4, 1993, magnetic storm - Wiley Online Library

JOURNAL OF GEOPHYSICAL

Multisatellite

observations

RESEARCH, VOL. 102, NO. A7, PAGES 14,123-14,140, JULY 1, 1997

of the outer

zone electron

variation

during the November 3-4, 1993, magnetic storm Xinlin Li, • D. N. Baker,• M. Temerin,2 T. E. Cayton,3 E.G. D. Reeves? R. A. Christensen, 3 J. B. Blake,4 M.D. Looper,4 R. Nakamura? and S. G. Kanekal 6 Abstract. The disappearance and reappearanceof outer zone energeticelectronsduring the November3-4, 1993, magneticstorm is examinedutilizing data from the Solar, Anomalous,and MagnetosphericParticleExplorer (SAMPEX), the Global Positioning System(GPS) series,and the LosAlamosNationalLaboratory(LANL) sensorsonboard geosynchronous satellites.The relativisticelectronflux dropsduringthe main phaseof the magneticstormin association with the large negativeinterplanetaryBz and rapid solar wind pressureincreaselate on November3. Outer zone electronswith E > 3 MeV measuredby SAMPEX disappearfor over 12 hoursat the beginningof November4. This representsa 3 ordersof magnitudedecreasedownto the cosmicray backgroundof the detector.GPS and LANL sensorsshowsimilar effects,confirmingthat the flux drop of the energeticelectronsoccursnear the magneticequatorand at all pitch angles.Enhanced electronprecipitationwas measuredby SAMPEX at L -> 3.5. The outer zone electron fluxesthen recoverand exceedprestormlevelswithin one day of the storm onsetand the inner boundaryof the outer zone movesinwardto smallerL (< 3). Thesemultiplesatellitemeasurementsprovide a data set which is examinedin detail and usedto determinethe mechanismscontributingto the lossand recoveryof the outer zone electronflux. The lossof the inner part of the outer zone electronsis partly due to the adiabatic effects associatedwith the decreaseof Dst, while the loss of most of the outer

part (thoseelectronsinitiallyat L -> 4.0) are due to either precipitationinto the atmosphereor drift to the magnetopause becauseof the strongcompression of the magnetosphere by the solarwind. The recoveryof the energeticelectronflux is due to the adiabaticeffectsassociated with the increasein Dst, and at lower energies(1.0 MeV) of the outer zone electrons. Introduction

Rapid flux variationsof relativistic(•>1 MeV) electronsin Earth's magnetosphereare one major consequenceof magnetic storms.However, the cause of the variations is still not well understood.This is partly due to the lack of highresolutiondata in the early times, for instance,the sampling interval (orbital period) was >1 day for Explorer 14 [Owens and Frank, 1968] and partly that there hasbeen little effort to conductdetailed studiesof relativisticelectronsduring magnetic stormsin recentyears.Here we presenta studyof the electronflux variation in responseto a major solarwind disturbance which occurred late on November 3, 1993. Data from SAMPEX, the GPS seriesof satellites,and the LANL sensors

•Laboratory for Atmospheric andSpacePhysics, University of Colorado, Boulder.

2Space Sciences Laboratory, University of California, Berkeley. 3LosAlamosNationalLaboratory, LosAlamos,NewMexico. 4Space Sciences Department, Aerospace Corporation, LosAngeles, California.

SSTEL,NagoyaUniversity, Toyokawa, Japan. 6NASAGoddardSpaceFlightCenter,Greenbelt, Maryland. Copyright1997by the AmericanGeophysicalUnion. Paper number 97JA01101. 0148-0227/97/97JA-01101$09.00

onboardgeosynchronous satellitesare used. Our purposein studyingthis stormis to determinethe causeof the relativistic electronflux dropsthat typicallyoccurat the beginningof the main phaseof stormsand the relativisticelectronflux enhancementsthat typicallyoccurlater. In particular,we are interested in determiningthe extent to which the flux decreasesat the beginningof the stormare due to adiabaticeffects(conserving all three adiabaticinvariants)or are due to real lossesof energeticelectronsfrom the magnetosphereand thus the extent to which the recoveryof the energeticelectron flux requiresthe energizationof new electrons.We find that for this magneticstormnearly all the energeticelectronsin the outer radiation belt were lost from the magnetosphereduring the main phaseof the storm.Both adiabaticeffectsand lossfrom pitch anglescatteringare knownto affectthe measuredflux of radiation belt electronsduring quiet times [Mcllwain, 1966, 1996]. However, there has not been a clear understandingof the relative importanceof various lossprocessesduring the rapid decreasethat occursduringthe main phaseof magnetic storms[Rinaldiet al., 1994]. The November1993magneticstormhasbeen selectedas a specialstudyintervalfor the National SpaceWeather Initiative [Knippet al., 1995]. Thus much relevant data are available. Currently, the data are available at http://www.ssc.igpp. ucla.edu/gem/event_nov93.html on the World Wide Web. At

14,123

14,124

LI ET AL.: OUTER

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2156 UT on November 3, 1993, IMP 8, which was in the

(sampledevery0.1 s) andE > 3 MeV (sampledevery6 s) are

magnetosheath on the nightside,((X, Y, Z) - (-30, 23, 5) in GSE coordinates),detecteda southwardturning of the interplanetarymagneticfield (IMF). IMP 8 measuredthe largestsouthwardcomponentof the magneticfield (-36 nT Bz in GSM coordinates)at 2336UT on November3, 1993,together with a stronglyenhanceddynamicpressure.The observations showedthat the pressurein the magnetosheath increasedby over an order of magnitudedue to the enhancementof density and velocity related to a recurring, high-speedsolar wind stream(J. Foster,privatecommunication, 1995). Exceptfor a

setforHILT SSDs.The16solid-state detectors (10cm2in area

few brief northward

excursions IMP

8 continued

to measure a

strongsouthwardmagneticfield until 0100 UT on November4, 1994.Instrumentson the Geotail spacecraft206 R•r downthe tail alsorecordedthe densityenhancements andmagneticfield increases[Nakamuraet al., 1997].It is evidentthat the magnetic storm is primarily causedby the large negativeBz coupled with the large pressureincreasedue to the solar wind

and2 mm thick) are placedin a squarearrayof four rows;each

of thefourrowsof thearrayhasa geometric factorof -25 cm2 and a view angle of 68ø x 68ø.The L valuesat SAMPEX are determinedby mappingthe field line usingthe International GeomagneticReferenceField (IGRF) model 1990 extrapolated to the time of observation. No external fields are included in the calculation

of L.

We use data from the two geostationarysatellitescarrying LANL instrumentswhich have completecoverageduring the period of interest. The sensorscover a very broad energy range, -0.2 to -2.0 MeV [Higbieet al., 1978;Belian et al., 1992]. We also use data from four of the seven electron channels

that havesignificantcountson the four GPS satellitescarrying particledetectors[Feldmanet al., 1985].These four omnidirectionalelectronchannelscoverthe energyranges0.2-0.4,

densityincrease.A suddencommencement (SC) wasobserved 0.4-0.8, 0.8-1.6, and 1.6-3.2 MeV. The detectors on each at 1720UT on November3 and geomagneticactivityincreased spacecraftare nominallythe sameand the channels,basedon after that, but the main phaseof the stormdid not beginuntil pulseheight analysis,have similargeometricfactors.In actu-2230 UT [Knippet al., 1995]. One of the geosynchronous ality there are some differencesin the relative responsebesatellitessituatedat localnoon(1989-046),observedthe mag- tweendetectorsand channels.The L valuesusedin presenting netopauseto move inside of geosynchronous orbit late on the GPS data are calculatedusingan updatedIGRF modelfor November 3 and early on November 4 (M. Thomsen and the internal field and the Tsyganenko89 model [Tsyganenko, J. Borovsky,privatecommunication, 1996). 1989]at quietconditions (Kp = 0+) fortheexternal field. Energeticelectronsare importantboth for their geophysical and practicaleffects.It hasbeensuggested that energeticelecObservations trons affectthe atmosphereby penetratingto lower altitudes than most other magnetospheric particles[Bakeret al., 1987; Figure 1 showsdaily averagedenergeticelectronsin the Calliset al., 1991].More specifically, Thorne[1977,1980]sug- magnetosphere andtheDst andKpindices forthesecond half gestedthat precipitatingrelativisticelectrons,throughthe pro- of 1993.The variationsare mainlydueto recurrenthighspeed ductionof odd hydrogenand odd nitrogen,couldlead to local streamsin the solar wind, which are a typicalfeature of the ozone(03) depletions in the 40-80 km regionof the middle decliningphase of the solar cycle [e.g., Baker et al., 1994]. atmosphere.The relativisticelectron componentis also of Typically,the flux of relativisticelectronsdropsat the onsetof practicalimportancebecauseof its deleteriouseffecton space- intensemagnetospheric activityand recoversin a few hoursto craft subsystems [Gussenhoven et al., 1987]. Recent observa- daysafter the initial drop.Higher-energyelectronstake longer tions indicate that the fluctuation of the relativistic electron than lower-energyelectronsto approach,or sometimessurflux,in additionto its sheerintensity,canbe a causeof space- pass,their prestormlevels.Suchis the casefor stormsaround craft operationalanomalies[Tschan,1996]. days227, 245, and 308 of 1993. This generalpattern is well known from the earliest daysof satellitemeasurements,for example,Forbushet al. [1961, 1962]. Instrumentation The two larger enhancementsin the SAMPEX measureThe electrondatausedfor thisstudyare from (1) SAMPEX, mentsat L = 6.6, 6, and 5 arounddays220-240 and 300-320 whichwas launchedon July3, 1992,into a nearlypolar orbit maybe due to SAMPEX's orbit beingnearthe noon-midnight with an altitude of 520 x 670 km, an inclination of 82ø, and an plane during these periods.However, despiteorbital differorbitalperiodof about95 min [Bakeret al., 1994];(2) Four of encesand despitethe differencesin the energyrangesof the the GPS constellationof satellites(NS18, NS24, NS28, and respective detectors the overall temporal variations at NS39). The GPS satellitesare in a circular,12 hour orbit at 4.2 SAMPEX and at geosynchronous are well correlated. We now focus on variations in the outer radiation belt elecRE with an inclinationof 55ø,whichpassescloseto the peak intensityregion of the outer radiation belt at low latitudesas tron around the November3-4, 1993, magneticstorm. This well as larger-L magneticfield linesat higherlatitudes[Drake storm is the fourth of a series of five associated with the et al., 1993];(3) The LANL sensors on a seriesof geosynchro- passageof a coronalhole and associated recurrenthigh speed nous satellitesat r = 6.6 R•r. solarwind stream[Knippet al., 1995],andit is the focusof the SAMPEX's complementof four energeticparticle instru- specialstudyinterval. Figure 2 showsthe orbitally averaged ments includesHILT, the heavy ion large telescope.HILT integralelectron fluxfromHILT, Dst, andKpversus timefor consistsof a large drift chamber-proportional counterstele- November 1-8, 1993. The electron flux from SAMPEX is scope,with solid-statedetector(SSD) elementsat the backof shownfor severalL values.Peakswith a periodicityof about12 the telescope(cf. Kleckeret al. [1993]for details).The primary hours in the electron flux are due to the South Atlantic Anompurposeof HILT is to measureionsfrom helium to iron in the aly (SAA). In the SouthAtlantic region,the Earth'smagnetic energy range from 4 to 250 MeV/nucleon. However, at mid- field is weaker due to the offsetdipole, and more particles latitudes,the HILT SSDsrespondalmostexclusivelyto ener- precipitateinto the atmosphereor have their mirror points geticelectrons.Two electronenergythresholdsof E > 1 MeV belowthe spacecraft. In anyorbit,SAMPEX seesonlyparticles

LI ET AL.: OUTER RADIATION

lO6 lO5 19119-046

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BELT ELECTRON VARIATIONS

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Days of 1993 Figure 1. Time history of various parametersfor the secondhalf of 1993. (a) Daily averagesof the

differential flux(electron/(cm 2 s srMeV)) of electrons measured bytheLANL sensor onboard geosynchronoussatellite (1989-046), withtheenergy ranges of 0.7-1.8MeV (solidline)and1.8-3.5MeV (dottedline). (b)-(e) Daily averages of the integral flux of electrons(electron/cm-s-sr) 2 measuredby SAMPEX, solid (dotted)line is for >1 MeV (>3 MeV) electronsrespectively, at L = 6.6, L - 6, L = 5, L = 4, andL ---

3. (g), (h) Dst andKp indices.

that mirror

at or below the satellite.

Prior to the end of No-

vember3, there was little variationin the day-to-dayfluxesof energeticelectrons.However, right at the beginningof the magneticstormlate on November3, the energeticelectronflux decreasedprecipitously.At L = 3, however,the flux of >3 MeV electronsappeared.unchangedat the backgroundlevel until the end of November4 when it beginsto increase.Other importantfeaturesseenin Figure 2 are the delayin the recovery of the more energeticelectronswith respectto the less

energeticelectronsandthe resemblance of the temporalprofile of the energeticelectron flux to the Dst profile (e.g., L - 4), whichsuggests a significant correlationbetweenthem.

A more detailedview of the energeticelectronscan be seen by combiningdata from the GPS satellites.Plate 1 showsthe countsper secondfrom four GPS satellites,eachrepresented by a different color (NS18 = red, NS24 = green, NS28 purple, NS39 - black), in the energyrange of 0.8-1.6 MeV. The temporal resolutionof the measurementsis 96 s. Each satellite cuts four times each 12-hour orbit through a large range of L values above its minimum L value of-4.2. By combiningthe four satellitesone can get an almostcontinuous recordof the energeticelectronflux at eachL larger than L = 4.2. In Plate 1 eachL is actuallya plot of the countrate within 96-s bins when the GPS satellite is within 0.1L

of the nominal

14,126


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6

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8

1993

Figure 2. Energeticelectronsmeasuredby SAMPEX duringNovember1-8, 1993.The orbitallyaveraged

integralfluxisplottedversus timefordifferent L. Alsoplotted(bottompanels) areDst andKp indices during the sametime period. Solid lines, >1.0 MeV; dotted, •3.0 MeV.

value. Thus the plot at L = 4.5 representsthe data from the GPS satelliteswhen they are betweenL = 4.6 and L = 4.4. The magneticlocaltime (in hours)at midnightuniversaltime of the four GPS satellitesis approximately10.9for NS18 (red), 4.25 for NS24 (green), 17.2 for NS28 (purple), and 2.4 for NS39 (black). The magneticlocal time of each satelliteadvancesroughly2 hours for each real hour. Two featuresof particularnote in Plate 1 are the very good agreementbetweenchangesin the electronflux andchangesin Dst (thisDst has 10-minresolution,D. Knipp and B. Emery, private communication,1996), and the completedisappearance of the energeticelectronflux to the backgroundlevel of the detectors above L -

4.6 at about 0130 UT on November

4. The moststrikingand obviouscorrelationbetweenDst and the electronflux occursat the end of November 3 during the main phaseof the storm.The drop in the electronflux at the smallerL valuescoincidesexactlywith the decreasein Dst. There are alsocorrelationsbetweenthe smallerchangesin Dst and the electronflux. The increasein Dst up to 1600 UT on November

3 is reflected

in a small increase in the flux at L =

4.2 as is the increase between 0900 and 1200 UT on November 4. The decrease in Dst between 0330 and 0730 UT on November 4 is also reflected

in a decrease in the flux best seen at L =

4.5. On an even shorter scale one can see correlations

between

changesin Dst andthe fluxbetweenthe timesof outboundand inbound

orbits

of the same satellite

at L

-

4.5

and 5.0.

Superimposed on thesechangesis an overallrisein the flux on November4. By the end of November4, the flux at L -- 4.2 and 4.5 had recoveredor exceededprestormvaluesthoughDst

hasonly partiallyrecovered.The correlationbetweenDst and the electronflux at L = 4.2 and 4.5 can be explained,in part, by the adiabaticresponseof the electronsto changesin the magneticfield representedby Dst. This will be discussed later. The secondfeature of particular note is the steep radial gradient seen at L = 4.5 near 0200 UT on November4 as NS28 (purple) movesinward in L from 4.6 to 4.4. As this satellitemovesto smallerL before 0200 UT countsat all larger L (5.0, 5.5, 6.0, 6.5) were at background,2 to 3 order of magnitudebelowprestormlevels.More importantly,thissteep gradientand lack of flux at largerL wasalsoseenin the other energychannels(0.2-0.4 MeV, 0.4-0.8 MeV, 1.6-3.2 MeV). It appearsthat all energeticelectronsaboveL = 4.6 had been lost and that at this time L - 4.6 representsthe energetic electrontrappingboundary.One hour later whenNS28 (purple) movedoutwardto larger L the trappingboundaryhad moved outward

One

a little and occurred between L = 4.6 and 4.9.

can see that at L

=

5.0

the flux almost remains

at

backgroundlevels until -0800 UT on November 4 while at L = 5.5 the flux remainsat backgroundlevelsuntil -1600 UT. Another feature to note is that the gradientof the flux as a function of L at constantenergy is usuallyindicatedby the changein the flux as eachsatellitemoves0.2 L within eachL range.After the storm at L = 4.2 the peak in the observed flux typically occurred at a L smaller than the smallestL

encountered by GPS satelliteasindicatedby the "/V' shapeof flux as a function of time during each satellite's passage through this L range. This indicatesthat the peak flux at constantenergyoccurredat smallerL. However, before the

LI ET AL.: OUTER

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ELECTRON

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UT (hour) starting on Nov. 3, 1993 Plate 1. Countrate of electronsat variousL values in the energy range of 0.8-1.6 MeV from 4 GPS satellites,eachrepresented by a differentcolor(NS18 - red, NS24 = green,NS28 = purple,NS39 - black), are plottedversustime (tick marksin hours)startingon November3, 1993.The last panel is high time

resolution(every10-min)Dst indexobtainedfrom D. KnippandB. Emery(privatecommunication, 1996).

storm the flux was nearlyconstantduring each satellitespassagethrough the L range around L = 4.2. In fact, between 0400 and 0500 UT on November3 the flux actuallyhasa slight 5/shape indicatingthat the GPS satellitehad passedinwardin L of the flux peak. Thus, before the storm the peak flux occurrednear L = 4.2, while after the storm the peak flux

movedinward to smallerL. It shouldbe noted that comparisonsof the flux at differentL may also indicate pitch angle effectsas the larger L shellsare sampledat larger magnetic latitudes.

Before the main phaseof the magneticstorm fluxesat the variousGPS satellitesdo not showa largevariationwith lon-

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UT (hour) starting on Nov. 3, 1993 Plate 2. Countrate of electronsat variousL values in various energy rangesfrom 4 GPS satellites,each representedby a differentcolor (NS18 - red, NS24 = green,NS28 - purple, NS39 = black), are plotted versustime (tick marksin hours)startingon November3, 1993.The backgroundcountrate from eachenergy channelof eachsatelliteswassubtracted,multiplyinga factor (> 1) to lowercountrate channelsso the count rate is the same as the highest count rate channel, then a 0.11 was added to all channelsas a common background.The point of doing suchis to showdifferencesin the lossand recoveryof the energeticelectrons for differentenergies.The countratesfrom NS39 (black) for energychannelof 0.2-0.4 and 1.6-3.2 MeV are not plottedbecauseof instrumentmalfunctions. The hightime resolution(every10-min)Dst index(D. Knipp and B. Emery, privatecommunication,1996) is alsoplotted.


BELT ELECTRON

VARIATIONS

14,129

gitude or magneticlocal time. However,just before 0100 UT clearthat the electronflux leveldroppedat the end of Novemon November4 NS39 and NS18 differby more than a factor of ber 3, but by the end of November4 the flux had surpassed the 3 at L = 4.5 eventhoughmeasurements are made at the same prestormlevel. Startingabout 1800 UT of November3 (the time. The differenceis evengreater,about an order of magni- 12th panel), we seeenhancedprecipitationat L = 8.5, which tude, in the 0.4-0.8 MeV channel. Such differences are ex- movesto smallerL (down to L = 6) with time. This precippected since the ring current is not likely to be symmetric itation has the same characteras the "precipitationbands" initially, and thus the magneticfields and the energeticelec- discussed by Nakamuraet al. [1995].Precipitationis indicated trons are likely to be lesscylindricallysymmetric.Finite drift bysimilar fluxes in•!•etworows shown during these times times and strong convectionelectric field are also likely to suggesting thattheloss•one isfull. contributeto the increasedasymmetryof the lower-energy Plate4 shows thecourkt_ rateforthesouthern hemisphere electrons.One can alsoseesomeasymmetrynear 1200UT on night passes. Precipitation bands areseenat2310UT onNoNovember4 in the 0.4-0.8 and 0.8-1.6 MeV rangesduring a vember 3 atL = 4.2 andat/)220UT onNovember 4 atL = time when the overallfluxesare increasing. 3.7.Such precipitation bands appear tooccur atthetrapping Plate 2 showsthree other energyranges(0.2-0.4, 0.4-0.8, boundary oftheelectrons [Nakamura etal.,1995]. Additional 1.6-3.2 MeV) in additionto that shownin Plate 1. For Plate 2 "bursty precipitation" is seen at vahous L values after 1000 UT we have adjustedfor the slightlydifferentbackgroundcount here on November4 and alsoin •thenorthernhemisphere levels and effective geometric factors of the different GPS passes asseen in Plates 5 and6. Tl•edatagaps aredueto satellitesand channelsto producea more uniform presenta- spacecraft memoryquotas.It is interesting that substantial tion. The point of this figureis to showdifferencesin the loss and recoveryof the energeticelectronsfor differentenergies. flux is increasing.

bursty precipitation occurs atatime when the overall electron

We see that in the 0.2-0.4 MeV channel at L = 4.2, the flux

dropsless(only an order of magnitude)and recoversmore quicklyto prestormlevelsthan do the more energeticchannels. At 0.2-0.4 MeV the flux recoversto pre-stormlevelsby about 0900 UT on November4, while at 1.6-3.2 MeV only at -2100 UT did the flux reach pre-stormlevelsand by this time the 0.2-0.4 flux is aboutthree timesprestormlevels. Figure 3 showsthe count rate of >3 MeV electronsmeasuredby SAMPEX with a time resolutionof 6 s versusL. The first set of panels is for November 2, the secondset is for November3 and so forth. Each panelis for abouta quarterof one spacecraftorbit. Only data from the dayside southern hemisphereare plotted in order to make a better comparison betweendifferentpasses.The time intervalsfor eachpanel are shownin the right cornerof eachpanel.The peak at smallerL (1.2-3) is due to "contamination" from the inner protonradiation belt near the SAA, and there is a stable background probablyfrom penetratingcosmicraysat largerL. We can see that at the end of November

3 the >3 MeV

electron

flux falls

to the backgroundlevel (corresponding to a 3 ordersof magnitude decrease)for -12 hours.The >3 MeV electronsthen recover,surpassing their prestormlevelsin the end of November 4 and the peak flux movesto smallerL. Before the storm the flux peaked aroundL = 4 consistentwith the GPS data while after the stormthe flux peakedaroundL = 3.5. Plate 3 showsthe high time resolution>1 MeV count rate from HILT as a functionof L for November3 and 4. Only data from the daysidesouthernhemisphereare plotted in Plate 3 in order to make a better comparisonbetweendifferentpasses. The time intervalsfor eachpanel are shownin the right corner of each panel. Red and green dots correspondto electron counts measured

from two different

rows of the detector.

The

Figure 4shows theelectron differential flux (electrøns/( cm2-

s-sr-keV)) averagedover pitch angle,measuredby LANL instrumentsonboardtwo geosynchronous satellitesplotted versus universal time (tick marks in hours) starting from November 3. The two satellites'longitudesdiffer by -173 ø. The vertical

dotted lines and the dashed-dotted

lines indicate

localmidnightand localnoon,respectively. The electronfluxes at the bottom panel and the electron flux of 0.6-0.9 MeV electronsat the top panel have a peak around 1900 UT of November3, this maybe due to the compression of the magnetosphereby the solarwind,whichis alsoindicatedby the Dst profilearoundthe sametime (seeFigure2). The sharpdropin the electron flux at the end of November

3 is evident in both

plots even though one spacecraft(1984-129)was near local midnightandthe other(1989-046)wasnearlocalnoon.Plasma instrumentonboard1989-046measureda few sharpenhancements of tens to hundreds

2310 UT of November

of eV electrons and ions between

3 and 0030 UT of November

4 indicat-

ing that the spacecraftmovedin and out of the magnetosphere into the magnetosheath (M. ThomsenandJ. Borovsky,private communication,1996). As shownin both plots,the lessenergeticelectronsrecoveredbefore the more energeticelectrons. One alsomay noticethat there wasa generaldiurnalvariation of the electronflux,more evidentin the higherenergychannel. Higher flux occursat localnoon due to the day-nightasymmetry of the Earth's magneticfield causedby the compression by the solar wind of the daysidemagnetosphere[Paulikasand Blake, 1979]. However,overall,the relativisticelectronflux in all energychannelssurpassed the originalor pre-stormlevelby the end of November

4. This is in contrast to the GPS data for

the L = 6.5 field line for which the count rate is still below the

detectoris dividedinto four rowswith slightlydifferentaverage pre-stormvalue by the end of November4. This may indicate look directions.An isotropicfluxwill producesimilarcountsin incorrectfield-line mappingduring the storm or pitch angle the two rows shownwhile an empty losscone may produce, distributionsenhancednear 90ø at the equator. Pitch angle dependingon the angleof the magneticfield with respectto sorted data from LANL sensors on 1989-046 show that 0.75the detector look direction and the level of saturation, different 1.1 MeV field-alignedflux (pitch angles 78.5ø)is abouta factorof 3 larger energeticinner zone protons,appearwhen SAMPEX wasnear than prestormvalues.Ingrahamet al. [1996] showedthat at the SouthAtlantic region and on theseorbits on November 3 least sometimeswhen the energeticflux is recoveringat geosynchronous orbit the recoveryof the flux near 90ø precedes the detectorwas alsosaturatedat largerL (L = 3.5-5.5). Comparingcorrespondingpanelsin November3 and 4, it is the recoveryat smallerpitch angles.The flux from the space-

14,130


Nov.

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Figure3. Countrateplottedversus L forelectrons withenergy>3 MeV. Eachpanelisfor abouta quarter of onespacecraft orbit(orbitperiod-95 min),onlydatafromthedayside southern hemisphere areplotted in orderto makea bettercomparison between different passes. Thetimeintervals for eachpanelisindicated bythenotation on eachpanel.Thestablebackground at allL values indicated bythesmooth linenearly identical forallpanels isduetothegalactic cosmic rayparticles. Theinnerpeak(leftmost) ismostly fromvery energetic protons(> 100MeV). The outerbelt(rightmost) wasnotseenin thefourthpanelfor November 2 and3 because the spacecraft wasin thestrongest magnetic fieldregionduringthesepasses.

LI ET AL.: OUTER

RADIATION

BELT ELECTRON

VARIATIONS

14,131

1.e+04 In

MeV

5.e+02 0.6-0.9

1984 -129

GLON=8.3 ø 2.e+01

1.4-2.0

1.e+00

1 .e+04

In

MeV

0.75-1.1 5.e+02

1.10-1.5 1989 -046

GLON--165

ø 2.e+01

1 .e+00 0

UT

Figure4. Pitchangleaveraged electrondifferential flux (particles/(cm2-s-sr-keV)) measured by LANL instruments on boardtwo geosynchronous satellitesplottedversusuniversaltime (in hours)startingfrom November3 to 5. The two spacecraftare ---173 ø apart in longitude.The verticaldotted lines and the dash-dotted linesindicatelocalmidnightandlocalnoon,respectively. The correspond energyrangefor each curveis denotedon the right-handsideof the plots.

craft 1984-129(top) is underestimated becauseof the decayof the detector'ssensitivity.

• = p2_k/2moB,

(1)

which is alsocalledthe particle'smagneticmoment.The second adiabatic invariant associated with the bounce motion:

Discussion

We now discussthe mechanisms that may contributeto the disappearance and reappearance of the relativisticelectrons. Mechanisms Contributing to the Flux Reduction Some of the different mechanisms that can be used to ex-

plain the initial decreaseare: adiabaticresponseof the electrons,radial diffusionor transport,and lossof the outer portion of the electronsto ionosphericprecipitationor transport throughthe magnetopause. The moststraightforward mechanismis the adiabaticeffect

J-• Pll ds,

(2)

whichis the integralof the particle'sparallelmomentumalong its bouncetrajectory.The third adiabaticinvariantassociated with drift motion:

ß=•BdA,

(3)

(conserving all threeinvariants)dueto the changein the mag- whichis the magneticfluxcontainedin the particle'sdrift path. The ring currentcausesa decreaseof the magneticfield in netic field from the ring current.As a reminder,we list the three adiabatic invariants. the innermagnetosphere. This canbe measuredon the ground The first adiabaticinvariantassociated with the gyromotion: andisusuallyquantifiedby theDst index.If the third adiabatic

14,132


BELT ELECTRON

VARIATIONS

invariant is conserved,the electron drift orbit around the earth

before the storm of the relative fluxes at 3.8 and 4.2. We notice

mustmoveoutwardduringring currentenhancement.Conservation of the first two adiabaticinvariantsthen impliesthat the electron losesenergy [Desslerand Karplus, 1961; Mcllwain, 1966]. If the electronphasespacedensityat fixed • and J is increasingwith L and the electrondistributionis steeplyfalling with energy,astheyusuallyare, a fixed-energydetectorwill see a drop in the flux when the ring current is enhanced.We call

that the flux at larger energieshasa larger relative decreasein agreementwith data. The relative flux at L - 3.8 is about a factor of 3 less than at L = 4.2 before the storm estimated

from SAMPEX measurements (e.g., Plate 3). Thus we expect the Dst effect to give slightly over an order of magnitude decrease in the 0.8-1.6 MeV channel at L -

4.2. However,

the relative flux at L - 3.8 before the storm is steeplyrising this the "Dst effect." As shown in Plates 1 and 2, there was with L, and thus differencesin the actualchangein the magalmosta 2 order-of-magnitudedrop in the electronflux at L netic field will producelarge differencesin the expectedflux 4.2 and 4.5 in the electronflux above0.8 MeV after the strong decrease.Nevertheless,the actual observedchangeis larger, compression by the solarwind at the end of November3, 1993. and thus our best estimate of the Dst effect alone cannot The initial dropof the electronfluxcoincides with the decrease explainthe actual lossof the electrons. of Dst. It is alsoclear, however,that the Dst effectcannotby The overallscenariothat canexplainthe datais that, on the theelectrons moveoutward andloseenergy. Onthe itself completelyexplain the changein the electron distribu- average, tion. From the Dst effectwe would expectthe inner boundary averageelectronsat L > 4 movebeyondL = 4.5 and are lost. of the outer electron belt to move outward. Instead it moves However, the increaseof Dst is not symmetricand involves inward.The inwardmotion is apparentlya commonfeature:it temporalvariationson a shorttimescaleleadingto radial difwas noted, for instance,by Forbushet al. [1961, 1962] from fusioninward at smallerL valueswhere the phasespacedenExplorer 7 data. Nor can the Dst effectsufficientlyexplainthe sity has an especiallylarge outward radial gradient. Outside sharp drop seenin the GPS data at L = 4.5 at 0200 UT on L - 4.5 mostof the flux is lostby precipitationto ionosphere November 4 in Plate 1. or drift throughthe magnetopause. Thus radial diffusionoutNevertheless,we can estimate the magnitude of the Dst ward also leads to further losses of electrons between about effect given the initial radial profile of the electron spectrum L = 4 and L - 4.5 as they diffusebeyondL - 4.5 and are and the changein the magneticfield. We do not know the lost.Takinginto accountthat mostof the phasespaceis in the changein the magneticfield, but we can estimatethis from the outer portion of the radiationbelt, especiallywhen the radiachangein Dst and the correlationbetweenDst and the change tion belt is accounted for at constant • and J, leads to the in the equatorialmagneticfield. Sucha correlationwas com- conclusion that almost all the outer radiation belt electrons are piled by H. Singerand J. Hughesfrom CRRES satellitedata lost. Only the inner portion survivesand this on the average between L = 3 and 5 within 10ø of the magnetic equator losesenergy and is redistributedin L. Other estimatesmade (H. Singer,privatecommunication, 1996), and they showthat from data from the CRRES satellite[Rinaldiet al., 1994] also •B - 1.2 Dst - 43.4. As a specificexample,let us consider showthat the Dst effect can usuallyonly explainpart of the the changeexpectedin the 0.8-1.6 MeV electronsat L = 4.2 drop in the electronflux associated with magneticstorms. due to the main phaseof the storm.We will usethe resultthat The precipitationbands observedby SAMPEX provide a /SB = 1.2 Dst - 43.4 between L = 3 and 4.2. Inside L - 3 natural explanationfor the loss of the electrons.The most we will estimatethat the changein magneticfield is 2/3 of Dst. intense of the precipitationis seen in the south-nightplot Using a dipole model for the backgroundequatorialfield, we between 2306 and 2308 UT where the flux above 1 MeV

haveapproximately Bo = 30,000L-3 nT for thebackground reaches over104/cm2-s. (Theplotshows counts persecond and field. Then usingthat Dst changesfrom + 10 to -80 nT during the geometric factorof eachrowis -25 cm2 andthe accepthe main phaseand that the flux insidethe drift orbit remains tanceangleis 68ø x 68ø.)During the main phaseof the storm constant, we havethat electrons originallyat L - 3.8 moved DMSP data showedintense precipitatingauroral electrons outwardto L = 4.2 and that the magneticfield at the electron down to 59ø magnetic latitude and large convectiveelectric positionchangedfrom 515 to 265.5 nT. The decreasein the fields indicated by the uplift of the F region were seen by magneticfield is due to both the local decreaseat L = 4.2 and Millstone Hill incoherentradar down to L - 2 (J. Foster, the outward motion of the electrons.Now for simplicitywe privatecommunication,1996).Thusit is possiblethat the preconsideronly equatoriallymirroringelectronsand assumethat cipitationmay occur on auroral field lines sinceboth the authefirstadiabatic invariant isconserved. Thenusingthatp•/B = rora and the lossof energeticelectronswere observedbetween const and convertingmomentum to kinetic energy we have L = 4 and 5. The intensewave activityusuallyassociated with that 0.8 MeV electronsat L - 4.2 after the main phaseof the auroral field lines then providesa natural explanationfor the stormwere originally 1.25 MeV electronsat L = 3.8 and 1.6 pitch anglescatteringneededto producethe observedprecipMeV electronsat L - 4.2 were originally2.39 MeV electrons itation. The actual fluxes observed however are not sufficient to at L = 3.8. The relativisticelectronflux can often be approxproducethe requiredloss.Typicalfluxesabove1 MeV at L -imated by an exponentialof the form j = Jo exp (-E/Eo). Using suchan exponentialfit to the 0.8-1.6 MeV and 1.6-3.2 4.5 at equatorare about106electrons/cm2-s. Thusthe obMeV GPS channelsat L = 4.2 before the storm givesE o = servedfluxesare about 2 orders of magnitudelessthan that 0.59 MeV. Then assumingthe samevalueof Eo andjo at L = expectedfrom the strongdiffusionlimit. The strongdiffusion 3.8 and usingLiouville'stheoremthat the phasespacedensity limit at L - 4.5 givesa lifetime of the order of 1 min [Kennel alonga trajectoryis constantand that the phasespacedensity, andPetschek,1966].However,it is likelythat SAMPEX did not fp, is relatedto theflux,j, byfp = j/p2, wherep is the crossthe precipitationband at the longitude or time of the electron's momentum, we have that the differential flux at 0.8 strongestflux since,of course,the precipitation,itself, would MeV shoulddecreaseby a factor of 4.16 timesthe ratio before have reducedthe electroncontentof the flux tube by the time the storm of the relative fluxesat 3.8 and 4.2, while the flux at SAMPEX crossed that flux tube. 1.6 MeV should decreaseby a factor of 7.4 times the ratio Even if the electronsdo not precipitateinto the ionosphere


BELT ELECTRON

manyof them couldbe lost by drifting throughthe magnetopause.Early on November 4, 1993, the magnetopausewas observedto moveinsideof geosynchronous orbit (M. Thomsen and J. Borovsky,privatecommunication,1996). The magnetic fieldwaslikelyenhancednear the subsolarpointjust insidethe magnetopausedue to the compressionby the solarwind. During compressed conditionselectronsfrom smallerL from the nightsidewill movefurther out near the subsolarpoint in order to conserve the first adiabatic

invariant

in addition

to their

outward motion from the Dst effect. We have, however, no

direct evidencethat electronscrossedthe magnetopausenor do we knowexactlyhowfar insideof geosynchronous orbit the magnetopausemoved. Assuming,however,that the magnetic field beyondL -- 5 remainsat its dipolarvalue and assuming the sameDst effect as before on the magneticfield inside of L = 5 implies that just from the Dst effect alone electrons originallyfrom beyondL = 4.6 would move to the geosynchronouspositionat L - 6.6. Energization Mechanisms

VARIATIONS

14,133

in the outer magnetospheresuchas reconnectionin the tail. It has been suggestedthat solarwind electronseither from the Sun or from Jupiter [e.g.,Baker et al., 1979] can providethe source of the outer radiation

belt electrons.

Such solar wind

electron,however,do not havesufficientphasespacedensityto be the sourceof the radiationbelt regardlessof whether they enter directlyor are energizedwithin the magnetosphere while preservingtheir first two adiabaticinvariants[Li et al., 1997]. Radial diffusionprovidesan incompletedescriptionof the formation of electronbelts. It works best during periodsof relativemagnetospheric calmwhenradial transportduringone drift period is insignificant.A dramaticexampleof a different processis the CRRES eventof March 24, 1991,when a strong interplanetaryshockstruckthe Earth's magnetosphere generating intenseflux of electronsand protonsat L --- 2.5 in 90 s [Vampolaand Korth, 1992;Blake et al., 1992;Li et al., 1993; Hudsonet al., 1995].This eventhasbeenmodeledby usinga magnetosonicpulse to producegood agreementbetweenthe observedand simulatedelectrondrift echoes[Li et al., 1993]. In this event a new electron and ion radiation

belt was formed

The restoration of the outer radiation belt occursquickly at L = 2-3 in lessthanonedrift period(150 s) of the 15 MeV after its disruption.The energizationprocesses responsiblefor electrons. the recoveryof the energeticelectronfluxesare not asclear as The electronsin the outer radiationbelt can alsobe signifthoseleadingto their loss.Perhapsthe centralproblemis why icantlyaffectedby muchsmallerpulsesthanthose modeledin thereis radiationbelt at all at geosynchronous orbit. Substorms March 24, 1991, event. Such magnetosonicpulsesare associimpulsivelyinjectelectronsup to ---300keV at geosynchronous ated with both interplanetaryshocks[seeHudsonet al., this The timescalefor the radial transportof orbit [Bakeret al., 1989;Li et al., 1996].Suchinjectionscanin issue]and substorms. generalbe understoodasthe resultof the inwardconvectionof the electronsdue to suchmagnetosonicpulsesis much faster plasmasheet electrons.However, electronsare also seen at than the radialdiffusiontimescale.However,the energygainof geosynchronous orbit above MeV energiesand there is no the electronduringradial transportis still limited because/• is obviousreason why they should be there. Here we discuss usuallyconserved. Inward radial diffusionrequires that there be a outward somepossibleenergizationmechanisms for this stormin particular as well as for a more general situation. radial gradientof the phasespacedensityat constant/• andJ. Ring current (Dst) effect. Again, the most straightforward It is interestingto seewhether this is the casein our example. mechanism is the Dst effect. Since this is an adiabatic effect For simplicity,we will consideronly /•, and thus we should due to the ring current, the subsequentdecay of the ring consideronly equatoriallymirroring electrons.However, if we current should, in the absence of other effects, increase the

use GPS data, we must note that GPS data are omnidirectional

magneticfield in the inner magnetosphereand restore the originalenergeticelectrondistribution.It shouldbe noted that

and detect equatoriallymirroring electronsonly at L = 4.2. However,if pitch angleanisotropiesare not large, fluxesaway the Dst effect does not involve the loss of the electrons but from the equator are representativeof equatorialfluxes.For simplyan adiabaticreversiblechange.For manysmallerstorms definiteness, we considerthe relativephasespacedensityof 1.6 it is often the case that after Dst has recovered the smaller L MeV electronsat L = 4.2 and of electronswith similar • at portionof the outer electronbelt looksvery similarto what it L -5. Before the storm on November 3 near 0900 UT the meawasbeforethe storm.This iswhatwouldbe expectedif the Dst effect was dominant

and there was little real loss of electrons.

As noted above,it is evidentthat duringthis storm there was significantlossof electronseither to the atmosphereor to the magnetopauseand thus there must also have been significant electron energizationnot related to the Dst effect. One can noticein Figure2 that the Dst indexhasnot fully recoveredat the end of November7 thoughthe electronflux hassurpassed prestormlevels.Clearly,thisrequiresothereffectsbesidesDst. Radial diffusion. The classicexplanationfor the sourceof the outer electron

radiation

belt is radial

diffusion.

The ten-

dencyof radial diffusionis to equalizethe phasespacedensity of electronswith the samevalue of/• andJ [Schulzand Lanzerotti,1974].Sincethe phasespacedensityat a given/• andJ usuallyincreaseswith increasingradial distance,the usualeffect of radial diffusionis to increasethe flux at smallerL at any givenenergyand positionby radially diffusingelectronsfrom largerL. This meansthat it is necessaryfor energeticelectrons to be availablein the outermagnetosphere. The sourceof such electronsmay be electronsthat are energizedby someprocess

sured electron flux in the 0.8-1.6

MeV

and 1.6-3.2

MeV

chan-

nel on NS28 (purple,asshownin Plate2) at L = 4.2 couldbe fit byj = 19800 exp (-E/0.59) electron/MeV-s,whereE is in MeV, while the flux at L = 5 couldbe fit byj -- 8910 exp (-E/0.565) electron/MeV-s.The model magneticfield indicatesa equatorialmagneticfield of 406 and 234 nT at L = 4.2 andL -- 5, respectively. We find that an 1.13MeV electronat

L = 5 hasthesamemagnetic moment(p2/B) asthe1.6MeV electronat L -- 4.2 andthat the phasespacedensity(fp j/p2) is1.6timeslargeratL = 5 thanatL = 4.2 forthisvalue of the magneticmoment. However, at L = 5, B/Bo = 2.15 (the ratio of the local magneticmodel field to the equatorial magneticfield alongthe samefield line) and since,on average, electronmodels(e.g.,AE-8 [seeVette,1991])showthat the flux shouldbe smallerby about 25% at B/Bo = 2.15 than at the equator,we estimatethat the phasespacedensityat L - 5 at the equatoris twiceaslarge as at L - 4.2. Thus, as expected, the phasespacedensityat constant/• is larger at larger L. It is of coursemore interestingto perform thisexercisewhile

14,134


the flux at 1.6 MeV is increasingasit is after the main phaseof the storm.After all, thisis when radial diffusionshouldactually be doing something.We considerthe 1.6 MeV electronsat L = 4.2 at around 1500 UT on November 4. We find j = 28564 exp (-E/0.42) electron/MeV-sat L = 4.2. The flux at L = 5 is still very low but increasingso we use the closest in time measurement

at L = 5 after the measurement

at L =

4.2, whichis ---7timeslarger than the flux measuredat L = 5 just before. (Of course,if we expectradial diffusionto be working,then the flux at largerL shouldprecedein time the flux at smallerL but, aswe shallsee,eventhislarger flux is not sufficient.)The flux at L = 5 is j = 2112 exp (-E/0.36) (note that the characteristic energies,E o, are smallerafter the storm)and the equatorialmodelmagneticfieldsare 405 nT at L -- 4.2 and 209 nT at L -- 5. Thus we find that an 1.05 MeV

electron at L = 5 has the samemagneticmoment as an 1.6


lower-energyelectronsprovidesa hint that perhapswavesare importantwhich we now discuss. Wave heating. Another possiblesourceof the more energeticelectronsis heatingby wavesITemerinet al., 1994].Whistler wavesplay an important role in the theory of trapped magnetosphericelectrons,but they are usuallyinvoked as a lossmechanism[Kenneland Petschek,1966]. By pitch angle scatteringelectronsinto the losscone,whistlerwaveslead to the decayof the outer electronbelt. To understandhow whistler wavesmay energizeradiation belt electrons,one shouldconsiderthe normaltheoryof waveparticleinteractionof trappedelectrons.The interactionsbetween the wave and the electron occur when the cyclotron resonancecondition,

•o- kllvll= Nfi/•/,

(4)

is satisfied,where 12 is the electrongyrofrequency,•/is the phasespacedensityis actually2.8 timeslarger at 1.6 MeV at relativisticmasscorrection,N is an integer,parallel subscript meansparallel to the magneticfield. For wavespropagating L = 4.2 than at 1.05 MeV at L = 5. Radial diffusion should begoing backward withanetdiffusion outward butthatcannot parallelto the magneticfield, only the N = 1 resonantinterexplainthe increasein the fluxsincethe fluxat smallerL is also action, that is, where the electron and wave are going in oppositedirectionsalongthe magneticfield,produceschangesin very smallat the end of main phaseof the storm. There are some uncertaintiesin the above analysis.The the electronpitch angleand energy.In thiscaseandfor typical model magneticfields are not right sincewe have not taken plasmaparametersat sayL = 3 the dominanteffectof the interactionis pitch angle scattering[Lyonset al., 1972], and into accountthe ring current.We couldtake into accountthe while it is acknowledged that pitch anglescatteringnecessarily ring currentas before and that might be appropriatefor L = involvessomeenergychange,it canbe shownthat thischange 4.2, but it is not likely to be right for L = 5 sinceit wouldgive is small.Furthermorethe energychangeis correlatedwith the a smallervalue of the magneticfield there than is measuredat pitch anglechangeso that a pitch angledecreaseis correlated geosynchronous by GOES (H. Singer,privatecommunication, with an energy decrease.Thus any large changein energy 1996). We did this anywayand found that the phase space involvesa larger relative changein pitch angle, but a large densitieswere then aboutequalat L = 5 and 4.2. In addition, changein pitch anglewill put the electroninto the losscone it maywell be that pitch angleanisotropiesare muchlarger as where it will be lost. the electronflux is recovering.As mentionedalready,the MeV However, if the whistler wave is not propagatingexactly flux at the end of November4 had recoveredasseenby the Los parallel to the magneticfield then higherorder resonantinAlamos sensors but was still lower at the L = 6.5 field line as teractionscan occurwhen the Doppler-shiftedwavefrequency measuredby GPS suggesting suchan anisotropy. equalsanyinteger(includingnegativeinteger)multipleof the Performingthe same analysison the 0.4 MeV flux at L = gyrofrequency.For negativeinteger interactions,that is, for 4.2 shows,however,that the corresponding phasespaceden- the casewhen the velocitycomponentof the wave alongthe sityis larger at L = 5. Also, one shouldnote that there is an magneticfield is in the same directionas the velocityof the excess (with respectto the exponentialfit of the moreenergetic electron, there is an inverse correlation between the energy electrons)of lower-energyelectronsespeciallyafter the main changeand the pitch angle change,that is, a decreasein the phaseof the stormandthat theseelectronscannotbe fit by the pitch angleleadsto an increasein the energyof the electron. same characteristicenergy as the 0.8-3.2 MeV electrons. For relativisticelectronsthere are typicallyseveralharmonic There is little doubt that the source of 0.2-0.4 MeV electrons interactionscorrespondingto the first few positiveand negaat L = 4.2 are electronsradially diffusingor injected form tive integerswhere the resonantconditionmay be satisfied, larger L. At geosynchronous orbit, enhanced injections of perhapswith whistlerwaves,suchas chorusin the outer magelectronswith energiesof tens of keV up to 300 keV are netosphere. Sincefor relativisticenergiesthe gyroradius of the observedby the LANL sensorsafter the main phaseof this electronmay be on the order of or greaterthan the perpenstorm [Li et al., 1996]. These electronshave corresponding dicular wavelength,the strengthof the interactionsat the energiesthat are typicalof substorminjectedelectronsand the higherharmonicsare of the sameorder as at the fundamental large convectionelectricfieldsof the stormshouldbe able to (mathematicallythis involvesthe variousBesselfunctionsfor rapidlyconvectsuchelectronsinward. eachharmonicJ1, J2, etc. as a functionof (k 2_P),wherethe p The sourceof the more energeticelectronsis not as clear. is the electron'sgyroradius).Sincenow the energyand pitch Especiallysincethe SAMPEX data showthat burstyprecipi- anglechangeare no longer correlated,the electronsundergo tation is occurringas the flux is increasing.In addition,using more or lessindependentrandom walks in energyand pitch radial diffusionalone,it may be difficultto explainthe delayed angle in the presenceof whistler waves.Thus the electron recovery of the more energetic electrons.A recirculation distributionis heated even though someelectronsare lost to model has been suggestedto explain this delayedrecovery. the loss cone. Sincethis modelwaswell discussed elsewhere[Nishida,1976; The relative magnitudeof pitch angle and energychanges Fujimoto and Nishida, 1990a; Fujimoto and Nishida, 1990b; dependson the phasevelocityof the waves.Just as in the Baker et al., 1986, 1989], we do not discussit further here. centerof massframe there is no changein the kinetic energy However,the presenceof burstyprecipitationand an excessof due to elastic collisionsbetween two objects,there is little MeV electron at L = 4.2. However, now we find that the

L1 ET AL.: OUTER RADIATION

BELT ELECTRON

VARIATIONS

Nov.

14,135

4

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Plate 3. High time resolution(0.1 s, >1 MeV) count rate from two rows of HILT which have relatively different view angles to the local magnetic field. Only data in the daysidesouth hemispheresector on November 3 and 4 are plotted in order to make a better comparisonbetween different passes.The time intervalsfor each panel is indicatedby the notation on each panel.

14,136

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LI ET AL.: OUTER

Nov.

100

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14,138


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Plate 6. Sameas Plate 3 exceptthat only data in the daysidenorth hemispheresectoron November3 and 4 are plotted. The data gapsare due to spacecraftmemory quotas.


BELT ELECTRON

VARIATIONS

14,139

changein the energyof the electronsfrom wave-electronin-

old question:where do the energeticelectronsgo when they disappearand how do they come back? We have discussed there can be a relativelylarge changein energyif the wave severalmechanismsthat may contributeto the initial drop, phasevelocityis large. Suchcan be the caseat L = 4.2 after recovery,and further energizationof relativisticelectrons.Efthe main phase of the storm if much of the plasmaspheric fortsat quantifyingthe individualmechanismandsubsequently plasmais convectedawaysincelowerplasmadensitiesresultin incorporatingthem into a physicalmodel of energeticelectron larger phasevelocities.Thus the environmentafter the main dynamicsare being continuing. spaceof the November3 stormmayprovidea favorablefor the accelerationof the more energeticportion of the electrons. Acknowledgments.We acknowledgeuseful discussions with Dr. Whether this is really the caserequiresdata on the directions Richard Thorne about the ion cyclotronwave heating of relativistic and magnitudesof the propagatingwaveswhich we do not electrons.We also thank D. Knipp, B. Emery, J. Foster,H. Singer, have but at least the burstyelectronprecipitationshowsthat M. Thomsen,J. Borovsky,and SteveCummer,for providingrelevant data and discussions. The work at Universityof Coloradowas supwave activity of some sort is present. Ion cyclotronwavesalsointeractwith electronsand produce ported by NASA grant NAGW-5152 and NAG5-2681 and by NSF grant ATM9224688. The work at Universityof California at Berkeley precipitationbut theywere estimatedto producelittle heating wassupportedby NSF grantATM-9224688.The work at LANL was [Thorneand Kennel,1971;Lyonsand Thorne,1972].However, supportedby the Department of EnergyOffice of BasicEnergy Sciit was recentlysuggestedthat ion cyclotronwavesmight also ence.The work at the AerospaceCorporationwassupportedby NASA provide a significantsourceof heatingin a similar manner of under CooperativeAgreement26979B.The work at STEL, Nagoya teractions in the wave frame. However, in the earth frame

was supportedby Grant-in-Aid for ScientificResearch the whistlerwaves(R. M. Thorne,privatecommunication,University, (CategoryC).

1996).

Summary and Conclusions

References

The variation of the relativisticelectronsin the magnetospherehasbeenwell measuredby multiplesatellitesduringthe November3-4, 1993, storm.There was an overall flux drop followingthe strongactivityassociated with largenegativeIMF B• and high solar wind pressure.In particular there was a depletion of MeV electron throughout the magnetosphere lastingfor longerthan 12 hoursbeginningearly on November 4. The electronfluxesthen increasedto higher than pr½storm levelwith the lower energyelectronsrecoveringfirst. The electrons alsomovedto smallerL (L = $) within a day. Some singlesatellitemeasurements[e.g., Owensand Frank, 1968] have previouslynot seensignificantreductionsof the relativistic electronflux at large L (>6) during magneticstorms. This maybe becauseby the time the spacecraftreturnedto the largeL the flux had alreadyrecovered. These multiple-satellitemeasurementsprovide a more detailed picture of the relativisticelectrondynamicssuchas the initial drop of the electronflux duringthe main phaseof this storm is indeed a global feature of the magnetosphereand is due to a real lossof electrons.The analysisof the electrondata from GPS and SAMPEX suggeststhat the lossof the inner part (L < 4) of the outer zone electronsis partly due to the

Baker, D. N., P. R. Higbie, R. D. Belian,and E. W. Hones,Do Jovian electronsinfluencethe terrestrialouter radiationzone?J. Geophys. Res., 84, 531, 1979.

Baker, D. N., J. B. Blake, R. W. Klebesadel,and P. R. Higbie, Highly relativisticelectronsin the Earth's outer magnetosphere,1, Lifetimes and temporalhistory1979-1984,J. Geophys.Res.,91, 4265, 1986.

Baker, D. N., J. B. Blake, D. J. Gorney,and P. R. Higbie, Highly relativisticmagnetospheric electrons:A role in couplingto the middle atmosphere? Geophys. Res.Lett., 14, 1027,1987. Baker, D. N., J. B. Blake, L. B. Callis,R. D. Belian, and T. E. Cayton, Relativisticelectronsnear geostationary orbit:Evidencefor internal magnetospheric acceleration,Geophys. Res.Lett., 16, 559, 1989. Baker, D. N., J. B. Blake, L. B. Callis,J. R. Cummings,D. Hovestadt, S. Kanekal, B. Blecker, R. A. Mewaldt, and R. D. Zwickl, Relativistic electron accelerationand decaytime scalesin the inner and outer radiationbelts:SAMPEX, Geophys.Res.Lett., 21,409, 1994.

Belian, R. D., G. R. Gisler, T. Cayton, and R. Christensen,High Z energeticparticlesat geosynchronous orbit during the great solar proton eventof October 1989,J. Geophys. Res.,97, 16897, 1992. Blake,J. B., W. A. Kolasinski,R. W. Fillius,and E.G. Mullen, Injection of electronsandprotonswith energiesof tensof MeV into L < 3 on March 24, 1991, Geophys.Res.Lett., 19, 821, 1992. Blake,J. B., M.D. Looper,D. N. Baker,R. Nakamura,B. Klecker,and D. Hovestadt,New high temporaland spatialresolutionmeasurements by SAMPEX of the precipitationof relativisticelectrons, Advancesin SpaceResearch,18, No. 8, page 171-186, 1996. Callis,L. B., D. N. Baker, J. B. Blake, J. D. Lambeth, R. E. Boughner, adiabatic effects associated with the decrease of Dst while the M. Natarajan, R. W. Klebesadel,and D. J. Gorney, Precipitating lossof mostof the outer part, thoseelectronsinitiallyat L -> relativisticelectrons:Their long term effect on stratosphericodd 4.0 are due to either precipitationinto the atmosphereor drift nitrogenlevels,J. Geophys.Res.,96, 2939, 1991. to the magnetopause becauseof the strongcompression of the Dessler,A. J., and R. Karplus,Someeffectsof diamagneticring currents on Van Allen radiation,J. Geophys.Res.,66, 2289, 1961. magnetosphere by the solarwind. Drake, D. M., T. E. Cayton, P. R. Higbie, D. K. McDaniels, R. C. The adiabatic effects associated with the increase in Dst Reedy,R. D. Belian,S. A. Walker, L. K. Cope, E. Noveroske,and definitelycontributeto the recoveryof the energeticelectron C. L. Baca,Experimentalevaluationof the BDD-I dosimeterfor the flux. In additionfor lowerenergies( 1.0 MeV) of temporalvariationsof the Van Allen radiationbelt, October1959to the outer zone electrons.

One of our intentionsin presentingthis studyis to raisethe

December 1960,J. Geophys.Res.,67, 303, 1962. Fujimoto,M., andA. Nishida,Monte Carlo simulationof energization

14,140

LI ET AL.: OUTER

RADIATION

BELT ELECTRON

of Joviantrapped electronsby recirculation,J. Geophys.Res., 95, 3841, 1990a.

VARIATIONS

Nakamura, R., D. N. Baker, J. B. Blake, S. Kanekal, B. Klecker, and D.

Hovestadt,Relativisticelectron precipitationenhancements near the outer edgeof the radiationbelt, Geophys.Res.Lett., 22, 1129,

Fujimoto,M., and A. Nishida,Energizationand anisotropization of energeticelectronsin the Earth's radiation belt by the recirculation process,J. Geophys. Res.,95, 4265, 1990b. Gussenhoven, M. S., E.G. Mullen, R. C. Filz, D. H. Brautigam,and F.

Nakamura,R., S. Kokubun,T. Mukai, and T. Yamamoto,Changesin the distanttail configuration duringgeomagnetic storms,J. Geophys.

A. Hanser, New low-altitude dose measurements,IEEE Trans. Nucl. Sci., NS-34, 676, 1987.

Nishida, A., Outward diffhsion of energetic particles fromtheJovian

1995.

Res., 102, 9587, 1997.

Higbie,P. R., R. D. Belian,andD. N. Baker,High-resolutionenergetic radiationbelt, J. Geophys.Res.,81, 1771, 1976. particlesmeasurementsat 6.6 Re, 1, Electron micropulsations, J. Owens, H. D., and L. A. Frank, Electron omnidirectionalintensity Geophys.Res.,83, 4581, 1978. contours in the Earth'souterradiationzoneat the magneticequaHudson, M. K., A.D. Kotelnikov, X. Li, I. Roth, M. Temerin, J. tor, J. Geophys.Res., 73, 199, 1968. Wygant,J. B. Blake, and M. S. Gussenhoven, Simulationof proton Paulikas,G. A., and J. B. Blake, Effectsof the solarwind on magneradiationbelt formationduringthe March 24, 1991 SSC, Geophys. tosphericdynamics: Energeticelectronsat the synchronous orbit, in Res. Lett., 22, 291, 1995. Quantitative Modelingof Magnetospheric Processes, Geophys. Monogr. Hudson,M. K., S. G. Elkington,J. G. Lyon,V. A. Marchenko,I. Roth, Set.,vol. 21, editedby W. P. Olsen,p. 180,AGU, Washington,D.C., 1979. M. Temerin, J. B. Blake, M. S. Gussenhoven, and W. R. Wygant, Simulationsof radiation belt formation during storm suddencom- Rinaldi, M. A., W. Nightingale,Y. T. Chiu, and M. Schulz,Short-term mencements, J. Geophys. Res.,this issue. responseof outer-belt relativisticelectronsto Dst variations,Eos Trans.AGU, 75, 545, 1994. Ingraham, J. C., T. E. Cayton, R. D. Belian, R. A. Christensen, F. Guyker, M. M. Meier, G. D. Reeves,D. H. Brautigam,M. S. Schulz,M., and L. Lanzerotti,ParticleDiffusionin theRadiationBelts, Gussenhoven,and R. M. Robinson, Multi-satellite characterization Springer,New York, 1974. of the largeenergeticelectronflux increaseat L = 4-7, in five-day Temerin,M., I. Roth, M. K. Hudson,andJ. R. Wygant,New paradigm period followingthe March 24, 1991, solar particle event, in AIP for the transportand energizationof radiationbelt particles,Eos Trans.AGU, 75, 538, 1994. Conference Proceedings 383 of Workshop on theEarth'sTrappedParticleEnvironment,edited by G. D. Reeves,p. 103, AIP, New York, Thorne, R. M., Energeticradiation belt precipitation:A natural de1996. pletion mechanismfor stratosphericozone,Science,21, 287, 1977. Kennel,C., andH. Petschek,Limit on stablytrappedparticlefluxes,J. Thorne, R. M., The importanceof energeticparticleprecipitationon Geophys. Res.,71, 1, 1966. the chemicalcompositionof the middle atmosphere,PureAppl. Geophys.,118, 128, 1980. Klecker,B., et al., HILT: A heavyion largeareaproportionalcounter telescopefor solarand anomalouscosmicrays,IEEE Trans.Geosci. Thorne, R. M., and C. F. Kennel, Relativisticelectronprecipitation Remote Sens.,31, 542, 1993. duringmagneticstormmain phase,J. Geophys. Res.,76, 4446, 1971. Knipp,D., Coordinatedstudyof Nov. 3-4, 1993magneticstorm,paper Tschan,C., Directionsfor magnetospheric modelsandsensors: An Air presentedat the GEM meeting,Snowmass, Colo.,June28, 1995. Force perspective,paper presentedat NSF/GEM GGCM CamLanzerotti, L. J., C. G. Maclennan, and M. Schulz, Radial diffusion of paign/SECSpaceWeatherWorkshop,Boulder,Colo.,Jan. 11-12, 1996. outer-zoneelectrons:An empiricalapproachto third-invariantviolation, J. Geophys.Res., 75, 5351, 1974. Tsyganenko,N. A., A magnetospheric magneticfield model with a Li, X., I. Roth, M. Temerin, J. Wygant,M. K. Hudson,and J. B. Blake, warpedtail currentsheet,Planet.SpaceSci., 37, 5, 1989. Simulationof the prompt energizationand transportof radiation Vampola, A. L., and A. Korth, Electron drift echoesin the inner particlesduring the March 23, 1991 SSC, Geophys.Res.Lett., 20, magnetosphere, Geophys. Res.Lett., 19, 625, 1992. 2423, 1993. Vette, J. I., The AE-8 trappedelectronenvironment,Rep. WDC-ALi, X., D. N. Baker, M. Temerin, T. E. Cayton,E.G. D. Reeves,R. A. R&S 91-24,Natl. SpaceSci.Data Cent., Greenbelt,Md., 1991. Christensen,and J. B. Blake, A source of outer radiation belt elec-

trons: Substorminjectionsand further accelerations,paper presented at the Chapman Conferenceon the Earth's Magnetotail: New Perspectives, AGU, Kanazawa,Japan,Nov. 5-9, 1996. Li, X., D. N. Baker, M. Temerin, D. Larson, R. P. Lin, E.G.

D.

Reeves,M.D. Looper, S. G. Kanekal, and R. A. Mewaldt, Are energeticelectronsin the solarwind the sourceof the outer radiation belt? Geophys.Res.Lett., 24, 923, 1997. Lyons,L. R., and R. M. Thorne, Parasiticpitch angle diffusionof

D. N. Baker and X. Li, Laboratoryfor Atmosphericand Space Physics,Universityof Colorado, 1234 Innovation Drive, Boulder, CO

80303-7814.(e-mail:[email protected]) J. B. Blake and M.D. Looper, SpaceSciencesDepartment,AerospaceCorporation,Los Angeles,CA 90009-2957. T. E. Cayton,R. A. Christensen, and E.G. D. Reeves,Mail Stop D-436, LOsAlamos National Laboratory,Los Alamos, NM 87545. S. G. Kanekal, NASA Goddard Space Flight Center, Code 690,

radiationbelt particlesby ion cyclotron waves,J. Geophys. Res.,77,

Greenbelt, MD 20771.

5608, 1972.

R. Nakamura, STEL, Nagoya University, Toyokawa,Aichi 442, Japan. M. Temerin, SpaceSciencesLaboratory,Universityof California, Berkeley, CA 94720.

Lyons,L. R., R. M. Thorne,and C. F. Kennel,Pitch-anglediffusionof radiationbelt electronswithin the plasmasphere, J. Geophys.Res., 77, 3455, 1972.

Mcllwain, C. E., Ring currenteffectson trappedparticles,Geophys. Res. Lett., 71, 3623, 1966.

Mcllwain, C. E., Processes actinguponouter zone electrons,in Radiation Belts:Models and Standards,Geophys.Monogr.Set., vol. 97, editedby L. F. Lemaire et al., pp. 15-26, AGU, Washington,D.C., 1996.

(ReceivedMay 22, 1996;revisedFebruary10, 1997; acceptedMarch 3, 1997.)