Magnetospheric dynamics and mass flow during the November 1993 storm

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL 103, NO All, PAGES 26,373-26,394, NOVEMBER 1, 1998 Magnetospheric dynamics and mass flow during the November 1993 storm Joseph E Borovsky, Michelle F Thomsen, and David J McComas Space and Atmospheric Sciences Group, Los Alamos National Laboratory, Los Alamos, New Mexico Thomas E Cayton Astrophysics and Radiation Measurements Group, Los Alamos National Laboratory, Los Alamos, New Mexico Delores J Knipp Department of Physics, US Air Force Academy, Colorado Springs, Colorado Abstract The National Space Weather Program (NSWP) Storm that occurred in November 1993 is examined with the use of plasma and energetic-particle measurements on three satellites in geosynchronous orbit Geosynchronous orbit affords a powerful perspective on magnetospheric dynamicsince both tail and dipole processes can be regularly seen, as well as nightside and dayside processes The major magnetospheric regions analyzed before, during, and after this storm are the outer plasmasphere, the ion plasma sheet, the electron plasma sheet, and the outer electron radiation belt Ionospheric outflows into the magnetosphere are also observed, and during the storm the magnetosheath and the low-latitude boundary layer are both seen briefly The geosynchronous observations indicate that prior to the storm the magnetosphere was very quiet and the outer plasmasphere was filled out to beyond geosynchronous orbit Extremely large anisotropies were seen in the ion plasma sheet during a compression phase just prior to storm onset During the storm's main phase the drainage of the outer plasmasphere to the dayside magnetopause was observed, a superdense ion plasma sheet was tracked moving around the dipole, and a superdense electron plasma sheet was seen The anomalously large plasma pressure on the nightside led to a 1 situation at geosynchronous orbit The 1 region spread around the dipole with the superdense ion plasma sheet The magnetic-field tilt angle at geosynchronous orbit indicated that strong cross-tail currents were present very near the Earth These currents appear to be associated with plasma diamagnetism Geosynchronous observations indicate that magnetospheric convection was extremely strong In the electron plasma sheet, severe spacecraft charging occurred The density of relativistic electrons was observed to peak very early in the storm, whereas the flux of these relativistic electrons peaked much later in the aftermath of the storm 1 Introduction The geomagnetic storm of November 3-4, 1993, is sometimes known as the GEM storm or as the National Space Weather Program (NSWP) storm It is the focus of the National Space Weather Initiative, wherein large amounts of data were collected and analyzed for one magnetosphere/ionosphere event [D J Knipp et al, An overview of the early November 1993 geomagnetic storm, submitted to Journal of Geophysical Research, 1997] The NSWP Storm was a major magnetic Copyright 1998 by the American Geophysical Union Paper number 97JA /98/97JA $0900 storm, as determined by the magnitude of its Dst perturbation (-116 nt), and it was a storm that resulted in significant fluxes of relativistic electrons in the magnetosphere The Kp and Dst signatures of this storm can be seen in the first two panels of Figure 1 The NSWP Storm was driven by a high-speed stream in the solar wind On the leading edge of a high-speed stream, the solar-wind velocity v is high and the magnitude of the interplanetary magnetic field B is high, so the energy input to the magnetosphere vb8outh tends to be high In the top panel of Figure 1, the Kp signature of the storm can be seen; it has a sawtooth shape, rising fast and declining slowly, which is typical for a high- speed stream [eg, Wilcox et al, 1967] The solar-wind density is also high on the leading edge of a stream, which will lead to a plasma sheet with high density, 26,373

2 26,374 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 20 8 main phase Kp Index 6 i I t 2 3øe recovery 0,: I', o c= -20, -40 C oo -12o MPA Data Coverage (Alaska Meridian) (Greenland Meridian) (Novaya Zemlya meridian) Day of November 93 Figure 1 The Kp index (top panel) and Dst index (middle panel) are plotted for the interval containing the NSWP Storm In the bottom panel the data coverage of the MPA on three geosynchronou satellites is indicated which probably acts as a source to make a strong ring current and hence a strong Dst perturbation This paper will elaborate on the view of the magnetosphere during the NSWP storm obtained with multiple satellites at geosynchronous orbit in the equatorial plane (For a similar view of a CME-driven storm, see MF Thomsen et al, The magentospheric response to the CME passage of 1/10-11/97 as seen at geosynchronous orbit, submitted to Geophysical Research Letters, 1997) From geosynchronous orbit (r = 66 Rs) much of the magnetosphere can be seen: plasma and energetic particles being fed into the dipole from the magnetotail are regularly seen, plasma flows from the ionosphere into the magnetosphere are regularly seen, and the passage of plasma through and around the dipole to the dayside is seen With multiple satellites in geosynchronous orbit, deviations from normalcy can be perceived for several plasma populations of the magnetosphere: eg, the outer plasmasphere, the ion and electron plasma sheet, the outer radiation belts, the magnetospheric trough From measurements made at geosynchronous orbit, a large number of indicators of magnetospheric dynamics can be obtained (as will be detailed in later sections): for examples, the amount of field-line stretching in the near-earth magnetotail can be monitored, the rate of magnetospheric convection can be estimated from the position of where the inner edge of the electron plasma sheet crosses geosynchronous orbit, and the rate of convection can be estimated from the local-time position of the plasmaspheric bulge and the presence of plasmaspheric drainage plumes crossing geosynchronous orbit Hot-plasma motions around the dipole are indicators of solar-wind entry of material into the magnetosphere, the injections of energetic ions and energetic elections are indicators of substorm occurrence, and the rapid dipolarization of the nightside magnetic field is an indicator of substorm occurrence The data most utilized in this study will be from the Magnetospheric Plasma Analyzer (MPA) and the Synchronous Orbit Particle Analyzer (SOPA) on three satellites in geosynchronous orbit (r = 66 Rs) The three satellites are at a longitude of -165 ø (on the Alaska meridian of longitude), at a longitude of-37 ø (Greenland meridian), and at a longitude of +70 ø (Novaya Zemlya meridian) The MPA counts electrons and ions with an electrostatic analyzer segmented into six look directions [Bame et al, 1993; McComas et al, 1993], producing a sequence of two- and three-dimensional distribution functions every 86 s, with each individual distribution function being produced during a single 10-s spin of the satellite Moments of the velocity-space distribution functions are routinely calculated every 172 s The energy range of the detector is from 1 ev to 40 kev for both electrons and ions Because the MPA is on a negatively biased portion of the satellite, it is able to detect ambient ions with kinetic energies down to 0 ev The SOPA is an energetic-particle detector that produces spin-averaged differential fluxes of electrons and ions above 50 kev every 10 s Densities and temperatures for the energetic electrons and ions are obtained from Maxwellian fits to the measured SOPA fluxes [Cayton et al, 1989] Two relativistic Maxwellians are simultaneously fit to the SOPA-measured electron fluxes and two Maxwellians are simultaneously fit to the SOPA-measured ion fluxes Information about the direction of the magnetic field at the satellites is obtained by determining the orientation of the symmetry axis of the three-dimensional distribution functions In the present study, only the field direction obtained from MPA particle distributions is used; in general this direction agrees quite well with the direction obtained from the three-dimensional SOPA particle distributions The data coverage for the three geosynchronousatellites during the early part of November 1993 is plotted

3 _ BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,375 in the bottom panel of Figure 1 As can be seen, the data coverage is best for Data sets other than the MPA and SOPA data sets will be utilized in this study and cited as they are used These include IMP- 8 plasma data in the solar wind, IMP-8 magnetic-field o 100 data in the solar wind, GOES-6 and GOES-7 magneticfield data at geosynchronous orbit, and an AE-type index produced by assimilated mapping of ionospheric electrodynamics (AMIE) c This report is organized as follows In sections 2, 3, and 4 the dynamics of the outer plasmasphere, the near- Earth ion and electron plasma sheets, and the outer 24 edge of the electron radiation belt, respectively, are discussed through the storm In section 5 the compression and erosion of the dayside magnetosphere is discussed The magnetospheric convection rate and the stretching of the nightside magnetic-field lines are discussed in _ sections 6 and 7 Section 8 examines ionospheric ion outflows into the equatorial magnetosphere In section 9 several mass-flow scenarios in the magnetosphere are viewed The results of this study are summarized in section 10, where a synopsis of the storm and a list of new aspects of magnetospheric dynamics are given 2 Plasmaspheric Dynamics through the cx Storm Plasmaspheric density Ii I Position of plasmasphere? - Level ß ß of ß The typical appearance of the plasmasphere as seen from geosynchronous orbit is an encounter with the o bulge of the plasmasphere once per day per satellite in Day of November 1993 the dusk sector of local time [Chappel! et al, 1970; Hige! and Lei, 1984; Moldwin et al, 1994] These encounters Figure 2 The density of plasmaspheric ions (energies < usually last for about 1-2 hours The appearance of the 100 ev) as measured by MPA on the satellite is bulge in the evening sector is taken to be a sign of mod- plotted in the top panel, the local-time location of plasmaest geomagnetic activity When geomagnetic activity is spheric material (cold-ion density > 10 cm -3) as seen by is plotted in the middle panel, and the Kp index very low for a few days, cool, dense plasmaspheric ma- is plotted in the lower panel terial is encountered over nearly all of geosynchronous orbit [eg, $ojka and Wrenn, 1985] This buildup of material in the outer plasmasphere is believed to be caused by cold outflows from the ionosphere into mag- cating normal convection in the magnetosphere during netic flux tubes that are on drift trajectories that do those days Beginning on October 30 (day-1), there is not encounter the magnetopause (see section 8) Hence a lull in activity and the outer plasmasphere begins to the density buildup is owed to the longevity of the flux refill; it refills through November 3 In the final stages tubes The appearance of plasmaspheric material at all of this refilling the outer plasmasphere is observed at all local times is taken to be a sign of very weak convection local times, as seen in the middle panel of Figure 2 The in the magnetosphere plasmaspheric material can be seen as the dense band In the top panel of Figure 2, the density of plasmas- of low-energy ions in the color spectrograms of Plate 1 pheric material as seen by the geosynchronous satellite At the end of November 3, with the rise in Kp, the outer is plotted as a function of time for 15 days plasmasphere is stripped off and convected toward the centered around the NSWP Storm The storm occurs dayside magnetopause (see below and see section 6) on November 3-4, 1993 A density greater than about As can be seen in Figure 2, in the days following this, 10 cm -3 is taken to mean that the satellite is in the plasmaspheric material is only seen once per day in the outer plasmasphere In the middle panel of Figure 2, dusk sector Hence, the plasmasphere returns to its the local-time positions where plasmaspheric material usual configuration with a duskside bulge is observed are plotted In the bottom panel, the Kp For about 6 hours after geomagnetic activity picks index is plotted As can be seen in the top and middle up at about 2200 UT on November 3, the plasmaspanels, during the final days of October 1993 the plas- pheric material seen by on the dayside of the maspheric bulge is seen daily in the dusk sector, indi- magnetosphere is material that is being stripped off of

4 ß,, = = 26,376 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW November 3, max 100 ½' 3 ø - 2, LU_j 0 UT LT i I : : '2-4 " min UT LT November 4, '4 3 e"lu o 2 o o,_,3, LU 2 LU 0 UT o, r max LT I : '2 o -- 1 o - 0 ß ' ' ' ' ' ' 025 min 000 UT LT I Plate 1 Color spectrograms from MPA measurements on the satellite (a) November 3, 1993 and (b) November 4, 1993 Each panel is 12 hours long, with an ion energy spectrogram above an electron energy spectrogram

5 ß I!! BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,377 Nov 3-4, \ 0 km/s Figure 3 The plasmaspheric flow vectors in the equatorial plane along the orbit are shown from 1600 UT on November 3 to 0347 UT on November 4 as measured by MPA on The flow velocities 12-min averages the outer plasmasphere and removed from the magnetosphere at the dayside magnetopause One indication that this material is draining is the fact that in the postnoon sector its convection velocity is westward and radially outward, toward the subsolar point on the nose of the magnetosphere This is shown in Figure 3, where the measured flow vectors of the plasmaspheric material are indicated along the orbit of during this time interval (see also section 6) An even stronger indication that this material is draining to the dayside magnetopause is the fact that when the magnetopause is encountered by at UT on November 3, at 2347 UT on November 3, and multiply at UT on November 4 (owing to the strong compression and erosion of the dayside magnetosphere, as discussed in section 5), plasmaspheric plasma is seen up adjacent to magnetosheath plasma each time This is observable in the ion spectrograms of Plate 1 and is discussed further in section 5 In Borovsky et al [1997] this event was discussed and it was shown that the plasmaspheric material adjacent to the magnetosheath is accelerated in a direction that is consistent with reconnection of the plasmaspheric flux tubes at a dayside neutral line This reconnection of plasmaspheric flux tubes is not unexpected, since cold plasma convects with the field lines and at least some of the field lines on the dayside must be convecting into the dayside neutral line 3 Plasma-Sheet Dynamics through the Storm As plasma-sheet material convects earthward in the near-earth magnetotail, the gradient- and curvature- oo 22 drift effects differ for plasma-sheet ions and plasmasheet electrons The warm ions drift duskward as they enter the dipole, and the warm electrons drift dawnward as they enter the dipole As a result, the plasma sheet of the tail separates into an ion plasma sheet and an electron plasma sheet near the earth, and the two plasma sheets have very different evolutions 31 The Ion Plasma Sheet As material moves sunward from the magnetotail, the ion plasma sheet penetrates deeper into the dipole than geosynchronous orbit Under usual circumstances, a geosynchronous satellite is in the ion plasma sheet at all local times, with the density and temperature of the ion plasma sheet being lower and cooler on the dayside than it is on the nightside [eg, DeForest and Mcllwain, 1 1] In the top panel of Figure 4 the ion pressure /%, as determined from the combined MPA and SOPA mea- surements on is plotted as a function of time for 10 days commencing with November 1, and in the bottom panel of Figure 4 the hot-ion density ni as mea- sured by the MPA detector on is plotted (The correction for the ion density owed to the SOPA measurements is slight) Because strong fluxes of ambi ø ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' '"1'"1'"1'"1'"1'"1'"1"'1,,,I,,, Ion Pressure _ '' '- ' " ß, i ' : Hot-Ion Density c "' :!:,:) :':'-,:t ":i" ' ß ' ', -- :' ' ": '& ß ': :, 17' -,' ' 4 I: ''; ß :: ß :" : ', ß,: ß,:, -: :,, :I,,', 0,,,' 1,,, I,,, I,, I,,, I,,, I,, I 1,,, I,,,/ S Day of November 1993 Figure 4 From the sate]lite , (top) the ion pressure from MPA and SOPA is plotted and (bottom) the hotion density from MPA is plotted The ion pressure is only plotted when penetrating-radiation backgrounds are suffi- ciently low as compared with the ion count rates

6 ß ß 26,378 BOROVSKY ET AI' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW Ion Pressure 4 (MPA range only) ß rt 3 22 LT, :' C Q:- 2 /i,", ' \ ß I ß ß o " ""1"" '1 " "1" ""1"'""" Ion Pressure (MPA range only) : a 3 18 LT ½';' ' C t, ': r ß r I,x' " Ion Pressure 4 (MPA range only) Q- 3 (- 14 LT,: n'- 2 :' UT of November 3, 1993 [hr] Figure 5 The ion pressure of a superdense plasma sheet moving around the dipole is plotted as it encounters the three geosynchronousatellites The ion pressure is measured by the MPA only The local time of each satellite when the superdense plasma sheet encounters it is indicated in each panel is about the gradient-and-curvature-drift speed of an 8-keV proton, which is about the temperature of the ion-plasma-sheet at the moving high-density front What was not noted in Borovsky et al [1998a] is that the moving front was not only highly dense, but it was highly diamagnetic (high-fi) As can be seen in the bottom panel of Figure 4, in the 2 days prior to the storm the ion-plasma-sheet pressures and densities were low At the beginning of November 4, a few hours into the storm, the ion-plasma-sheet densities and pressures rise considerably The satellite was at 165 ø west longitude (Alaska), so is at local midnight at 1100 UT each day Typical values for the density and pressure of the ion plasma sheet at local midnight at geosynchronous orbit are n = 097 :E 047 cm -3 and P - 16 :E 15 npa [Borovsky et al, 1998b] The values of n and P attained on November 4 are considerably higher than those typical values The propagation of a high-pressure front around the dipole in the ion plasma sheet can be seen in the three panels of Figure 5, where the ion pressure in the energy range 40 kev as measured by MPA is plotted for the three geosynchronous satellites Note that this is only the partial pressure of ions, the total ion pressure is typically 16 times this partial pressure Plasma pressure corresponds to diamagnetism, and one should expect the magnetic field to Minimum-I estimate ent relativistic electrons produce a strong background of false counts in MPA following November 5 (see section 4), the measurements of the hot-ion parameters are more difficult, so the pressure is only plotted when the ion fluxes are much higher than background-count levels As can be seen in the bottom panel of Figure 4, the density of the ion plasma sheet is anomalously high on November 4 In Borovsky et al [1998a], the appearance of this superdense plasma sheet and its motion around the dipole during the November-1993 storm was discussed Four hours after a high-density front passed the Earth in the solar wind, the ion-plasma-sheet density increased at midnight at geosynchronous orbit as seen by the satellite Seven hours after the arrival of the solar-wind density front, the ion-plasmasheet density increased at dusk at geosynchronous orbit as seen by the satellite Thirteen hours after the arrival of the solar-wind density front, the ion-plasma-sheet density increased on the dayside at geosynchronous orbit as seen by the satellite The three-satellite observations of the density increase of the ion plasma sheet indicate that the front moved at a velocity of about 24 km/s around the dipole; this Minimum-I estimate!, ' minimum-i LT 10 I I, I t I t, I, + I, t I ; t I, I i :" '"% ' 14 LT '{ :, ß,,, ß, y - :, ' 6 12 I 18 I,,,, t 24 i UT of November 3, 1993 [hr] he h ee[eos ch o ous s e]]i[es is pio[[ed fu cqo s of ime fo 24 hours e [he o se[ of [he s o m

7 _ BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,379 be reduced inside the high-pressure plasma A measure of the degree of aliamagnetism is the ion of the plasma, phology to occur in the dipole when a high-/ plasma sheet moves in During these high-pressure times on i = 8 rnkbt,/b 2, which is related to the diamagnetic the nightside on November 4, the magnetic-field orisusceptibility X of the plasma, which is X = M/B = -nkbt,/b 2, where M is the magnetization (density of dipole moments) in the plasma Estimated lower limits to the plasma/ are plotted in the three panels of Figure 6 In the upper two panels, i was estimated by using the values of nkst measured by MPA for <40-keV ions and multiplying by 16 to approximately account for the contribution of the >40-keV ions In the bottom panel the values of nkst measured by MPA and SOPA were used In all three plots, the "unperturbed" dipole-field value B = 106 nt was used; when is high, the true field will be less than this value, so at those times the entation was extremely stretched away from a dipole geometry into a tail geometry (see section 7), and the H component (z component) of the magnetic field on the nightside, as measured by the GOES-6 and GOES- 7 satellites in geosynchronous orbit, went down to a few nanotesla on November 4 whereas a few tens of nanoteslas is more typical (data courtesy of H Singer, 1997) It is suspected that a superdense plasma sheet is a necessary prerequisite for obtaining a strong ring current and a large Dst perturbation This has been argued in Borovsky et al [1997], and numerical modeling of the generation and loss of the ring current for true value of/ will be considerably greater that the the NSWP Storm indicates that a high-density plasma value estimated in the plots As can be seen in Figure 6, the plasma behind the propagating density fronts can be unity or even substantially greater Thus, as a superdense plasma sheet moves around the dipole, one could expect a transition from low-/ to high-/ to occur In a high- plasma, confinement and control sheet at geosynchronous orbit is needed to obtain a ring current strong enough to account for the Dst perturbation of the storm [Kozyra et al, 1997] Other modeling of stormtime ring current found that an increase in the phase-space density of the ring-current source was necessary to account for the strong Dst perturbations of the plasma by the magnetic field will be lessened during storm times [Uhen eta!, 1994] The scenario One might expect a change in the magnetic-field mor- of a high-density plasma sheet leading to a strong ring current is supported by Figure 7, where the density of the ion plasma sheet, the Kp index, and the Dst index are plotted for 50 days commencing with November 1, 1993 (day 305) As can be seen from the figure, a strong (negative) value of Dst is associated with a I ; Ion-Plasma-Sheet: Density - _ high-density plasma sheet and strong geomagnetic ac- : ;, Dec tivity (Kp) The density of the ion plasma sheet at 3 geosynchronous orbit is largely a function of the density of the solar wind density [Borovsky et al, 1997, 1998a], with the sign of B in the solar wind playing a gating role High-density plasma sheet occurs when the solar wind has a high density during an interval of 7- - southward IMF According to the IMP-8 Solar Plasma Q 5 Faraday Cup data (courtesy of A Lazarus, 1997) and,' 4 the IMP-8 Magnetic Field data (courtesy of C Russell, o I I' I 1997) the solar wind was dense with B southwar during the superdense-plasma-sheet interval of November 3-4 Dst Index 0 32 The Electron Plasma Sheet Nov 4 Dec Day of 1993 Figure 7' The relationship betwee superdense plasma sheets and strong Dst perturbations is shown For 50 days, the ion-plasma-sheet density as measured by MPA on and , the Kp index, and Dst are plotted The relationship between superdense plasma sheets and strong Dst perturbations is shown The electron plasma sheet is strongly deflected dawnward as its material enters the outer dipole from the magnetotail Consequently, the electron plasma sheet is seen only in the premidnight-to-dawn sector of geosynchronous orbit [McComas et al, 1993; Thomsen et al, 1998] As a geosynchronous satellite travels in its orbit from the dayside, around the duskside, into the nightside, it usually encounters the electron plasma sheet in the dusk-to-midnight sector and finds that the electron plasma sheet has a sharp edge in that sector One such distinct edge can be seen in the electron spectrogram in Plate lb at about 0400 UT on November 4 In Figure 8 the hot-electron density on three different days as

8 , 26,380 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW o dusk midnight dawn - October 29, 1993 (normal activity) i I i i i I,- I 1, I November 2, 1993 (very quiet) I I I I 1 November 4, 1993 Note in Figure 8 that whereas the electron plasma sheet tends to exhibit a sharp edge when the geosynchronous satellite enters it, it has a very gradual falloff in intensity when the satellite exits it This gradual falloff is caused by electron precipitation into the atmosphere as the electron plasma sheet moves around the dawnside of the dipole [Thomsen et al, 1998] This precipitation gives rise to the diffuse aurora and converts the electron plasma sheet into the magnetospheric trough In Figure 9, the hot-electron pressure, temperature, and density as measured by MPA on the satellite are plotted as functions of time for l0 days commencing on November 1 Toward the end of this time interval, the strong diurnal variation in the hot-electron properties is seen as the satellite enters the electron plasma sheet in the dusk-to-midnight region and gradually exits the electron plasma sheet into the magnetospheric trough in the dawn-to-noon sector On November 2 and 3, prior to the storm, the electron pressure Hot-Electron Pressure UT [hrfrac] Figure 8 The local-time profile of the electron plasma sheet is at geosynchronous orbit is displayed for three days: (top) a day long before the NSWP Storm, (middle) a day during the quiet interval just prior to the NSWP Storm, and (bottom) a day during the main phase of the NSWP Storm In all three panels, the hot-electron density is measured as a function of time by MPA on the satellite '--' ,_ 7 ) measured by MPA on is plotted The first day (top panel) is well before the NSWP Storm, the second 0 day (middle panel) is in the calm interval just prior to the storm, and the third day (bottom panel) is during the main phase of the storm The electron plasma sheet 25 is of more-or-less typical density, local-time width, and : location on October 29 The satellite encounters the edge at about 1930 LT (0630 UT), which is usual On November 2, the electron plasma sheet is very weak and the satellite does not encounter its edge until al- :;, :œ '[ ß ß most 2300 LT (On November 2, the hot-electron tem- 05 perature was also extremely cool, only a few-hundred ',:i i ','k :, ; ;, ;,: : ev) On November 4, during the storm, the electron plasma has a very high density and is very wide in lo- Day of November 1993 cal time The satellite encounters its edge at 1700 LT Figure 9 From the satellite , the electron pres- As will be discussed in section 6, the position of the sure from MPA (top panel), the electron temperature from electron-plasma-sheet edge is a measure of the strength MPA (middle panel), panel and the hot-electron density of convection in the magnetosphere from MPA (bottom panel) are plotted 3 2

9 BOROVSKY ET A1' NOVEMBER 1993 STORMmDYNAMICS AND MASS FLOW 26,381 and hot-electron density both remain quite low; this in- superdense electron plasma sheet by a satellite must dicates that convection in the magnetosphere was very wait for that satellite to reach the nightside low and that the electron plasma sheet did not extend The large fluxes of hot electrons in the electron plasma all the way in to geosynchronous orbit from the mag- sheet leads to spacecraft charging and to differential netotail Indeed, geomagnetic activity was very low on charging Often in the electron plasma sheet at geosynthese 2 days prior to the storm, as can be discerned from chronous orbit, the flux of plasma-sheet electrons to a the plot of the Kp index in Figure 1 On November 4, satellite can be larger than the flux of photoelectrons off during the main phase of the storm, the electron plasma the satellite This drives the potential of the spacecraft sheet is very robust as can be seen from Figure 9: it has negative with respect to the potential of the ambient an exceptionally high density, a high temperature, and plasma As the absorption of electrons drives the poa very high pressure, and it is exceptionally wide in local tential of a satellite to negative values in the electron time A synopsis of the electron-plasma-sheet dynamics plasma sheet, ambient cold ions fall across the potential through the NSWP Storm is that the electron plasma drop of the spacecraft's sheath and are collected by the sheet was very weak at geosynchronous orbit prior to MPA, showing up in the energy spectrograms as an "ion the storm because of very weak magnetospheric con- line" with a kinetic energy equal to the spacecraft povection, that the electron plasma sheet was very robust tential with respect to the ambient plasma This quan- on November 4, and that the electron plasma sheet returned to normal on November 5 The electron plasma sheet was quite dense on November I and on November 4, as can be seen in the bottom panel of Figure 9 The density of the electron plasma sheet at geosynchronous orbit is highly correlated with the density of the ion plasma sheet at geosynchronous orbit, and the density of the ion plasma sheet at geosynchronous orbit (and in the magnetotail) is highly correlated with the density of the solar wind [eg, Borovsky et al, 1997, 1998a] Hence, it is likely that the high density of the electron plasma sheet on November 1 and 3 is caused by a high-density solar wind; indeed an examination of the IMP-8 Solar Plasma Faraday Cup data [courtesy of A Lazarus, 1997] shows that the solarwind density was high on both November I and 4 The electron-plasma-sheetemperature was high on November 4 and again on November 6 The cause of these high temperatures is not known The temperature of the ion plasma sheet at geosynchronous orbit (and in the magnetotail) is highly correlated with the velocity of the solar wind [eg, Borovsky et al, 1998a], but the temperature of the electron plasma sheet at geosynchronous orbit is poorly correlated with the temperature of the ion plasma sheet at geosynchronous orbit (which is very different from the case of the plasma sheet in the magnetotail where the electron and ion temperatures are generally well correlated [Baumjohann et al, 1989]) As was discussed in section 31, the ion plasma sheet is superdense on November 3-4 and multisatellite observations show the onset of the superdense ion plasma sheet propagating around the dipole from the nightside, toe the duskside, and to the dayside (eg, Figure 5) The electron plasma sheet is also superdense on November 3-4, but the superdenselectron plasma sheet appears only on the nightside of the dipole Hence, the arrival of a superdense ion plasma sheet at a satellite is not always accompanied 3OOO ' I ' I ' I ' I ' I ' I ' I ' I ' i ' I ' I ' I ' I ' I ' Measured Spacecraft Potential _ o - I I I I ' I ' I I ' I ' I - -Measured Minimum Differential Charging _ 200 L - - _ - _ o 1- Hot-Electron Pressure :i :,L,,1,,I, o8- -, ß t 04, I, i ' Day of November 1993 by arrival of superdenselec- Figure 10 For 15 days, the following quantities are plot- tron plasma sheet On the nightside of the dipole, the ted: the spacecraft potential q s/c as determined by the ion arrivals are approximately simultaneous, but away from line in MPA (topanel), the minimum differential spacecraft potential Aq as determined by the secondary and photoelecthe nightside, superdense ion plasma sheets can be seen tron spectra in MPA (middle panel), and the hot-electron on a satellite long before the superdense electron plasma pressure as determined by MPA (bottom panel) All quan- sheet is seen by that same satellite; the sighting of the titles are from

10 26,382 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW tity is denoted as ½s/c Examples of ion lines can be seen volts to a kilovolt or so in the electron plasma sheet on in the ion spectrograms in Plate lb at UT the nightside During the NSWP Storm on November and at UT on November 4 The spacecraft 4, the spacecraft potential (and differential potential) potential ½s/c of the satellite as determined by was a factor of 2 or so above normal, owing to the ro- MPA when an ion line is found is plotted for 15 days bust electron plasma sheet during the storm Spacecraft in the top panel of Figure 10 Note that the spacecraft potentials of kv or more in sunlight at geosynchronous potential can be nonzero at other times, but it is not dis- orbit are reported to be rare [eg, DeForest, 1972; Reacerned unless an ion line is present (Ambient cold ions soner et al, 1976; Whipple, 1981] Such high levels of in the electron plasma sheet are discussed in section 8) spacecraft charging can lead to interruptions in space- Note also that the large spacecraft potentials plotted craft operations [eg, Fredricks and Scarf, 1973] in Figure 10 are not associated with spacecraft-eclipse intervals A Ininimum value for differential charging 4 Penetrating Radiation through the onboard the satellite is also measured by MPA On the Storm electron spectrograms in Plate 1, a population of lowenergy electrons can be seen at almost all times; for It is known that high-speed-stream-driven geomagexample, this population intensifies at about 0930 UT netic activity to result in enhanced fluxes of relativistic and at about 1700 UT on November 4 This population electrons at geosynchronous orbit [Paulikas and Blake, consists of spacecraft-generated photoelectrons and sec- 1979; Belian et al, 1996] Relativistic electrons can ondary electrons that leave the spacecraft surface and penetrate into spacecraft and enhanced fluxes of these are collected by the MPA When there is differential penetrating electrons can lead to interruptions of space- charging of the satellite, then photoelectrons and secondary electrons that are emitted from regions of the spacecrafthat are at more-negative potentials than is the region where the MPA resides will show up at MPA with extra energy [eg, Whipple, 1976] In the electron spectrograms (eg, Plate 1), the upper edge of this lowenergy-electron population is a measure of how negative some areas of the satellite are with respect to the MPA area Hence, the energy of that edge is a lower limit to the amount of differential charging on the spacecraft (It is a lower limit to the differential charging because there are regions of the spacecraft more positive than the MPA region which cannot be sensed via measurements of cool electrons) The estimate of the differential craft operations [Wrenn and Sims, 1996] Plotted in the top panel of Figure 11 is the count rate of channeltron avalanches caused by penetrating radiation into MPA on the satellite as a function of time for the first 11 days of November 1993 In the bottom panel the ambient flux of electrons with energies of MeV charging obtained from MPA by discerning the upper edge of the photoelectron and secondary-electron pop- 400 ulation is denoted A½ In the second panel of Figure 10, the differential charging A½ is plotted for 15 days as 200 measured by MPA on the satellite Typically, the daily maximum value of A½ reaches 100 V or so, but 0 during the main phase of the storm on November 4 A½ reaches about 250 V As stated above, these are minimum values for the differential charging, other experiments at geosynchronous orbit have directly measured differential potentials much larger than these hundredsof-volts levels [eg, Inouye, 1976] In the bottom panel of Figure 10, the hot-electron pressure Pe = nekbt is plotted As can be seen, the times when there is high charging and high differential charging correspond to times when the hot-electron pressure is high Indeed, when an ion line is found, the measured spacecraft potential / has a linear correlation coefficient of +82% o with P As can be seen in the top two panels of Fig- Day of November 1993 ure 10, the differential-charging potential A tracks the Figure 11 The false count rate on MPA owed to penetratspacecraft potential / The correlation coefficient being radiation (top panel) and the differential flux of ambient tween the two values is about +70%, and typically A electrons in the MeV band as measured by SOPA is about 1/6 of / As can be seen in Figure 10, the (bottom panel) are plotted for 10 days Both quantities are spacecraft potential regularly reaches several-hundred from the geosynchronousatellite ooo 8OO 600 Penetrating background in MPA instrument

11 _ BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,383 as measured by SO PA on is plotted As can be seen, the two curves track each other extremely well, indicating that the false counts on MPA are caused predominantly by environmental electrons with energies of about MeV The penetrating-radiation background on MPA becomes severe during the recovery phase of the NSWP Storm, particularly on November 6-10 The high background hinders the measurement of ambient ions on those days because the penetratingradiation count rates become comparable to the count rates of ambient plasma ions Note that although the Dst perturbation of the storm peaks on November 4 (see Figure 1), the relativisticelectron fluxes do not reach their peak until November 8 This is explored further in Figure 12, where in the top panel the fluxes of relativistic electrons in four energy bands as measured by SO PA on are Energetic-Electron Density (running 6-hr average) 7 Kp Index _ _ 2000 ' i, i, i, i, i, i, ' i ' i ' ß, m 1000! E x '500 o lo -3! E t- 10_ " 1000 IO0 10 Energetic-Electron Differential _ ø Fluxes - _ MeV ' '8' 3'5 MeV xlo, ' ' i ' I i,' - i ' i Energetic-Electron Density' ::,,', : l' I' I' I' I' I' Energetic-Electron Temperature, I, I, I i I, I, I, I, I, I Day of November Dst Index I ' ' -5o i - I -75_ u) 31 Jan Nov 19 / I, Dec 8 Dec % % - - Nov 3 Dec 3 - _ -150,,, I,,, I,,, I,,, I, Day of 1993 Figure 13 For 90 days starting with November 1, 1993, the density of energetic electrons at geosynchronous orbit as measured by SOPA on is plotted (top panel), the Kp index is plotted (middle panel), and the Dst index is plotted (bottom panel) plotted As can be seen, the fluxes in all four bands peak on about November 8, which is day 4 or day 5 of the storm This few-day lag between the onset of geomagnetic activity and the peaking of energetic electron fluxes is well known [eg, Nagai, 1988; Baker et al, 1990] The SOPA electron fluxes are routinely fit with a bi-maxwellian distribution [Cayton et al, 1989], and the fits yield a density and a temperature for each of the two Maxwellians The fluxes of 1-MeV electrons are dominated by the properties of the hotter of the two Maxwellians In the middle and bottom panels of Figure 12 the density n and temperature T of the hotter energetic-electron Maxwellian are plotted As can Figure 12 For 10 days, the differential fluxes of energetic be seen, the density of energetic electrons at geosynelectrons in four energy bands (top panel), the density of energetic electrons (middle panel), and the temperature of chronous orbit rises very early in the storm (middle energetic electrons (bottom panel) are plotted All quanti- panel), reaching a peak on day 1, whereas the fluxes ties are measured by SOPA on at geosynchronous peak much later (top panel) This implies that the orbit population of relativistic electrons was injected by the

12 26,384 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW storm into the magnetosphere very early in the storm Then the population was heated or energized so that the temperature increased by about a factor of 2 over several days, resulting in the flux peak on November 8 Hence, to discern the cause of the flux increase of several orders of magnitude from the beginning of the storm to November 8, the key issue may not be finding an acceleration mechanism that can cook particles for several days and change their energies by orders of magnitude, rather the key issue may be finding an injection mechanism that can act early in the storm to inject or capture a large population of energetic electrons The energization is only by a factor of 2 with several days to operate The fast response of the density of energetic electrons at geosynchronous orbit to changes in geomagnetic activity is explored further in Figure 13 In the top panel the energetic-electron density n (of the hotter Maxwellian, with a temperature typically of kev) as measured by SOPA on is plotted for 90 days commencing with November 1, 1993; in the second panel the Kp index is plotted; and in the bottom panel Dst is plotted, with strong minima labeled As can be seen by comparing the first and second panels of Figure 13, the logarithm of the energetic-electron density tracks the Kp index quite well: sharp rises in Kp usually have sharp rises in n and the profile of log n is similar to the profile of Kp (note especially the intervals day 302 to day 312 and day 375 to day 390) A close inspection of the n and Kp data finds that the time lags between n and Kp for sharp rises, for sharp falls, and for maxima are 12 hours or less This is considerably less than the - 2-day time lag reported between the energetic-electron flux and Kp [Nagai, 1988; Baker et al, 1990] Note that the size of the time lag from Kp to n may depend on the local-time position of the geosynchronous satellite when n is measured; ie, some of the up-to-12-hour time lag may be caused by diurnal variations in n owed to orbital motion 5 Compression/Erosion of the Dayside Magnetosphere Unfortunately, during the NSWP-Storm interval solarwind data was scarce According to IMP-8 observations (courtesy of A Lazarus, 1997), which were intermittent, the solar-wind number density was high, but not extreme, on second half of November 3, and the solarwind velocity was modest The interplanetary magnetic field (courtesy of C Russell, 1997), when observed by IMP-8, showed little southward Bz until about 2200 UT on November 3 From about 2200 UT on November 3 until 0100 UT on November 4, Bz of the solar wind was -15 to-30 nt (southward) and the solar-wind density reached 75 to 100 cm-3 During the second half of November 3, prior to the onset of strong geomagnetic activity, Dst was positive for about a 15-hour period This can be seen in the bottom panel of Figure 14 This positive Dst implies that the magnetosphere was compressed by the solar wind [eg, McPherron et al, 1986] A compression of the magnetosphere is consistent with magnetic-field observations by the GOES-7 satellite in geosynchronous orbit (courtesy of H Singer, 1997): during this time interval the H component (northward component) of the magnetic field reached 150 nt from its more usual value of about 90 nt In the top panel of Figure 14 the hot-ion anisotropy of the ion plasma sheet is plotted as a function of time for the satellite ; the hot- ion anisotropy Tñ/T]i is measured with MPA only, so it excludes ions with energies above 40 kev Typically, Tñ/Tii for the ion plasma sheet at geosynchronous orbit [eg, Thomsen et al, 1996a, Figure 2], but during the latter parts of the positive-dst phase on November 3 the anisotropy becomes extremely strong, with Tñ/Tii exceeding 3 This extreme anisotropy associated with a greatly reduced flux of plasma-sheet ions with pitch angles along the magnetic field and an unchanging flux of plasma-sheet ions with pitch angles normal to the field During this extreme-anisotropy phase from 1200 to 2000 UT on November 3, the satellite travels from 0100 to 0900 LT; ie, it is in the postmidnight to prenoon region of local time During the same time interval, the satellite was in the to 1900-LT region (prenoon to dusk) and did not see an anomalously high anisotropy in the ion plasma sheet Because the anisotropy did not appear on both and in different regions of local time, the reduction of the field-aligned plasma-sheet ions seen on the dawnside was probably not a global loss of field-aligned ions from the magnetosphere (ie, a pitch-angle dependent charge-exchange loss); rather it may be a shell splitting effect (ie, an orbital selection effect) wherein those field-aligned ions are excluded from the dawnside at geosynchronous orbit At the onset of the storm at the end of November 3, compression and erosion of the dayside magnetosphere [cf Rufenach et al, 1989; McComas et al, 1994] brings one of the geosynchronous satellites out of the magnetosphere and into the shocked solar wind The excursions of out of the magnetosphere can be seen in the ion and electron spectrograms of Plate i at UT on November 3, at 2347 UT on November 3, and multiply at UT on November 4 In the excursion beginning at 2307 UT on Novem- ber 3, one sees (in Plate 1) the magnetospheric populations (the hot plasma-sheet ions and the cool plasmaspheric ions) vanish and the magnetosheath populations of electrons and ions appear This is an excursion out into the magnetosheath During this excursion, the peak magnetosheath density seen by MPA is 190 cm -3, which occurs at 2321 UT on November 3, when is at 1219 LT In the multiple excursions beginning at 0006 UT on November 4, one sees in the spectrograms of Plate I that the magnetosheath populations appear but that the ion plasma sheet does not disappear Hence, these multiple excursions are into

13 ß BOROVSKY ET A1- NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26, ' I ' I ' I ' I ' I ' I ' I ',' Ion-Plasma-Sheet Anisotropy ;> ß,i ':,: '[,;: i i '" i!,! : ß ß, ß o 0 ' I ' I ' I ' I ' 2O Dst Index -2O -40-6O -8O -1 oo -12o 4 s Day of November 1993 Figure 14 The anisotropy of the ion plasma sheet as determined by MPA on is plotted for 10 days (top panel) and the Dst index is plotted for the same 10 days (bottom panel) a boundary layer region and not completely into the magnetosheath For all of these excursions, when exits from the magnetosphere and enters back into the magnetosphere, plasmaspheric material is seen adjacent to magnetosheath material This adjacency has been noted before [eg, Elphic et al, 1996] For the NSWP Storm, the flow velocities of the plasmaspheric material at the outer edge of the magnetosphere have been analyzed and the flows were found to be consistent with plasmaspheric material being accelerated along the magnetopause as plasmaspheric flux tubes are opened at the dayside neutral line [Borovsky et al, 1997] 6 Magnetospheric Convection Rate The strength of convection in the magnetosphere affects the positions of boundaries in the magnetosphere From a satellite in geosynchronous orbit, two prominent observations are indicators of the convection rate: the local-time position where the satellite enters the electron plasma sheet and the local time at which plasmaspheric material is observed by the satellite These two observations are discussed in the following two paragraphs As was discussed in section 32, as electron-plasmasheet material convect sunward from the magnetotail into the nightside dipole, the gradient and curvature drifts turn the hot electrons dawnward When convection is weak the gradient and curvature drifts become dominant over the electric-fiel drift in the outer dipole and as a consequence the electron plasma sheet barely reaches geosynchronous orbit before turning dawnward (eg, middle panel of Figure 8) During ordinary levels of convection, the electron plasma sheet reaches geosynchronous orbit on the nightside, with a sharp edge to the electron plasma sheet appearing in the premidnight region of geosynchronous orbit (eg, top panel of Figure 8) When convection is very strong, the electron plasma sheet sweeps deeply into the dipole before turning dawnward and consequently the sharp edge of the electron plasma sheet at geosynchronous orbit is located near dusk, with the whole nightside region of geosynchronous orbit being within the electron plasma sheet (eg, bottom panel of Figure 8) In the top panel of Figure 15

14 , 26,386 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW Local-Time Position of E lecton :Plasm as heet ' ß 7o : ' ' ß O - ' ' ß: '''' ' ' ß ' e, ß ' ' " ' " - - ' ', ee e: ;'; "2 ' ß - ß : ß _ :: _ O ' Auroral Boundary Index - : [ : = 50 - : := : 45r,,,,,,,,,i,,,,,,,,,,,,,,, 1 Z 3 4 S l0 11 Day of November 1993 ibure lb In the top p nel the local time t which the three tellite lg8g-040, lgg0-0gs, lggl-080 ee the electron pl m heet re plotted function of time for 10 d y nd in the bottom p nel the P hillip L bor tory uror l boundary index i plotted for the me 10 d y For this plot, the electron pl m heet i t ken to be 3/2 125 x 104 cm-3ev 3/2 the local-time positions at which electron-plasma-sheet material is seen by the three geosynchronousatellites , , and are plotted for l0 days The local time of the edge of the electron plasma sheet is the bottom edge of the plotted points in the panel As can be seen, on November 1 the edge appears very close to local midnight, indicating that convection was weak on that day On November 2 and early on November 3, the electron plasma sheet is barely seen at geosynchronous orbit, indicating that convection was very weak in that interval (Examine, also, the electron energy spectrograms for November 3 in Plate 1) On November 4, the electron-plasma-sheet edge was seen at 1700 LT (see also Plate 1), past the dusk terminator, indicating extremely strong convection As can be seen in Figure 15, from November 4 through November l0 the edge of the electron plasma sheet moves from dusk toward midnight, indicating that the convection rate decreases to a more-usual value during that interval In the bottom panel of Figure 15 the Phillips Laboratory auroral boundary index [Gussenhoven et al, 1983] is plotted for the same l0 days The auroral boundary index is a measure of the lowest dipole latitude at which diffuse auroral precipitation is detected by lowaltitude satellites As can be seen by comparing the two panels in Figure 15, there is a strong correspondence between the local-time position of the electron- plasma-sheet edge at geosynchronous orbit and the auroral boundary index, both quantities being measures of how deep the electron plasma sheet penetrates into the nightside dipole As was discussed in section 2 (where the dynamics of the outer plasmasphere was explored), the local times at which plasmaspheric material is seen at geosynchronous orbit is an indicator of the convection rate in the magnetosphere These local times are plotted in the middle panel of Figure 2 The interpretation of Figure 2 leads to the same conclusions about convection as does the

15 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26, O 1 ooo o 1 o UT of November 3 [hrfrac] Figure 16 The flow velocity of plasmaspheric material (left axis) as measured by MPA on and the 34-station AE index (right axis) are plotted as functions of time for 18 hours commencing at 1000 UT on November 3, 1993 The AE(34) index is courtesy of B Emery (1997) interpretation of Figure 15 In the last days of October, the plasmaspheric bulge is seen each day for about I - 2 hours of local time near dusk, indicating the magnetospheric convection was normal on those days From about the last day of October to November 3, the localtime extent of the plasmasphere at geosynchronous orbit steadily increased, until plasmaspheric material was seen at all local times on November 3 This indicates that convection was at very weak levels so that the outer plasmasphere refills out beyond geosynchronous orbit, ie, that the magnetic flux tubes at geosynchronous orbit were on closed drift trajectories within the magnetosphere so that cool ionospheric outflows had time to build up to plasmaspheric densities in the magnetosphere On November 4, the built-up outer plasmasphere disappears, indicating that the outer plasmasphere was drained by a turnon of convection From November 5 through November 11 the plasmaspheric bulge is seen again each day for about I - 2 hours of local time near dusk, indicating more-usual convection levels during those days When plasmaspheric material is seen by MPA, the local convection velocity can be measured A plot of the convection velocity as seen by at the onset of the storm is shown in Figure 16 Here, the magni- in the outer plasmasphere at LT between dawn and noon and during the second burst of activity from UT the satellite was in a plasmaspheric drainage plume at LT across the afternoon sector After 0347 UT on November 4 plasmaspheric material was no longer present at the location of In the first burst of activity at LT, the plasmaspheric flow is predominately eastward and radially outward; in the second burst of activity at LT the plasmaspheric flow is predominantly westward and radially outward These flow directions can be seen in Figure 3, where the flow vectors measured along the orbit of are indicated Hence, during both inte 'vals the flow of plasmaspheric material is directed approximately toward the no e of the magnetosphere during those intervals Since cold plasma follows the convection of field lines, and since the field lines on the dayside should be convecting toward the dayside neutral line, it is likely that the dayside neutral line was located near the nose of the magnetosphere The plasmaspheric convection velocities in the second burst of activity were about 20 km/s; for a 100 nt magnetic field at geosynchronous orbit this flow velocity corresponds to an electric-field strength of about 2 mv/m During the same time interval, the solar-wind tude of the measured flow velocity Vps p of plasmaspheric electric field was about 8 mv/m (400 km/s with Bz material in the x-y plane is plotted for 18 hours start- -20 nt) ing at 1000 UT on November 3 Also plotted (right axis) is the AE(34) index (courtesy of B Emery, 1997), constructed from 34 magnetometer stations of the 89 stations used for AMIE studies As can be seen, the 7 Stretching of the Magnetotail By determining the direction of the symmetry axis of plasmaspheric flow velocity Vps p shows elevated values a three-dimensional ion or electron distribution function when the AE(34) index is elevated From 1000 UT to measured by MPA, the direction of the ambient mag UT travels from about 2300 LT around netic field can be obtained [Thomsen et al, 1996b] In the dawn and dayside to about 1700 LT During the first burst of activity from UT the satellite was Figure 17, the meridional angle 0 of the magnetic-field direction is plotted as a function of time, as determined

16 26,388 BOROVSKY ET AI' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 150 '''1'''1'''1"''1'''1'''1'''1'''1'''1''' MPA i' 7 i';, ' ' 110 '; ß ;:, ;, ß ' "':' """ _i ' ' ß : ß ß : ' ;' ß,,-- ;,:, ' ß ß ;: ß ß ß ß ß : '- : " :',',' : : ;, " : " ' s Day of November 1993,, : /, Figure 17 The meridional angle 0 of the magnetic field at geosynchronous orbit as measured by MPA on is plotted for 10 days by MPA on the satellite A meridional angle 0 90 ø corresponds to a dipole geometry, and 0 > 90 ø corresponds to a stretching of the magnetic field into a tail-like geometry As can be seen in Figure 17, there is a diurnal variation to 0, with the field being more dipolar on the dayside (0 UT corresponds to 13 LT) and the field being more tail like on the nightside Looking at the maximum amount of stretching each day in Figure 17, it is seen that the quiet interval before the storm, November 3, is characterized by a lack of strong stretching on the nightside It is also seen that on the first day of the storm, November 4, the stretching is anomalously large In the recovery phase of the storm, after November 4, the stretching on the nightside relaxes to more-usual values Note that there are substorms occurring throughout the storm and on the days following the storm, and the magnetic-field direction at geosynchronous orbit often "dipolarizes" at the time of substorm onset, but the reduction in the angle 0 associated with a dipolarization is typically small compared with the diurnal change in 0; hence, a substorm dipolarization does not fully relax the nightside magnetic-field configuration to a dipole configuration There is a very striking correlation between the value of 0 on the nightside at geosynchronous orbit and the value of the plasma sheet ion pressure P on the night Magnetic-Field versus Plasma-Sheet Stretching Pressure ,' lo P [npa] i Figure 18 For 89 days from November 1, 1993 to January 31, 1994, the mean nightside value of the magnetic-field tilt angle 0 is plotted as a function of the mean nightside value of the ion-plasma-sheet pressure Pa All measurements are from MPA and SOPA onboard All averages are over 6 hours from 2100 LT to 0300 LT Each data point is one crossing of the nightside by The November 4 crossing is the upper-left data point and the November 3 crossing is the second point from the bottom

17 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,389 side at geosynchronous orbit This connection is demon- times and local times at which these cool ions appear strated in Figure 18, where the average value of 0 on at the equator may be affected by time-of-flight effects the nightside is plotted as a function of the average plus the requirementhat sufficient numbers of cool ions value of P on the nightside: each data point represents build up in magnetosphere before the ions can be seen one crossing of the nightside by the satellite by MPA Often these cool ions are seen only when MPA There are 89 crossings in the figure The linear correlation coefficient between 0 and log(p,) is +76% Almost without fail, on days when the ion pressure is low in the ion plasma sheet, the stretching angle 0 is weak, and on days when the ion pressure is high, the stretching angle is strong Stretching occurs on the nightside where pressure of ion plasma sheet is highest The stretching angle looks along the magnetic field The cool ions in the electron plasma sheet on November 4 were seen over a very wide range of local times; this may be owed to stronger auroral activity over a wide region of local time on November 4 These cool ions in the electron plasma sheet form the ion line that allows MPA to determine spacecraft potential (see section 32) The times during on the nightside at geosynchronous orbit may be related the NSWP-storm interval where these ionospheric outto the degree of plasma diamagnetism, which is proportional to the plasma pressure (see section 31) It is sometimes envisioned that convection drives the current system that produces the magnetic-field-line stretching flows occurred can be discerned from the top panel of Figure 10 where spacecraft charging is measured when the ion line is present in the electron plasma sheet An outflow of cool ions from the ionosphere to the on the nightside at geosynchronous orbit [eg, McPher- equator that is associated with the filling of the outer ron et al, 1973] The maximum stretching angle on the nightside at geosynchronous orbit is also correlated to Kp, with 0 being larger when Kp is larger, which supports a convection-driven-stretching picture, but the 0- plasmasphere is seen at geosynchronous orbit across the dayside These ion outflows usually appear in local time from dawn to the edge of the plasmaspheric bulge near dusk, and the density of this cool-ion population steady Kp correlation (+67%) is weaker than the O-P, correla- increases from dawn across the dayside [eg, Chappel! et tion(+76%), implying that plasma-sheet pressure (diamagnetism) plays an important role in the stretching al, 1971; Thomsen et al, 1998] The magnitude of this density is thought to be an indicator of the "age" of the flux tube, ie, how long the convecting flux tube has had 8 Ionospheric Outflows to the Equator at least one footpoint in a sunlit ionosphere Typically the refilling-ion population is cool ( - ev) and magnetic- At geosynchronous orbit, two types of ionospheric field aligned, but it can be warm (many ev) and peaked outflows of cool ions are typically seen: (1) ionospheric perpendicular to the magnetic field When the ions are outflows into the postmidnight electron plasma sheet warm and perpendicular, it is thought that the popufollowing substorm onsets and (2) ionospheric outflows lation has undergone heating by plasma waves that are into the dayside magnetospheric trough associated with driven by anisotropies in the hot ion-plasma-sheet ions refilling of the outer plasmasphere In the following two [Gary et al, 1997] In the time interval surrounding paragraphs, these outflows are discussed in the context the NSWP Storm, the refilling-ion population is seen by of the NSWP Storm MPA as expected' before the interval where the plas- Examples of ionospheric ion outflows into the elec- masphere is built up (which is prior to the storm) refilltron plasma sheet are seen in the ion spectrograms of ing is seen, and after the storm clears out the built-up Plate I on November 4 at UT (at about 100 outer plasmasphere more refilling is seen On NovemeV), at UT (at about I kev), and at 14:30- ber 4, when one expects to see refilling on the dayside, 18:00 UT (at about 100 ev) Typically these outflows MPA does not detect the refilling-ion population This are seen in the postmidnight regions of local time; as a satellite moves from the dayside around dusk into the absence can be seen in the ion spectrograms on November 4 after about 1700 UT in Plate 1 It might be that nightside, it typically enters the electron plasma sheet convection was so strong on November 4 (where Kp was at about 2100 LT and it typically begins to observe _ 5) that the flux tubes convected across the frontside the ion outflows at about 0100 LT, which is four hours of the dipole too fast for sufficient densities of refilling later Because these cool-ion outflows are more likely ions to build up so that MPA could detect them to be present when Kp is higher and because they are observed in the electron plasma sheet, it is likely that 9 Mass Flow into and through the they are auroral outflows associated with increased au- Magnetosphere roral activity The temperatures and densities of these ions are difficult to discern because the ions are accel- Using satellites at geosynchronous orbit, several aserated through a spacecraft sheath with a sheath potential much greater than the temperature of the ion population (In addition, sheath-focusing effects and the increased ion collection area of the expanded highvoltage sheath [Chen, 1965; Hershkowitz, 1989] further complicate any attempt to analyze these cool ions) The pects of mass flow in the magnetosphere can be viewed Four themes dealing with mass flow during the NSWP Storm are outlined in the following four subsections; the first two deal with movements of particles through the magnetosphere and the last two deal with the evolution of particle populations in the magnetosphere

18 26,390 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 91 Movement of the Outer Plasmasphere 93 The Plasma Sheet as a Ring-Current Source From geosynchronous orbit, the buildup, residence, and loss of the outer plasmasphere are regularly seen This is an aspect of mass flow in the sense that one [eg, Elphic et al, 1996, 1997] And as discussed in population of particles evolves into another population section 2, these aspects of the outer plasmasphere are When convection in the magnetosphere undergoes a viewed very clearly during the NSWP-storm interval strong surge, plasma-sheet ions are convected into the Because it represents a significant amount of mass trans- dipole (and energized) where they form the ring current port in the magnetosphere [Borovsky et al, 1997], the [eg, Smith et al, 1979; Chen et al, 1994] movement of the outer plasmasphere during the NSWP This happens during the NSWP Storm, where nustorm is revisited here merical calculations show that the population of ions Cold ions from the ionosphere are seen flowing into that is measured by MPA at geosynchronous orbit on the equatorial region of geosynchronous orbit several the nightside is convected inward to form the ring curd ys prior to the storm (see section 8), and the accurent at lower L shells [Kozyra et al, 1997] According mulation of these ions leads to the build up of the outer to the modeling, the enhanced density of the plasma plasmasphere prior to the storm commencing The outer sheet in its superdense phase during the NSWP Storm plasmasphere is built up at all local times by November is a prerequisite for obtaining the measured Dst values during the storm 2 (see Plate 1) When the storm commences on the end The superdense plasma sheet being, in part, responof November 3, the outer plasmasphere begins to drain off and plasmaspheric material is seen flowing toward sible for large perturbations of Dst during storms has important consequences for how the solar wind drives the dayside neutral line (see section 6) and it is seen storms Since the density of the plasma sheet drives the right up to the magnetopause (see section 5) After its drainage on November 4, the outer plasmasphere does magnitude of Dst (see Figure 7), and since the density of the solar wind drives the density of the plasma sheet not rebuild again until November 12 [Borovsky et al, 1997, 1998a], it is reasonable to suspect that the density of the solar wind plays an important 92 Plasma Transport from the Solar Wind to role in the Dst perturbation produced during a storm the Plasma Sheet Statistical correlations have been established between 94 Relativistic-Electron Source the density of the solar wind and the density of the ion plasma sheet at geosynchronous orbit [Borovsky et The density of relativistic electrons at geosynchronous orbit increases early in the NSWP Storm and later deal, 1997, 1998a] These statistical studies have also clines slowly for several d ys as geomagnetic activity yielded transport times for solar-wind material to reach geosynchronous orbit on the nightside and around the dipole During the NSWP Storm, an increase in the solarwind density leads to a front of increasing plasma dendeclines This is a common pattern when a high-speed stream interacts with the Earth's magnetosphere A key to understanding the origin of the relativistic electrons is to find a source population that is captured into the outer dipole of the magnetosphere during sity that is observed moving past three geosynchronous the early phase of a storm This captured population satellites [Borovsky et al, 1997] The front reaches midnight about 4 hours after it is in the solar wind, it reaches dusk about 3 hours later, and it reaches the postnoon region after about 6 more hours These transport times of 4 hours to geosynchronous midnight, 7 hours to geosynchronous dusk, and 13 hours to geosynchronous prenoon are consistent with statistical measurements of the transport times from the solar wind to those locations [Borovsky et al, 1998a] In section 31 it was noted that the appearance of a superdense plasma sheet at a satellite not only brings on a transition to high density, it also brings on a transition to high plasma pressure In the case of the NSWP Storm, it is estimated that the magnetosphere becomes "high / " when the superdense plasma sheet arrives Hence, even at geosynchronous orbit, the plasma pressure in the ion plasma sheet exceeds the field pressure as a superdense plasma sheet moves in then undergoes modest (factor-of-2) energization over the next several days It is known that the plasma sheet typically has an anomalously high density during the early phases of a high-speed-stream-driven interval of geomagnetic activity [Borovsky et al, 1997] (The temperature of the plasma sheet remains normal during the anomalously high-density intervals) One can speculate that this anomalously high-density hot plasma is the source for high densities of relativistic electrons In the top panel of Figure 19 the daily averaged density of the electron plasma sheet at geosynchronous orbit is plotted as a function of time for 90 days: the NSWP Storm commences on d y 308 (November 3) In the bottom panel of Figure 19 the daily averaged density of relativistic electrons at geosynchronous orbit is plotted Note that in the top panel the amplitudes of the largedensity peaks are not measured well because the maximum number density of a superdense plasma sheet as

19 BOROVSKY ET A1' NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26, Electron-Plasma-Sheet 2 Density ' ' ' ' _ Relativistic-Electron ensity 305 I I - I I ß I ' Day of 1993 Figure 19 In the top panel a running daily average of the hot-electron density as measured by MPA on is plotted for 90 days commencing with November 1, 1993, and in the bottom panel a running daily average of the density of relativistic electrons as measured by SOPA on is plotted for the same 90 days ing, and after the National Space Weather Program (NSWP) Storm in November 1993 As seen from geosynchronous orbit, the NSWP was a robust storm: lots of relativistic electrons were produced, the plasma exceeded unity at geosynchronous orbit, the plasma sheet become superdense, magnetospheri convection was very strong, and the electron plasma sheet penetrated deeply into the dipole Several days prior to the storm (late October), the magnetosphere was in a normal state Once per day each of the geosynchronous satellites was encountering the plasmaspheric bulge indicating a normal rate of convection, the position and width of the electron plasma sheet indicated a normal convection rate, the ion-plasma-sheet pressure was normal, the density of relativistic electrons was declining, and the penetratingradiation backgrounds were low The amount of fieldline stretching on the nightside of the dipole was normal Spacecraft charging in the electron plasma sheet reached its normal values of-1000 V or so each day Cool-ion outflows from the ionosphere to the equatorial magnetosphere were seen on the dayside; these outflows will lead to plasmaspheric refilling as magnetospheric activity quiets before the storm Just before the storm (November 2 and 3), the magnetosphere becomes extremely quiet During this interval the outer plasmasphere refills out beyond geosynchronous orbit, so that the three geosynchronou satellites are bathed in plasmaspheric material at all local times Such a buildup is associated with a very seen by a satellite depends on the local-time position of the satellite when the superdense plasma sheet first ap- weak rate of magnetospheric convection The electron pears in the magnetosphere; a superdense plasma sheet plasma sheet barely reaches geosynchronous orbit from typically lasts less than a day and is confined to the the magnetotail owing to the very weak magnetospheric nightside and the orbital period of a geosynchronous convection and the ion-plasma-sheet pressure at geosynsatellite is 1 day As can be seen in Figure 19, the chronous orbit is very low The density of relativistic large increases in the density of relativistic electrons electrons at geosynchronous orbit becomes quite low tend to occur at instances when the plasma sheet is during this quiet interval and the penetrating-radiation superdense These large increases in the relativistic- background is very low The amount of field-line stretchelectron density also tend to occur when geomagnetic ing during the quiet interval was very small, with the activity increases Perhaps, as is the case for build- field remaining quite dipolar on the nightside at geosyning a strong ring current from superdense-plasma-sheet chronous orbit rather than becoming tail-like Spacematerial, both a high-density plasma sheet and strong craft charging reached only a few hundred volts geomagnetic activity are necessary to produce a high During the latter half of November 3, the magnedensity of energetic electrons at geosynchronous orbit tosphere undergoes a prolonged period of strong com- Note that high-density plasma sheet has its origin in pression This compression is seen in the Dst index, high-density solar wind [Borovsky et al, 1998a] There- which goes positive for about 15 hours, and it is seen in fore, if it is the case that a high-density plasma sheet the H component of the magnetic field as measured by contributes to make a high density of relativistic elec- GOES-6 and GOES-7 at geosynchronous orbit Durtrons, then the high density of the solar wind is an ulti- ing this compression phase, the ion plasma sheet on the mate cause for high densities of relativistic electrons in dawnside becomes extremely anisotropic, with Tñ/ i the magnetosphere exceeding 3 at times for the >40-keV ions At about 2200 UT on November 3 the IMF goes 10 Summary and Overview of the Storm strongly southward, as discerned by the magnetic-field experiment on IMP-8 The solar-wind density was mod- Many aspects of magnetospheric dynamics are ex- erately high at this time After Bz goes south, geomagplored by three geosynchronou satellites before, dur- netic activity commences and the dayside of the mag-

20 26,392 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW netosphere erodes away Near local noon, the geosynchronous satellite crossed the magnetopause into the magnetosheath and into a boundary layer several times between 2307 UT on November 3 and 0027 UT on November 4 Each time the magnetopause was crossed, plasmaspheric material was seen up against the magnetopause When geomagnetic activity commenced at the end of November 3, the material of the outer plasmasphere began to strongly flow toward the nose of the magnetosphere The flow velocities on the dayside exceeded 20 km/s, indicating a convection electric field of about 2 mv/m in the outer plasmasphere Material from the outer plasmasphere continued to drain sunward for at least 6 hours after the onset of activity During the main phase of the storm on November 4, ion plasma sheet was seen to be superdense, as was the electron plasma sheet This is consistent with the solar wind being dense and the IMF being southward The leading edge of the superdense ion plasma sheet was ob- Several new observations of magnetospheric dynamserved propagating around the dipole: first it was seen ics were made during the NSWP Storm One is the at local midnight, three hours later it was seen at local dusk, then 6 hours after that it was seen in the afternoon sector The electron plasma sheet was very hot (as well as very dense) on November 4, and it was very wide and cut deeply into the dipole on the nightside, indicating that magnetospheric convection was very strong Cool ionospheric ions, presumably from auroral processes, were seen flowing into the equatorial electron plasma sheet for several hours on November 4 During observation that as a superdense plasma sheet sweeps around the dipole, it brings not only a transition to higher plasma density, but also a transition from/ < 1 to > I (see section 31) The superdense plasma sheet can be highly diamagnetic, owing to its large particle pressure One should expect a morphological transformation in the magnetosphere as a superdense plasma sheet spreads through it A second new observation is that, whereas superthe main phase of the storm (November 4), the pressure dense ion plasma sheets and superdenselectron plasma of the ion-plasma-sheet on the nightside of the dipole sheets occur simultaneously, they are not necessarily at geosynchronous orbit was very high, exceeding even colocated around geosynchronous orbit (see section 32) the pressure B2/8 r of the unperturbedipole magnetic Typically they arrive simultaneously on the nightside at field Hence, the plasma- exceeded unity at geosyn- geosynchronous orbit But on the duskside and dayside chronous orbit on November 4 The stretching of the magnetic field on the nightside at geosynchronous orbit of the dipole, a superdense ion plasma sheet can appear without an electron plasma sheet The superdense was very large on November 4, indicating strong cross- electron plasma sheet, like the ordinary electron plasma tail currents very close to the Earth The density of relativistic electrons rose very fast on November 4, reaching a large value by l100 UT on sheet, seems to reside only on the nightside and dawnside of the dipole A third new observation is that the amount of fieldthat day Although the density of relativistic electrons line stretching on the nightside at geosynchronous orbit reaches its maximum on November 4, the flux of rela- is related to the ion-plasma-sheet pressure on the nighttivistic electrons will not reach its maximum for a few side (see section 7) Hence, the magnetic-field stretch- more days Owing to the hot, dense electron plasma sheet, spacecraft charging was severe on November 4 The potential of the spacecraft exceeded-2500 V with respect to the ambient plasma in sunlight Penetrating radiation levels remained low on November 4 During the recovery phase of the NSWP Storm, beginning on November 5, the outer plasmasphere returns to its normal configuration having a duskside bulge that crosses geosynchronous orbit, indicating normal levels of magnetospheric convection The width and position of the electron plasma sheet at geosynchronous orbit slowly returns to normal values over the few days following November 4, indicating a steady reduction in the amount of magnetospheri convection down toward normal levels The ion-plasma-sheet density and ionplasma-sheet pressures are normal during the recovery phase The density of relativistic electrons remains high on November 5 and 6, then it undergoes a several-day decline The temperature of the relativistic-electron population at geosynchronous orbit increaseslowly during the recovery phase, and the fluxes of these relativistic electrons increase to a maximum value on November 8 and then decline thereafter As the fluxes increase, the penetrating-radiation background at geosyn- chronous orbit worsens, reaching severe values on the days around November 8 During the recovery phase, the stretching of the magnetic field on the nightside lessens, and the levels of stretching are normal to weak by November 7 Likewise, spacecraft charging in the electron plasma sheet reduces back to normal values after November 4 ing appears to be largely associated with aliamagnetism During the main phase of the NSWP Storm, the stretching is extremely large and the ion-plasma-sheet pressure is also extremely large A fourth new observation is that the well-known in- creases in the fluxes of relativistic electrons at geosynchronous orbit that occur several days after the commencement of a storm can be described as a slow, mod- est heating of a population of energetic electrons that is injected very early in the storm (see section 4) Thus, to explain the origin of the high fluxes of relativistic electrons in the outer magnetosphere after a storm, one need not look for a powerful energization mechanism acting over several days in the latter phases of a storm,

21 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW 26,393 rather one should look for a source population that supplies energetic electrons in the very early stages of a storm A fifth new observation is that spacecraft charging in the electron plasma sheet at geosynchronous orbit can typically bring a satellite to potentials of-1 kv or more with respect to the ambient plasma, in sunlight (see section 32) During a storm, when the electron plasma sheet is hotter than usual and superdense, this charging potential can be several times larger than the-1 kv Comparing with previous measurements of spacecraft charging on other satellites in sunlight, where potentials much smaller than 1 kv were typical, one can surmise that the severity of spacecraft charging is a function of the design of the satellite, in particular, it is probably sensitive to the presence of nonconducting surfaces on the spacecraft Acknowledgments The authors with to thank Joachim Birn Rod Christensen, Dot Delapp, Barbara Emery, John Freeman, Gary Hoogeveen, Rick Elphic, Larry Kepko, Janet Kozyra, Evan Noverosky, Howard Singer, and Loretta Weiss for their help and to thank A1 Lazarus and Chris Russell for access to satellite data This work was supported by NASA, the US Department of Energy, and the US Air Force The editor thanks H Opgenoorth and L Lyons for their assistance in evaluating this paper References Baker, D N, R L McPherron, T E Cayton, and R W Klebesadel, Linear prediction filter analysis of relativistic electron properties at 66 RE, J Geophys Res, 95, 15133, 1990 Bame, S J, D J McComas, M F Thomsen, B L Barraclough, R C Elphic, JP Glore, J T Gosling, J C Chavez, E P Evans, and F J Wymer, Magnetospheric plasma analyzer for spacecraft with constrained resources, Rev Sci Instrum, 6, 1026, 1993 Baumjohann, W, G Paschmann, and C A Cattell, Average plasma properties in the central plasma sheet, J Geophys Res, , 1989 Belian, R D, T E Cayton, R A Christensen, J C Ingraham, M M Meier, G D Reeves, and A J Lazarus, Relativistic electrons in the outer-zone: An 11 year cycle; their relation to the solar wind, in Workshop on the Earth's Trapped Particle Environment, edited by G D Reeves, p 13, Am Inst Phys, Woodbury, NY, 1996 Borovsky, J E, M F Thomsen, and D J McComas, The superdense plasma sheet: Plasmaspheric origin, solar-wind origin, or ionospheric origin?, J Geophys Res, 102, 22089, 1997 Borovsky, J E, M F Thomsen, and R C Elphic, The driving of the plasma sheet by the solar wind, J Geophys Res, in press, 1998a Borovsky, J E, M F Thomsen, R E Elphic, T E Cayton, and D J McComas, The transport of plasma-sheet material from the distant tail to geosynchronous orbit, J Geophys Res, in press, 1998b Cayton, T E, R D Belian, S P Gary, T A Fritz, and D N Baker, Energetic electron components at geosynchronous orbit, Geophys Res Left, 16, 147, 1989 Chappell, C R, K K Harris, and G W Sharp, The morphology of the bulge region of the plasmasphere, J Geophys Res, 75, 3848, 1970 Chappell, C R, K K Harris, and G W Sharp, The dayside of the plasmasphere, J Geophys Res, 76, 7632, 1971 Chen, F F, Electr c probes, in Plasma Diagnostic Techniques, edited by R H Huddlestone and S L Leonard, p 113, Academic,San Diego, Calif, 1965 Chen, M W, L R Lyons, and M Schultz, Simulations of phase space distributions of storm time proton ring current, J Geophys Res, 99, 5745, 1994 DeForest, S E, Spacecraft charging at synchronous orbit, J Geophys Res, 77, 651, 1972 DeForest, S E, and C E McIlwain, Plasma clouds in the magnetosphere, J Geophys Res, 76, 3587, 1971 Elphic, R C, L A 5¾eiss, M F Thomsen, D J McComas, and M B Moldwin, Evolution of plasmaspheric ions at geosynchronous orbit during time of high geomagnetic activity, Geophys Res Left, 23, 2189, 1996 Elphic, R C, M F Thomsen, and J E Borovsky, The fate of the outer plasmasphere, Geophys Res Left, 24, 365, 1997 Fredricks, R W, and F L Scarf, Observations of spacecraft charging effects in energetic plasma regions, in Photon and Particle Interactions with Surfaces in Space, edited by R J L Grard, p 277, Reidel, Norwell, Mass, 1973 Gary, S P, M F Thomsen, J Lee, D J McComas, and K R Moore, Warm protons at geosynchronous orbit, J Geophys Res, 102, 2291, 1997 Gussenhoven, M S, D A Hardy, and N Heinemann, Systematics of the equatorward diffuse auroral boundary, J Geophys Res, 88, 5692, 1983 Hershkowitz, N, How Langmfiir probes work, in Plasma Diagnostics, vol 1, edited by O Auciello and D L Flamm, p 113, Academic, San Diego, Calif, 1989 Higel, B, and W Lei, Electron density and plasmapause characteristics at 66 Rs: A statistical study of GEOS 2 relaxation sounder data, J Geophys Res, 89, 1583, 1984 Inouye, G T, Spacecraft potentials in a substorm environment, in Spacecraft Charging by Magnetospheric Plasmas, edited by A Rosen, p 89, MIT Press, Cambridge, Mass, 1976 Kozyra, J U, V K Jordanova, R B Horne, and R M Thorne, Modeling of the contribution of electromagnetic ion cyclotron (EMIC) waves to stermtime ring current erosion, in Magnetic Storms, Geophys Monogr Ser, vol 98, edited by B T Tsurutani, W D Gonzalez, Y Kamide, and J K Araballo, p 187, AGU, Washington, DC, 1997 McComas, D J, S J Bame, B L Barraclough, J R Donart, R C Elphic, J T Gosling, M B Moldwin, K R Moore, and M F Thomsen, Magnetospheric Plasma An- alyzer: Initial three-spacecraft observations from geosynchronous orbit, J (7eophys Res, 98, 13453, 1993 McComas, D J, R C Elphic, M B Moldwin, and M F Thomsen, Plasma observations of magnetopause crossings at geosynchronous orbit, J (7eophy$ Res, 99, 21249, 1994 McPherron, R L, C T Russell, and MP Aubry, Satellite studies of magnetospheric substorms on August 15, 1968, 9, Phenomenological model for substorms, J Geophy3 Res, 78, 3131, 1973 McPherron, R L, D N Baker, and L B Bargatze, Linear filters as a method of real time prediction of geomagnetic activity, in Solar Wind-Magnetosphere Coupling, edited by Y Kamide and J A Slavin, p 85, Terra Sci, Tokyo, 1986 Moldwin, M B, M F Thomsen, S J Bame, D J McComas, and K R Moore, An examination of the structure and dynamics of the outer plasmasphere using multiple

22 26,394 BOROVSKY ET AI: NOVEMBER 1993 STORM--DYNAMICS AND MASS FLOW geosynchronous satellites, J-Geophys Res, 99, 11475, 1994 Nagai, T, "Space weather forecast": Prediction of rela- tivistic electron intensity at synchronous orbit, Geophys Res Lett, 15, 425, 1988 Paulikas, G A, and J B Blake, Effects of the solar wind on magnetospheric dynamics: Energetic electrons at the synchronous orbit, in Quantitative Modeling of Magnetospheric Processes, Geophys Monogr Set, vol 21, edited by W P Olson, p180, AGU, Washington, DC, 1979 Reasoner, D L, W Lennartsson, and C R Chappel, Relationship between ATS-6 spacecraft-charging occurrences and warm plasma encounters, in Spacecraft Charging by Magnetospheric Plasmas, eduted by A Rosen, p 89, MIT Press, Cambridge, Mass, 1976 Rufenach, C L, R F Martin, and H H Sauer, A study of geosynchronous magnetopause crossings, J Geophys Res, 9, 15125, 1989 Smith, P H, N K Bewtra, and R A Hoffman, Motions of charged particles in the magnetosphere under the influence of a time-varying large scale convection electric field, in Quantitative Modeling of Magnetospheric Processes, Geophys Monogr Set, vol 21, edited by W P Olson, p513, AGU, Washington, DC, 1979 Sojka, J J, and G L Wrenn, Refilling of geosynchronous flux tubes as observed at the equator by GEOS 2, J Geophys Res, 90, 6379, 1985 Thomsen, M F, J E Borovsky, D J McComas, and M B Moldwin, Observations of the Earth's plasma sheet at geosynchronous orbit, in Workshop on the Earth's Trapped Particle Environment, edited by G D Reeves, p 25, Am Inst Phys, Woodbury, NY, 1996a Thomsen, M F, D J McComas, G D Reeves, and L A Weiss, An observational test of the Tsyganenko (T89a) model of the magnetospheric field, J Geophys Res, 101, 24827, 1996b Thomsen, M F, D J McComas, J E Borovsky, and R C Elphic, The magnetospheric trough, submitted to Geophysical Monogr Ser, edited by J Horwitz, AGU, Washington, DC, in press, 1998 Whipple, E C, Observation of photoelectrons and secondary electrons reflected from a potential barrier in the vicinity of ATS6, J Geophys Res, 81, 715, 1976 Whipple, E C, Potentials of surfaces in space, Rep Prog Phys, 44, 1197, 1981 Wilcox, J M, K H Schatten, and N F Ness, Influence of interplanetary magnetic field and plasma on geomagnetic activity during quiet-sun conditions, J Geophys Res, 72, 19, 1967 Wrenn, G L, and A J Sims, Internal charging in the outer zone and operational anomalies, in Radiation Belts: Models and Standards, Geophys Monogr Ser, vol 97, edited by J F Lemaire, D Heynderickx, and D N Baker, p 275, AGU, Washington, DC, 1996 J E Borovsky, D J McComas, and M F Thomsen, Mail Stop D466, Los Alamos National Laboratory, Los Alamos, NM ( jborovsky lanlgov) T E Clayton, Mail Stop D436, Los Alamos National Laboratory, Los Alamos, NM D J Knipp, Department of Physics, US Air Force Academy, Colorado Springs, CO (Received August 15, 1997; revised October 21, 1997; accepted October 21, 1997)