet and disturbed conditions. For a non-zero Bv

component of IMF, the two-cell pattern becomesasymmetric. The asymmetric two-cell pattern pre­sented by Heppner and Maynard2H for southwardIMF and for both By > 0 and By < 0 are shown in Fig11.

3.1 Northward IMF

During quiet period or when the IMF is northward,the distribution of electric field, field-aligned cur­rents, particle precipitation, etc. are even more com­plex. It was first recognized by Maezawa29 that plas­ma convection does not have a simple two-cell cur­rent system in ionosphere. He deduced a new currentsystem that can be formed with increase in the nor­thward component and there should exist sunwardplasma convection above the pole. In Fig. 12 it isshown from the satellite observations that there actu­ally exists sunward flow in the polar cap and the cur­rent system is interpreted as the four-loop currentsJo•

Asimple illustration that may describe the processqualitatively when the IMF has Bz pointing northwardis shown in Fig. 13. Suppose there is some reconnec­tion at N, then the associated electric field EF isdirect­ed into the paper. This, when mapped into the ionos­phere, produces sunward flow over the pole. This pat­tern of current system may explain the phenomena oftheta aurora as observed. However, it raises manymore questions and further work will be necessary.

In addition to convection, other two importantcoupling mechanisms are auroral particle precipit­ations and Birkeland currents. The energetic precipi­tated electron is a source of optical emissions, and ionproduction due to impact ionization is a source ofbulk heating for ionosphere as well as for atmos­phereJU2. The Birkeland currents have been studiedfor many years now and statistical pattern is available,especially for a southward IMF componentJJ ~ J5.

lhese currents flow into and out of the ionosphereand are connected via horizontal currents that flow in

the lower ionosphere.

249

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

OPEN t'IELD LINE

RECONNECTED TO

IMF AND DRAGGED

INTO TAIL LOBE

f

MAGNETOPAUSE

\/lL\IlA TIC

,EARTHWARD INJECTION

OF PLASMA PARTICLES. -----_\LOSS OF PLASMOID

FRQM MAGNETOTAIL

PARTICLE MOTION IN VIOLATION

OF GUIDING CENTER APPROXI

ELECTRONS AND IONS

INWARD CONVECTION AN D ENERGIZATION

OF ELECTRONS TRAPPED IN CllRRENT SHEET

NON,ADlAB\TIC\A/lIABATH:

PARTICLE PRECIPITATION WITH FULL LOSS CONES

AT ENERGIES UP TO AT LEAST SEVERAL HUNDRED keV

'\

INCOMING

SOLAR WIND

PARTICLES..

Fig. 1O(b)-Two adiabatic regions, one in the near magnetotail and another in the distant magnetotail, are separated by a region of non­adiabatic behaviour(after Baker etaI.'· and Lyons").

4 Emission processes in the magnetosphereWave-particle interaction in the radiation belt of

the magnetosphere is one of the important plasmaprocesses leading to many different types of emis­sions in the magnetosphere, The particles in the radia­tion belt arc well described in terms of adiabatic in­

variants, However, the particle trajectories arc affec-

ted by the electromagnetic field and over a long timeeven a weak electromagnetic disturbance can be im­portant. Electromagnetic disturbance should, there­fore, playa significant role in determining the form ofradiation belt and in explaining some of the phenome­na observed in the radiation belt. There are two ap­proaches to this problem: (i)study the effect of pcrtur-

250

1,lilllii III ilHi 1·11"'fl IPI 1111 II I' H~Hlld;iHI 111,*1I11~1 lilll~,11 I

DAS: MAGNETOSPHERIC PHENOMENA

By>O12 h

SOUTHWARD 1M FBy<0

12 h

18h

On

6h18 h 6h

/

Fig. II-Asymmetric two-cell current pattern presented by Heppner and Maynard28 for southward IMF and for both By > 0 and By < 0

bation on the trajectory of a particle by using the con­cept of adiabatic invariants, and (ii) study the effect ofcollisionless plasma on the disturbance which per­mits a wide variety of instabilities. There are four im­portant emission processes in the magnetospherethat can be described on the basis of these instabilitiesand they are (i) VlF emissions in the whistler mode,(ii) auroral kilbmetric radiation, (iii) electrostaticnoise, and (iv) micropulsations.

4.1 Very low frequency emissions

Very low frequency (VlF) emissions are certainlyvery puzzling electromagnetic phenomena and arebelieved to originate in the magnetosphere. An exten­sive study has been done by Helliwell's group at Stan­ford. There are two basic classes of emissions: (i)spontaneous, and (ii) triggered. Triggered emissionscan be generated even by man-made signals and a ty­pical emission pattern is shown in Fig. 14 (Ref. 36).The figure shows a very complex frequency-timestructure, viz. risers, falling tones, hooks, invertedhooks, and long enduring oscillations. The main goalof these active experiments is to understand the inter­action between coherent VLF waves and energeticparticles in the magnetosphere leading to the whistlermode instability through which both natural and arti­ficially stimulated VLF emissions are produced3? - 39.Recently some new observations in the magnetos­phere of non-ducted coherent VLF waves from'aground transmitter and associated sidebands andVLF emissions40 have enhanced our understandingof the physical processes leading to emissions. The in­jected signals and the associated VLF emissions tra-

vel to the ionospheric region conjugate to source andcan be observed on the ground ..

It is now believed that the main mechanism ofVlFemissions is the gyro-resonance interaction betweenwhistler mode wave and energetic particles41 - 48.Par­ticle trapping in the wave plays an important role. Inthe discussion of particle trapping, Dungey41 pointedout that each trap causes an eddy in phase space andthat this leads to eddy diffusion or stirring similar topitch angle diffusion for quasi-linear theory. Das45in­cludes loss cone in the distribution and discusses

crude models of distribution on the basis of trappingand obtains enough amplification of wave packet foremissions to be triggered.

The important point to be noted in the model pro­posed by Das is that the growth rate is enhanced at fre­quencies slightly above and slightly below the centralfrequency or in other words it is capable of producingsidebands. Following this, a number of theoreticaltreatments for the sideband instability have been giv­en by many workers49 - 51.Nunn48 studied the effect ofsecond order resonance due to inhomogeneity of themagnetic field in a narrow band whistler wave for gen­eration ofVLF emissions. He also obtained equationwhich can show the time development of wave fieldbecause of resonant particle current.

The other two important aspects of emissions arethe preferential triggering of emissions at half theelectron gyro-frequency and the frequency-timestructure. Following the fact that the speeds of Lan­dalJ resonant and gyro-resonant particles are thesame if w = ! Qe (w is the frequency of the whistler

251

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

Dawn-Dusk Meridian

1800

1811190065329

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2400Fig. 12-Satellite measurements of the convection electric field along a dawn-dark orbit in the sunlit southern polar region. The bottom

picture shows the inferred four-cell convection (after Burke et af.3").

mode wave and Qe is the electron gyro-frequency),Ashour-Abdalla46 showed that the gyro-resonantand the Landau-resonant diffusions add up in theireffects at this frequency so as to produce a steep slotin the distribution which gives enhanced growth. Vy­as and Das52 pointed out that for Landau resonanceall the waves in a narrow band wave packet will re-

sonate with the particle at the same time providedVII cm e = vph = vg ( vph and vg are phase and group vel­ocities of whistlers and e is the angle made by thepropagation vector with the magnetic field) whichcan happen only when OJ = 1- Qe. As a result, Lindauresonance effect is cnhanced considerably and themechanism of slot formation becomes more convinc-

252

III !·!HI' llillill \jl 1111 II "'tl I I Ilill

DAS: MAGNETOSPHERIC PHENOMENA

NorthwardIMF

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Fig. 13-Possibleconfigurations forreconnection with northward IMF and sunward plasma flow in polar region

kc/sle:s-,t-,

-!14.5- __ !

t +

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(EW)

I3

I4

I5

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Fig, 14-Typical spectrum of artificially stimulated YLF emissions (after Helliwell")

ing and satisfactory. Another model for emissions athalf the equatorial gyro-frequency is discussed byVyas and Das53 on the basis ofthe fact that the phasevelocity becomes maximum at w = 1- Qe. The Voyag-er-2 plasma wave instrument also detected whistlermode emissions near Q/2 at magnetic equatorcrossing near the outer edge of the IO plasmatorus 54. Das and Kulkarni'" solved numerically the

equation describing the time evolution of the ampli-tude and phase of the wave packet by computing thesecond-order resonant particle current. The fre-quency-time structure is shown in Fig. 15.

A number of ground-based observations of side-band generation in VLF transmitter signals propagat-ing through the magnetosphere in the whistler modehave been reported in the past 56 - 60. Recently, side-

253

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

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Fig. 15- Frequency-time structure ofVLF emissions obtained by a theoretical model. Hooks, inverted hooks, ris­ers, falling tones and long enduring oscillations are shown. R is the amplitude of the waves, a the frequency, and Zthe distance from the centre of the interacting region; all these parameters are expressed in dimensionless units (af-

ter Das and Kulkarni55).

bands have also been observed in the high altitude sa­tcllite from the high amplitude ground transmitterwhieh provides information regarding the wave andparticle properties in or near interaction region, In ad­dition to sideband signals, VLF emissions associatedwith the transmitted pulses have also been oberved,suggesting that the nonlinear stage of whistler insta­bility described above has been reached. Furtherdevelopments have been made in the theoretical stud­ies of wave-wave particleinteraetion whieh ean simul­taneously take care of competing processes involvingboth wave-particle interactions and wave-wave inter­actions. As an example, in a computer experiment in

254

the whistler mode ion beam instability, a serial-paral­lel cascading of wave-wave interaction took place in­stead of ordinary three-wave coupling as a result of amodification by particle heating due to nonlinearwave-particle interaction61•

4.2 Auroral kilometric radiation

The discovery of auroral kilometric radiation(AKR) is one more milestone of earth's magnetos­pheric research. It has also considerably enhancedthe understanding of radiation processes detected inother magnetized planets like Jupiter and Saturn byPioneer and Voyager.

\. III I," '"'' '" 'I' "I' "!flil \1'" , 1"1'

DAS: MAGNETOSPHERIC PHENOMENA

The first comprehensive study on these radiationswas made by Gurnett62 and the most of our knowl­edge of kilometric radiation has been derived fromthis study. Two important findings for theoreticalun­derstanding are that (i) the radiation is right-handedpolarized (extraordinary mode), and (ii) the frequen­cy range is from 50 kHz to 500 kHz. Several studiesmade afterwards have supported these findings ofGurnett's pioneering study. The role offield-alignedcurrents was pointed out by Green et al.63 Tht: observ­ations made by ISEE-satellite have also revealed thatAKR is generated in the extraordinary mode andpropagates nearly perpendicular to the magneticfield.

On the basis of these observations, several theorieshave been proposed. Apparently, these theories areclassified in two basic classes: direct and indirect gen­eration mechanisms of an extraordinary mode. In thedirect generation mechanism, Melrose64 suggested aplasma instability process due to large anisotropy inthe distribution resulting from mirror effects of con­verging magnetic field lines. Cyclotron maser insta­bility was first considered by Wu and Lee65 in whichthe loss cone distribution of mirror reflected elec­trons play an important role and it was shown to becapable of explaining emissions at different wavemodes and harmonics. The indirect generation me­chanism involves the linear or nonlinear conversion

of •electrostatic waves into an electromagneticwave66-68. However, it works in two steps; first theelectrostatic waves need to be excited by high energyelectrons or currents and then conversion must take

place. One of the crucial requirements of this me­chanism is the existence of unstable electrostaticwaves. As fqr electrostatic ion-cyclotron waves areconcerned, there exists both theoretical and experi­mental support69, 70,

Emissions of the type mentioned above are gener­ated in the magnetosphere of Jupiter and Uranus.Curtis et al.71 have suggested that cyclotron maser in­stability generated by energetic electrons supplied bythe outer radiation belt of Uranus is responsible forUranus kilometric radiation (UKR). Nonlinear waveinteraction or nonlinear conversion of waves may al­so play role in producing UKR (Ref. 72).

Micropulsations and electrostatic noise in the mag­netosphere are also very important magnetosphericphenomena. Each of these processes forms an inde­pendent topics of review and is beyond the scope ofthis review. These are, therefore, not being includedin this article.S Discussion and conclusions

The magnetic reconncction at the magnetopausc isthe principal mechanism for many magnetosphericphcnomcna, It has heen shown heyond any douht that

the interplanetary magnetic field plays a very import­ant role in the transport of energy from the solar windto the magnetosphere. The role of southward inter­planetary magnetic field in producing loop currentsystem, convection and field-aligned currents is un­derstood reasonably well. However, the effect of nor­thward turning ofIMF appears to be quite complicat­ed and further studies in this direction may be essen­tial.

Most of the magnetospheric processes are treatedas quasi-steady processes. The quasi-steady modelshave been successful in explaining a number of basicfeatures of the magnetosphere which include daysidemagnetopause boundary, etc. However, there existmany evidences which suggest that the solar wind­magnetosphere system is intrinsically non-steady.

We have just discussed in detail that the interactionbetween solar wind and magnetosphere is controlledby the IMF, and the rates of transport of plasma, mo­mentum and energy from the solar wind to magnetos­phere depend on the IMF condition. However, IMFitself is highly variable and magnetosphere would notbe able to reach an equilibrium state. There existmany other phenomena that suggest non-steadystructure of magnetosphere. This becomes also animportant problem in magnetospheric physics whichneeds special attention.

Wave-particle interactions in the magnetosphereare believed to be the basic processes ofVLF and ELFwave emissions. Controlled injection of VLF wavesinto magnetosphere has recently brought many newresults which may be understood from theoreticalmodels on sideband instability in the whistler mpde.More attention has been given recently to make stud­ies on complex processes involving two competingprocesses, viz. wave-particle int~factions and wave­wave interactions. The results obtained so far are en­

couraging and much more is needed to be done in fu­ture.

Significant progress has been made during the lastdecade with regard to our understanding of magne­tosphere-ionospl1ere coupling. Some of the import­ant issues need to be resolved in near future are thefollowing.

The convectitm pattern for northward componentof IMF should be established with different values of

By. A detailed study has to be made to establish thetime constant for changing from one convection pat­tern to another as the IMF changes. This appears to beimportant because many ionospheric as well as mag­netospheric phenomena seem to correlate with thischange. Special structure of plasma convection, parti­cle precipitation, field-aligned currents, are a fewmore processes that may be considered in more de­tail.

255

INDIAN J RADIO & SPACE PHYS, AUGUST 1990

References 33 Potemra RA, Iijima T & Saflehos N A, Dynamics of Magnetos-phere(D Riedel, Hingam, Massachusetts, USA), 1979.

~ Dung~y J W, Phys Rev Leu( USA), 6 (1961) 47. 34 lijima T & Shibaji T, J Geophys Res (USA), 92 (1987) 4467.2. VasyhunusVM,RevGeophys&spacePhys(USA), 13(1975) 35 BurchJL,ReiffPH, Weimer DR&WinninghamJD,JGeo-

303. ,. ,.', physRes(USA),90(1985) 1577.3 Dungey J W, Cosmic Electrodynamics (Cambndge UmvesIty 36 Helliwell R A, Whistlers and Related Ionospheric Phenomena

Press, London), 1958. (Stanford University Press, Stanford, California, USA), 1965.4 CowleySWH, Cosm~cElectrodyn(Netherlands),2(1971)90. 37 Helliwell R A & Katsufrakis J P, J Geophys Res (USA), 795 Fairfield D H & Cahll L J, J Geophys Res ( USA), 71 (1966) (1974) 2511.

155. 38 Koons H C Dazey M H, Dowden R L & Amon L E S, J Geo-6 Arnoldy R L, J Geophys Res (USA), 76 (1971) 5189. phys Res ( USA), 81 (1976) 5536.7 Sonnerup B U 0, Paschmann G, Papamastorakls I, Sckopke 39 D d R L McK A C Amon L E S Koons H C & Dazey

N,Haer:ndel G,BameSJ,AsbridgeJR,GoslingJT & Russel MO~, ;~eophYs Re;( USA), 89 (1978) i69.C T, J Geophys Res (USA), 86 (~981 ) 10,049. 40 Bell T F, J Geophys Res ( USA), 90 (1985) 2792.

8 Levy R H, Petschek H E & Slscoe G L, AIM J (USA), 2 41 D J W G h ' d't d b C DeWI'tt and J HI'eblot1964 2065 ungey ,eop YS1CS, e 1 e y( ) , (Gordon and Breach, New York), 1963.

9 Haerendel G, Paschmann G, Sckopke N, Rosenbauer H & 42 Liemohn H B, J Geophys Res( USA), 72 (1967) 39.Hedgecock PC, J Geophys Res ( USA), 83 (1978) 3195. 43 Helliwell R A, J Geophys Res (USA), 72 (1967) 4773.

to Russell C T & Elphlc R C, Geophys Res Leu( USA), 6 (1979) 44 Dowden R L, J Geophys Res ( USA), 67 (1962) 2223.33. -45 DasAC,.lGeophysRes(USA),73 (1968) 7457.

11 Schindler K, J Geophys Res (USA), 79 (1974) 2803. / 46 Ashour-Abdalla M, Planet &Space Sci( GB), 20 (1972) 639.

12 Galeev AA & Zeleny L M, Sov Phys-JETP( USA), 43 (1976) 47 Brice N M, J Geophys Res (USA), 68.(1963) 4626.1113. 48 NunnD,Planet&SpaceSci(GB),22(1974)349.

13 Fi~n J M & Kaw P K, Phys Fluids.( USA), 20 (1977) 72. 49 Brice N M, J Geophys Res (USA), 72 (1967) 5193.14 Pntchett P L & Wu C C, Phys FlUids ( USA), 22 (1979) 2140. 50 Budko N I, Karpman VI & PokhotelovOA, Cosmic Electro-15 Coroniti F V, J Geophys Res ( USA), 90 (1985) 747. dyn (Netherlands), 3 (1972) 147.

/,16 Sundaram AK, Das AC & SenA, Geophys Res Leu( USA), 7 51 LashinskyHT,RosenberT J & DetriekDL, GeophysResLeu(1979)921. (USA) 7(1980)837.

17 Wu C C. LeboeufJ N, Tajima T & DawsonJ M, UCLAPPG- 52 Vyas N K & Das AC Nature (GB), 253 (1975a) 29.551 (Centre for Plasma Physics and Fusion Engineering, Uni- /53 Vyas N K & Das A C: J Atmos &Terr Phys( GB), 37 (1975b)versity of California, Los Angeles, USA), 1980. / 1171.

18 Richard R L, Walker R J, Sydora R D & Ashour-Abdalla M, 54 CoronitiFV,ScarfFL,KenneICF,Kurth WS&GurnettDA,Preprint UCLA PPG-l,140, IGPP-~094 ,(Centre .for Plasma Geophys Res Leu( USA), 7 (1980) 45.

PhYSICSand FUSIOnEngmeenng, Umverslty of Cahforma, Los /5 5 Das A C & Kulkarni V H, Planet &Space Sci (GB), 23 (1975)Angeles. USA), 1988. I 41.

19 BakerDN,BameSJ,BirnJ,Feldm~WC,Gosling~T,Hon~s 56 Bell T F & Helliwell R A, J Geophys Res ( USA), 76 (1971)(Jr)E W,Zwickl RD,SlannJ A,SmIlhEJ, TsurutamBT &SI- 8414.

beck D G. Geophys Res Leu( USA), 11 (1984) 1042. 57 Likhter Y I. Molchanov 0 A & Chmyrev V M, Sov Phys-20 Hones (Ir) E W, BirnJ, BakerDN, BameSJ, Feldman W C, JETP(USA), 14 (1971) 325.

McComas D J & Zwicke R D, Geophys Res Lett(USA), 11 58 ParkCG&ChangDCD, GeophysResLett(USA),5 (1978)(1984) 1046. 861.

21 Mauk B H & Meng C I, Preprint APUJHU 88-13 (The John 59 Helliwell RA, cited in Wave Instabilities in Space Plasma, ed-Hopkins University, Baltimore, Maryland, USA), 1988. ited by P J Palmadesso & K Papaddopoulos (D Reidel, Hingh-

22 Lyons L R & Speiser T W, J Geophys Res (USA), 87 (1982) am, Massachusetts, USA), 1979, 191.2276. 60 ChangDCD,HelliwellRA&BellTF,JGeophysRes(USA),

23 Martin (Jr)R F,JGeophysRes(USA),91 (1986) 11985, 85(1980) 1703.24 Baker D N, BameSJ,McComasDJ,Zwickl RD. SlavinJ A& 61 Matsumoto H, Abstract URSI WIPP89, 1989 (Private com-

Smith E J, cited in Magnetotail Physics, edited by A T Y Lui munication).(The John Hopkins University Press, Baltimore, Maryland, 62 Gurnett D A, J Geophys Res (USA), 79 (1974) 4227.USA). 1987, 137. 63 Green J L, Safleko N A, Gurnett D A & Potemra T A, J Geo-

25 Lyons L R, J Geophys Res (USA), 89 (1984) 5479. phys Res (USA), 84 (1979) 5216.26 Axford W I & Hines C 0, Can J Phys (Canada), 39 (1961) 64 Melrose D B, Astrophys J( USA), 207 (1976) 651.

1433. 65 Wu C S & Lee L C, AstrophysJ( USA), 230 (1979) 621.27 Axford W I, J Geophys Res ( USA), 67 (1962) 3791. 66 Benson R F, Geophys Res Lett( USA), 2 (1975) 52.28 Heppner J P& MaynardN C,JGeophysRes( USA), 92( 1987) 67 Oya H, Planet &Space Sci( GB), 22 (1974) 687.

4467. 68 Roux A & Pellat R, J Geophys Res (USA), 84 (1979) 5189.29 Maezawa K,J Geophys Res (USA), 81 (1976) 2289. 69 Kindel J M & Kennel C F, J Geophys Res (USA), 76 (1971)30 Burke W J, Kelley M C, Sagalyn R C, Smiddy M & Lai S T, 3055.

Geophys Res Leu( USA), 6 (1979) 21. 70 MozerFS,CarisonCW,HudsonMK, TobbertRB,ParadyB,

31 Evans B S, Proc Quantitative Models of Magnetosphere- YatteauJ & Kelley M C, Phys Rev Lett( USA), 38 (1977) 292.Ionosphere Coupling Processes (University of Kyoto, 71 Curtis SA, DeschM D & Kaiser M L,J Geophys Res( USA), 92Kyoto, Japan), 1987,325. (1987) 15199.

32 Hardy D A, Gussenhaen M S & Holeman E, J Geophys Res /72 Buti B & Lakhina G S, Geophys Res Lett (USA), 15 (1988)(USA),90(1985)4228. 1149,

256

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