IOURNAL OF GEOPHYSICAL RESEARCH, VOL. ].02, NO. AS, PAGES 9553-9S72, MAY L 1997 /:/S

/

NASA-CR-205269 / -

Geotail measurements compared with the motions of high-latitude auroral boundaries during two substorms

N. C. Maynard, 1 W. J. Burke, 2 G. M. Erickson, 3 M. Nakamura, 4 T. Mukai, s S. Kokubun, 6 T. Yamamoto, r B. Jacobsen, s A. Egetand, s J. C. Samson, g D. 1_. Weimer, 1 G. D. Reeves, l° and H. L6hr ll,12

Abstract. plasma and field measurements at -95 RE are compared with extensive _ound-based, near-, and geo_ynchronous measurements to study relationships between auroral activity and magnetotail dynamics during the expansion phases of two suhstorms. The studied intervals are representative of intermittent, moderute activity. The behavior of the and the observed effects at Geotail for both events are harmonized by the concept of the activation of near-Earth X lines (NEXL) after substorm onsets, with subsequent discharges of one or more plasmoids down the magnetotail. The plasmoid_ must be viewed as three-dimensional structures which are spatially Limited in the dawn-dusk direction. ALso, recormection at the NEXL must proceed at variable rates on closed magnetic field lines for significant times before be_nmng to reconnect lobe flux. This implies that the in the near-Earth magnetotail is relatively thick in comparison with an embedded current sheet and that both the NEXL and distant X line can be active simultaneously. Until recomaection at the N'EXL engages lobe flux, the distant X line maintains control of the poleward auroral boundary. [f the NE,V_ remains active after reaching the lobe, the aurora[ boundary can move poleward explosively. The dynamics of high-latitude aurora in the midmght re, on thus provides a means for monitoring these processes and indicating when significant lobe flux reconnects at the NE_Y_L.

Introduction derstanding of magnetospheric processes ;hrough their ionospheric signatures. Such techniques have been de- veloped and utilized to expand understanding of the Variations of the high-lar2tude are reg- dayside [Newel/, et a£, 1991; Sandholt ularly monitored by sensors at ground facilities and on satellites in tow-Earth rather than at their et aL, 1990] and of sutmtorm onset; [McPherrvn et aL, 1973; Elphinstone et aL, 1995]. This study concerns source regions in the magnetosphere. Thus it is useful the dynamics of the night.side, poleward boundary of to develop remote sensing techniques that improve un- the auroral oval. It is based on simultaneous ground ' Mission ResearchCorporation, Nashua,New Hampshire. and space measurements taken on January 14, [994, zPhillips Laboratory, Hansom Air Force Base, Massachusetts. during two subs_rrrm extending from 0600 to 0730 and JC¢nter for Space Physics, Boston UniversitT, Boston, Massachusetts. from 1900 to 2200 LIT. Throughout this day _he Geotail ' Department of Earth and Planetary Physics, University of Tok'yo, Tokyo. satellite was in or near the distant plasma sheet where . s Institute of S pace and Astronautical Scitmc_, Sagamihara, Japan. it observed substorm effects. Ground-based instrumen- s Solar Terrestrial Environment Laboratory,Nagoya University, Toyokawa, tation of the Canadian Auroral Network for the OPEN Japan. Program Unified Study (CANOPUS) and International Institute of S pace and Astronautical Science, Kanagawa, Japan. mDepartment of'Physics, University ofOs|o, Oslo, . Monitor for Auroral Geomagnetic Effecr-_ (IMAGE) sys- Department of Physics, Canadian Network for Space Research, Univer- terlm were operating in _he midnight sector dunng the sity 0fAlbcrta,Edmonton, Alberta, Canada. first and second subsr_rms, respectively. Auxiliary mea- =°Astrophysics and Radiation Measurements, Los Alamos Nauonal surements were also acquired by Defense Meteorological Laboratory, Los Alamos, New Mexico. "Institut fiir Geophysik and Mcmorotogie, Technical University of Satellite Program (DMSP) satellites in _he ionosphere Braunschweig, . and Los Alamos National Laboratory (LANL) satellites =,Z'Nowat Geo Forschungs Zenmtm, Postdam, Germany. a_ geosynchronous altitude. The poleward boundary of the auroral oval lies at or Copyright 1997 by the American Gcophysical Union. near the boundary between open and closed magnetic field lines. Under conditions of steady convection the Paper number 97JA00307. 0148-0227/97/97JA-00307509.00 locations of this boundary should vary smoothly with the potential imposed across the cap by the solar

9553 9554 MAYNARD ET AL.: GEADTA£L MEASUREMENTS AND AURORAL BOUNDARIES [S/scoe, 1982]. The basic assumption underlying regions,has been observed by IMP 8 [Slamn etaL, 1990] this pedagogical model, namely, that steady convection and LSEE 3 [Slm_n et aL, 1993}.There is a nearly one- is possible in a stably configured magnetosphere, may to-one correspondence between plasmoids observed in be incorrect [S/scoe and Cummings, 1969; Erickson and the fartailby ISEE 3 and earlierground onser_ [Mold- Wolf, 1980_. In ways still not fully understood, this de- urinand Hughes, 19931. Since the launch of Geot_il a viation from stable equilibrium gives rise to magneto- number of particleand fielddetectionsof plasmoid sig- spheric substorms. Figure 1 of Akasofu [1964} illustrates natures have been reported [Prank et aL, 1994; Kawano the phenomenology of substorms in the nightside auro- et aL, 1994; Mac/mia et aL, 1994; Nagai et aL, 1994; ral oval. The onsets of substorms are marked by the Nishida et aL, 1994a, b]. Of particular interest for in- brightening of the equatorwardmost auroral arc. This terpreting Geocail data in this paper are the heuristic is followed by a rapid polewaxd expansion of the ac- model of M_ et aL [1994] for satellite encounters tive region and the development of a bulge in the mid- with tallward propagating plasmoids and the classifica- night sector. Thus the poleward boundary, of auroral tion of variations in the magnetotail Bz aftex substorm luminosity expands to high magnetic latitudes during onsets developed by Naga£ et aL [1994]. The concept of the expansion phases and retreats equatorward during a quasi-stagnant plasrnoid introduced by Nishida et ai. the recovery phases of substorms [Craven am/Fra_ [1986] provides a context for slower moving plasmoids 1987]. The poleward boundary isalsomarked by a pair propagating tailward inside a distant X line. This has of oppositely directed sheet_ of field-alignedcurrents. recently been expanded to deduce simultaneously active The more poleward current sheet is directed into the near-Earth and distant X lines [see N/skuta at a/., 1996; ionosphere [Fujiiet al.,1994]. Optical techniqueshave Hoshino et aL, 1996; Kawano et aL, 1996}. been developed to monitor the locationsand motions of In the following sections we first briefly describe the the poleward boundary of the auroral oval [de laBeau- ground- and spac_based instrumentation which provide jerdidre et al., 1994]. From these the magnetotafl re- the data sources for our study and then present de- connection rates have been estimated [B/anchard el aL, tailed descriptions of measurements taken on January 19961. 14, 1994, during two intervals of moderate substorm The causal linkage between the magne_sphere and activity. In the discussion section we first compare the high-latitude ionosphere during substorms has been the Geotail measurements in these weak-to-moderate a subject of long and fruitful research leading to the de- events with predictions of the substorm plasrnoid model velopment of a model whose general features are widely [Hones, 1977]. The sequence of plasma flow directions accepted. Controversial detailsthat continue to be de- observed during the first event indicates that Geotail bated have been reviewed by Erickson [1995]. After exited a tallward moving plasmoid before its 0 line southward turnings of the interplanetarymagnetic field reached XGSM = --95 R.E and entered a part of the (IMF), merging with the Earth's fieldisenhanced along magnetotall that was largely unaffected by substorm the dayside . Magnetic fluxistransferred activity. Geotail returned to the plasmoid after its 0 from the dayside magnetosphere to the nightsidemag- line passed. The width of the plasmoid is limited in netotail.During the substorm growth phase, intensified the YCSM direction. In the second substorm period, and reconfiguredmagnetotail currentscause the shapes Geetail encountered signatures of a plasmoid with three of fieldlinesin the nightside,inner magnetosphere to embedded 0 lines. It was only after a third electrojet stretchfrom dipolarto taillike[Kaufinann, 1987;Baker activation of this substorm that all previously closed and McPh_rro_ 1990]. Onset occurs on magnetic field field lines in the magnetotail were pinched off at the linesthat connect the inner edge of the plasma sheet near-Earth neutral line_ Subsequently, open flux from with the ionosphere [Maynard et aL, 1996]and are fre- the lobe rapidly reconnected and then, as activity, de- quently associatedwith northward turnings ofthe IMF creased, the X line moved tailward past Ceotail. Pole- [Ca_n et aL, 1975; Lyons, 1995]. They are marked by ward expansion of the auroral boundary occurs when rapid dipolarizationof near-Earth magnetic fieldlines open flux reconnects at a rate faster than steady state. and the development of westward electrojets spanning Significant reconnection must have occurred on closed a few hours of local time in the midnight sector [Singer magnetic field lines within the plasma sheet prior to en- et aL, 1983]. This results from the partial diversion gaging reconnection of lobe field lines in the near-Earth of cross-tail currents through the auroral ionosphere by region. When this new X line passed tailwaxd of Geo- means of filamentary field-aligned currents [Mc.Ph_v'rvn tail, the aurora had already begun to retreat equator- et aL, 1973]. ward, indicating that the period of rapid reconnection Hones [1977] outlined a global model of substorm dy- had ended. namics that considers the consequences of a near-Earth X line (NEXL) forming as substorm activity expands. A Data Sources byproduct magnetic O-type neutral linealsoforms and is expelled down the magnetotail as a plasrnoi& Sup- January 14, 1994, was the fourth most disturbed day portive evidence for the existenceof substorm-related of the month, with Kp ranging from 3+ to 5. It was the plasmoids in the magnetotail, as travelingcompression continuation of an extended period of magnetic activity MAYNARD ET AL.: GEOTAIL M_EASUREM_NTS AND AURORAL BOUNDARIES 9555

8 located at three CANOPUS stations along the mag- January 74 1994 netic meridian passing through Pinawa, Gillam, and Ranldn mitt. They are presented in a stacked-plot for- mat to provide a him:ory o{ _:he onset: and po/eward expansion of the subszorm. Magnetometer measm'e- _X ments, taken along the same meridian, help identify the time of onset and locate the electrojet currents. During 0 i the second interval, ground data came from the [M- AGE magnetometer chain [Lfihr et aL, 19841, which -2 ;- exr, ends from the archipelago norzh of Nor- way into central and from Trorn_ in northern 10_ ' _ ' _ Norway. Meridian-scanning photometer and a/l-sky im- ager measurements were available from Ny ,_esund at J Svalbard. The names and the geographic/geomagnetic locations of ground stal;ions used in our s_udy of the first and second subs_orm are _ven in Tables I and 2, respectively. In addition, supportive information is _y O. provided by energetic particle precipitation [Hardy et aL, 1984} and ion drift measurements [Green et aL, 1986] from the DMSP F8, F10, and Fll satdlites flying in circular, -synchronous at an altitude of 840 [ kin. White-light optical images acquired by DMSP F10 t have also been examined. Energetic ions and electrons 8 r .', from the LANL geosyaaehronous satellites 1990-095 and 1991-080 [Belian et aL, 19921 provided information on inner magnetosphere particle injections and constrained tl substorm onset definition. _Z The Geotail spacecragt is spin stabilized, completing 20 revolutions per minute. Its spin axis is maintained -2 !i A,i within 5° of perpendicular to the ecliptic plane. On January 14, 1994, Geotail was at apogee in the mag- ..a netotail near XCSM = --96 RE, YGSM = 9 R_, and a few R.¢ south of the GSM equatorial plane..Mlowing [ , , , , • i 16 18 20 22 24 for the 4° aberration angle of the , Geotail Time - UT was slightly to the eveningside of magnetic midnight during the periods of interest. Data from the magnetic Figure 1. Three components of the interplanetary field fiuxgate (MGF) instrument [Kokubun et al., 1994], field, ifi geocentric solar-magnetospheric (GSM) coor- the electric field detector (EFD) [Tsuruda et al., 1994], didates, measured by IMP 8 during the last 8 hours of and the low-energy par'title (LEP) experiment [Mukai et January 14, 1994. aL, 1994] are used in this study. There are _o triaxial fluxgate magnetometers on Ceota_il. They are located on a nonconducting mast as diat, anees of 5.12 and 7.15 that sZarted in the middle of January 11 [Nishida et aL, rn from the spacecraft. Measurements for each compo- 1995]. IMF data from _;he IMP 8 satellite have a gap nent were made with 15 bit resolution in one of seven during the first substorm interval. It is probable that dynamic ranges from 4-16 nT to 4-65,536 nT at rares IMF By was negative, since it had that polarity both of 16 s -t. These were averaged to produce one vec- before and a_ter the data gap. Figure i shows [MF tor every 3 s. EFD measurements come from a double measurements acquired during the last eight hours of probe thag uses spherical sensors. Each sphere is at- the day. All three components were highly variable. tached to and separated from the spacecraft body by a This is characteristic of the entire day, including times 50 m wire depioyer. Data axe sampled as rates of 32 prior to the first sub, otto. or 64 s-t. To avoid spurious effects due to asymmetric Ground-based and satellite measurements of the high- photoelectron currents, only data, axxtuired while the latitude ionosphere are correlated with observations in probes point in the ranges 60° to 120° and 240 ° to 300 ° the magnetotail from the C-eo_;ail spacecraft near XCSM with respect to the Sun, are used. These da_a are ;hen of --96 RE. During the interval of the first subs_orm, fit to sinusoids to produce one measurement of the elec- ground magnetometer data and optical measurements tric field components in the spin plane every 3 s. The came from CANOPUS stations in central Canada [Ros- LEP has three sensor components. Within the magne- toker et aL, 1994]. Meridian scanning photometers were tosphere the energy-p_r-mharge element is mtxv¢. ,,_ful. 9556 MAYNARD ET AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDABIE3

! 4/1994 X-corn >orient ( 200 nT/div)

RANK

FCHU -

GKL

_LL

PINA I i l f I I I 1 5:50 6:00 6:30 7,-00 7:._0 I •j, I I ---t---_ POR day 14 94 6300 i

75

(D = 70

o _J 65

6O

55

5:30 6:00 6:30 7:00 7:30 Time(UT)

Plate I. Magnetic and optical measurements from CANOPUS stations along the Kaakin-Pinawa magnetic meridian between 0530 and 0730 UT on January 14, 1994. The 630.0 nm emissions were measured by three meridional scanning photometers a¢ the Rankin Inlet, Gillam, and Pin_wa stations. The intensity versus magnetic latitude spectrogram was compiled assuming an emis- sion altitude of 230 krm The solid, dashed-dotteM and dashed lines beneath the rnagnet_grarns represent different Bz and Vx configurations encountered at Geot.ail and are described in the text.

It consists of two nested quadraspherical electrostatic upper pane/s present the X component magnetic fields analyzers. Directional differential fluxes for electrons measured along the CANOPUS meridian chain (Table and ions are sampled simultaneously in 32 energy steps 1). To assist the reader in comparing ground measure- and at seven spacecraft elevation angles. Electron ener- ment.s with those at Geotail, we have placed markings $ies are sampled from 8 eV to 38 keV. Ion energies per beneath the magnetograms of Plate 1, giving the rela- charge are measured from 32 eV/Q to 39 keV/Q. tive polarities of the earthward-tailward Vx component of plasma flow and the north-4outh Bz component of First Substorm Interval: 0600 - 0730 the magnetic field. Solid lines indicate periods when Bz UT > 0 (northward) and Vx < 0 (tailward). Dotted-dashed Plate 1 displays data from CANOPUS acquired dur- lines indicate times when Bz < 0 and V x < 0; dashed ing the first substorm event of January 14, 1994. The lines indicate times when Bz > 0 and Vx > 0. While MAYNARD ET AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES 9557

Table 1. Coordinates of CANOPUS Stations S_ation Abbreviation GLAT GLONG [LAT IV[LONG

RanRin Inlet RANK 62.8 ° 267.9 ° 73.5 ° 328.9 ° Eskimo Point ESKI 61.1 ° -065.9° 71.8 ° 336.5 ° Forz Churchill FCHU 58.8 ° 265.9 ° 69.5 ° 336.7" GiUam GILL 56.@ 265.@ 67.2° 336.2" IslandLake ISLL 53.9° 265.3° 64.7_ 336-4° Phaawa PINA 50.2° 263. 9° 6I.0° 335. i°

the instances Bz < 0 and Vx > 0 have been observed _udes of the magnetic deflections and the intensity of by Geotail [Nishida st aL, 19951, they were not found auroral emissions decreased slightly at the highex lati- during the periods of interes_ on January 14, 1994. tudes. However, opuica[ and magnetic activity contin- Substorm onset occurred at 0629 UT, jus_ south of ued along the tow-latitude border of the aurora near the Gillam station. It is marked by the star_ of negative 65 °. An intensification in the aurora aear 75° occurred bays at GILL and ISLL. This onset is also clearly seen as at 0650 UT. Shortly a_ter 0710 UT the intensity of op- an intensification of 630.0 nm emissions, commencing at tical emissions a_ 74 ° dropped to background levels, 0629 UT near 65 ° magne_-ic latitude (MLAT) then grad- characteristic of the polar cap. This indicates that the ually stepping poleward. A significant enhancement of photometer scan crossed the open/closed field Line and optical em/ssions occurred over Fort Churchill near 0636 auroral oval/polar cap boundaries[de la Beau}ardiere et UT. A decrease in the magnetic field X component also al., 1994; Blar_chard et ai, 1996]. We conclude that the began simultaneously at that s_ation. Optical activ- auroral brightening at 0641 UT (2340 magn_ic local ity progressed poleward to between 73° and 76 ° near time (MLT)) marks the polar cap boundary as being 0641 UT. Note that after the initial activity the anapli- neax 79 ° MLAT, the northern mos_ _ of optical

• ..... ,3dK L I ION L0 j 0mu. ion 0.I

IHI

90

-90 gr 05:_0 06:00 06:20 06:_0 07:'30 if/:?.0

P late 2..Measurements _om the LEP instrument on Geotail spanning the firm substorm interval in an energy-versus-_ime color spectrogram format. The top two plots give the omnidirectional, differential flux of electrons and ions. The lower spectrogram panes, f_om top to bosom give _he directional differential flux of ions moving tailward, dawnward, earthward, and duskward, respectively. The magnitude of B and the polar (theta) and azimuthal (phi) angles of B are provided for reference in the bottom two line graphs. 9558 MAYNARD Eq" AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES

t£ I_ 1.0 &l

I I I P

ION

_V

I_ [nl] IBI

.q0 the_ phi -'[I0 UT 20:00 20:20 20:40 21:00 21:20 21:a,0 22:00

Plate 3. Measurements from the LEP instrument on C_ail spanning the second suhstorm interval in the same format as Plate 2. MAYNARD ET AL.- GEOTAIL MEASUREMENTS AND AUI_OKAL BOUNDARIES 9559

Table 2. Coordinates of IMACE Stations Station Abbreviation GLAT GLONG MLAT MI, ONG

Ny Alesund NAL ,-8.9° [1.9° 74.9° [14.3° Lo nKyeaxbyn LYR 78.2° [5.3° 73.8° [15.3° Hornesuad HOP_ 77.0 ° L5.6° 72.8 ° L13.1 ° Hooch Island HOP 76.5 ° 25.0 ° 71.4 ° [07.0 ° S6r6ya SO[:_ 70.5 ° 22.2 ° 66.1 ° t10.3 ° Masi MAS 69.5 ° 23.7 ° 64.9° LI0.6 ° Muonio M-UO 68.0 ° 23.5 ° 63.5 ° 109.4 ° Pello PEL 66.9 ° 24.1 ° 62.4° 109.1° Ouluj_-'vi OUJ 64.5 ° 27.20 59.7 ° [10.2 °

coverage. The boundary retrea_ed equatorward in the indicated on the plate, the look directions were selected subsequent minutes. The data in Plate 1 indicate that to sample ions _reaming in the ta/lward, dawnward, at ..-0720 UT another activation of the substorm cycle earthward, and dusk'ward directions. With _he excep- initiated and la._ed beyond the limits of our study. tion of a brief excursion into the plasma sheet boundary A dawn-dusk pass of the DMSP F8 spacecraft (not layer (PSBL) at ---0540 UT, the spacecral% was in the • shown), near the time of s-uhstorm onset, crossed the plasma shee_ prior to substorm onset, as indicated by high-la:itude, auroral boundary at _78 ° MLAT and low magnetic fields and elevated levels of particle fluxes. 1800 MLT. l_apid, antisunward plasma flows were de- Thus the spacecraft was located in a region of closed tected just poleward of that boundary at 0631:30 UT. magnetic field lines, with the disr.anc X line ;ailward of The boundary transition was sharp, with no particle its position during the _ow_h and the early expansion precipitation on i_ poleward side. The polar cap po- phases of the substorm. tential measured during this pass was ---50 kV. The The ion flow turned tailward at 0637 UT (8 rain after DMSP Fll spacecraft crossed nightside, southern high onset), becoming _ronger and more energetic at 0642 latitudes at 0656 UT. However, it did no_ fully enter UT. After a decrease a second enhancemen_ was ob- the polar cap, detecting intense auroral precipitation served at about 0655 UT. During the period of strong up to its mo_t poleward excursion of-73.l°MLAT at tailward flow, Bz was, with a few minor exceptions, 2336 MLT. The [ocar.ion of the polar cap boundary and northward. Ey was negative, except when Bz briefly the most poieward precipitation observed by DMSP are changed sign, consistent with relatively steady tailward consistent with _he boundary of 630.0 nm emissions flow that lasted more than 20 rain. The positive 8z in- shown in Place [ for that time. We also note that the dicates that the satellite must have been on closed field LASqL satellite 1990-095 was near 0300 LT..__t 0632 UT lines while experiencing this tailward flow or was ;all- it de_cted an energy-dispersed electron injection, indi- ,yard of the O line within a plasmoid s_ruc_ure. No_e cating that the sateifite was [oca_ed to the eas_ of the that the beginning of the tailward flow at (3eo_ail oc- substorrn-activated region. curred "-8 min after mb_orrn onset, approximately the Figure 2 displays magnetic and electric field as well as time of auroral expansion to 70 ° MLAT (Plate l). The as ion bulk flow components measured by Geo'tail dur- increases in speed and intensity at 0642 and 0655 UT ing the first substorm event as functions of UT and the coincided with brightenings of the aurora in the vicinity GSM locations of the satellite. Figure 2a shows the Bx of 75 ° M:LAT. component (GEM) of the magnetic field and the total At 0659 UT the ion flow turned sunward and field magnitude of the magnetic field (dotted line). Figures aligned as the spacecraft entered the PSBL. At this 2b and 2c show Bz and the YGSM component of the point, Ev and Bz were both positive, consis_en_ with electric field, Ey, respectively. Figures 2d - 2g show the the observed earthward ion flow. The flow turned tail- vetociD, in the XGSM direction Vx, the component of ward as Bz briefly reversed sign and then returned velocity parallel to the magnetic field (positive toward earthward with positive Bz. During the second period the Earth), as well as the XcsM and Y_SM components of earthward flow, the plasma velocity in the XCSM di- of the ion velocity perpendicular to the magnetic field, rection was field aligned. From about 0710 to 0720 UT, respectively. Subs_orm onset was at 0629 UT as de_er- Bz turned negative, and the plasma velocity became mined from g-round measurements. Note that at this strongly eailward; Ey remained positive. In this in- time, and for most of the previous hour, the ion flow terval of Bz polarity changes, Vy was significant and was earthward at Geotail and Bz was positive. Plate 2 positive (dawn- to-dusk}. provides LEP mea._urement_ for the same period in an energy-versus-time spectrogram format. The top plot Second Substorm Interval: 1900 - 2200 gives the omnidirectional, differential flux of electrons UT with energies beVween 8 eV and 38 keV. The lower plo_ The second event did not have a clean onset. Figure represent directional differential fluxes of ions with en- 1 shows that after a period of primarily northward IMF, ergies per charge between 32 eV/Q and 39 keV/Q. As Bz turned southward near 1815 UT. Large-amplitude 9560 MAYNARD ET AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES

a _

-a b

4_

c

lOOO 500 d -500 -10_ 1ooo 500 e >= o -500 -IOOO I00(3 5OO o > -500 -I0(3(3 I000 500 o g > -500 -1000 UT 0530 0630 0730 XCSM -95.8 -95.8 -95.9 YGSM 10.1 9.8 9.5 ZCSM -3. I -3.6 -4.1

Fi&,ure 2. Magnetic field, electric field and ion drift velocities'measured by the magnetic field fluxgate (MGF), dectric field dete_or, (EFD), and low energy particle (LEP) instruments on Geotail between 0530 and 0730 UT on January 14, 1994. GSM components of vectors are presented- (a) The X and (b) Z of the magnetic field in nanoTeslas. (c) The dawn-dusk componet of the electric field in milliVolts/mer_r. (d) The earthward velocity component in kilometers per second. (e) The velocity components parallel to the Earth's magnetic field and (f) the earthward and (g) dawnward velocity components perpendicular co the magnetic field.

oscillations with periods between 27 and 31 rain fol- at Troms_ (not shown). Activity spread poleward, with lowed. Figure 3 gives the variations of the X compo- a second imtiation at 1925 UT. Table 2 indicates that nent of the Earth's magnetic fidd observed done the there is a 5.3 ° magnetic latitude difference between the IMAGE meridian chainof magnetometers. Activityini- SOR and Hopen Island (HOP) stations. The station at tiated near the S_rSya (SOR) and Masi (MAS) stations Bear Island (BJN) which usually bridges that gap was in northern Finland st -_1858 UT. It was alsodetected not operational on that day. A third intensification be- - MAYNABf) ET AL.: GEOTAXL MEASUR/_M_ENTS AND AUIAOEAL BOUNDAB/ES 9561

Z I"IQ G E MAGNETOrIETE@ [994-01-I_

H0R

-X POP

'S .--__--.___.._._.,,-=- ._..< _ - - _.w.%_ -- ...... _ - . I S0R

mmS

rIUO

>t i ...... - reEL

, 3U3

r

_.C3 :820 [840 [900 t920 [940 2000 2020 20_0 2100 U. 2"J 2!40 3200 t,r ------_-.-----_ ..... _.-_.-- -,--4

Figure 3, Variations of the X component of the Earth's magnetic field measured at stations along the IMAGE magnetometer chain during the second substorrn interval

gan near [950 UT at the SOR and MAS stations and (R. Smith, private communication, [996), confirming 2002" UT at HOP. A comparison with data in Figure l the sequence of events a_ Ny /Tdesund which are de- shows that that these activations occurred at_er north- scribed above. To help in comparing ground and Geo- ward turnings a.ssociated with the large oscillations of tail data, we have introduced the same symbolic mark- IMF Bz. Exact relationships between solar wind/IMF ings that appear in Plate i beneath the magnetograrns variations and the substorm activations shown in Figure in Figure 3 and beside the photometer scans in Figure 3 are well outside the scope of this report. 4. The only additions are heavy lines to indicate times Stack plots from the meridian scanning photometer when, as discussed below, s_rong earthward enhance- measurements at Ny ]klesund of 557.7 nm emissions are ments of plasma flow were detected by Ceotail. presented in Figure 4. At 1925 UT the aurora appeared Optical and particle data (not shown) reveal that on the southern horizon of Ny /ldesund and gradually DMSP F10 crns_ed the boundary of the polar cap into intensified, moving slightly poleward: After retreating the auroral oval a_; 2028 UT (TW M'LAT, 21.4 MLT). near 1942 UT it continued poleward, expanding rapidly The satellite passed to the west of Ny/_.lesund between after 1955 UT. It crossed zenith at about 2000 UT and Svalbard and Greenland. These DMSP measurements expanded to cover the visible sky with aurora until 2005 are consistent with our interpretation that the poleward UT, ,_l-sky images show that the decrease in the inten- auroral boundary at Ny/_desund reflects the location of sity of the emissions, observed bevween 2010 and 2020 the open/closed boundary [B/anehard et aL, 1996]. Po- UT in Figure 4, reflects a reduction m optical emis- lar cap potentials measured by the DMSP satellites in sions over the entire sky. This occurred while IMF this time interval were in the 50 to 70 kV range. The Sz was northward for more than I0 rain. At 2025 LANL satellite [991-080 was in the midnight sector. It UT the aurora began to retreat southward, reaching detected numerous, weak ion injections, the larges_ of and remaining near the horizon until 2140 UT when which were at 1852, [858, 1904, and 1909 UT. The most there was a brief, poleward excursion. We have also pronounced etececron injections were detected at _ [927, examined the meridian scanning photometer data from [941, 1951, and 2008 UT. Long:yearbyen located --_[00 km south of Ny _lesund Figure 5 displays the Geotail data for this event in 9562 MAYNARDETAL.:GEOTAILMEASUREMENTSANDAURORAL BOUNDARIES

Z = _,,,

z _ _._

_;sa

_ _=--=

_ = -_ =-'- ___

.,-_ _..

= _ --_-= o _=-_____ ¢

.... ¢ | ......

° _ i

g o - MAYNARD _r AL.: GEOTAIL MEASUR.BMENTS ,_ND AUR.OEAL BOUNDARIES 9563

12 _,...... 2 , ' "_'~':": "'; ": ...... ! _i":'-""[" ...... '" ' "t >( 03 -6 -12

d X >

e

>

200 L g _oo_ OL- > -500 }- -12oor

UT I800 I900 2000 2100 2200 X_.3,_ -96. I -96.1 -96.1 -96. i -96 1 ':"c,_,.' 9 t 9. i 9. t O.O 90 Z,,s,, -25 -2.I -I.g -i6

Figure 5. ._lagnetic fietd, electric field, and ion dril_ velocities measured by the MCF, EFD, and GEP instruments on C_otail between t8a0 and 2230 UT on January t4, t994, in the same format as Figure '2.

the same format as Figure 2. Omnidirectional electron toward dusk but with regions of nearly isotropic fluxes fluxes and quadrature flows of ions are shown in Plate 3. (Plate 3). At 1938 LIT t;he polarities of Bz, Ey, and To relate events at Geotafl to ground observations and V,r, reversed, Bx returned to near zero, and the plasma substorm phenomenology, we divide the Oeotail data flow again became strongly tailward. By was large, into four se_m'nents: (t) 1830 to 1925 UT, (2) 1925 to negative, and dominant in the next 3 min, after which 2000 UT, (3) 2000 to 202! UT, and (4) 2021 to 2150 it returned to near zero. This reversal in Bz is differ- UT. ent from the one in the previous substorm in that Ey In segment I, Geotail entered the plasma sheet from changed from negative to positive. It was positive on the southern lobe at 1830 UT. The flow was weak and both sides of the Bz reversal in the previous substorm variable in direction until 1915 UT, when the plasma period. There was no evidence of significant earthward flow velocity increased to ---250 km s-t and became flow. Between 1950 and 1955 UT, Vx peaked at > lO00 predominantly in the --XGsM direction. Bz remained km s -t, and Ey and Bz also maximized. At 1955 UT positive, indicative of closed field lines, and Ey became these quantities quickly decreased with the flow return- slightly negative (dusk to dawn). This initial effect at ing to a nearly constant 500 km s- t which was observed Geotail, relatable to ground activity, occurred -,, t7 rain through most of segment 3. after the first electrojet activation when the average en- In segment 3, BX became large and positive, indi- er_es of electrons and ions rose to a few hundred eV cating that Ceotail was in or near the northern PSBL. and a few keV, respectively. These values are consis- Since ion temperatures and densities remained high, the tent with Ceotail measurements in the plasma sheet. satellite did not enter the lobe. Data displayed in Plate In segment 2, By increased in magnitude in the nega- 3 show that both the flow speed and the thermal spread tive direction, becoming the dominant component. Be- of the ions decreased. Attention is directed to the Bz _veen 1932 and t938 UT, Bx became strongly nega- and Ey measurements in Figure 5 taken between 2000 tive, and the flow decreased to near zero as the satellite and 2020 UT. For the first half of _his period Bz was approached the southern PSBL. The primary flow was positive and became negative for the second half. The 9564 MAYNARD ET AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES polarity of E-¢ also changed from negative to positive enhanced transfer of flux from the day to the nightside consistent with continued tailward convection. These of the magnetosphere until the start of the substorm characteristics are similar to the polarity reversal at expansion phase. Hones [1977] proposed that substorm 1939 LIT. Recall also that it was in this interval that the expansion begins with the formation of a NEXL where most poleward excursion of the aurora was observed at magnetic reconnection proceeds on closed field lines, Ny ]_lesund. quickly cutting through to the lobes. This magnetic For segment 4, betnveen 2021 UT and 2030 UT, Bz topology requires that neutral lines in the near tall form was almost exclusively positive, and after a brief neg- in pairs. Because of the configuration of nearby field ative excursion Ey remained positive. From 2028 UT lines, neutral lines where merging occurs are called X until 2139 UT the dominant flow was earthward ex- lines. Tallward of the X line, is a closed structure with cept for near-zero-velocity episodes when the satellite an O-type neutral line near its center. Plasma trapped crossed into the southern lobe between 2031 and 2043 in the O configuration constitutes a plasmoid which con- UT. Earthward flowing plasma and positive Bz indi- vects down the tail away from the Earth. In the case cate that the satellite was again on closed field lines. illustrated by Hones [1977] all of the closed magnetic Upon reentering the plasma sheet at 2043 LIT, the ion flux initially extending tailward of the NEXL becomes fluxes detected by Geotail were variable. A series of pinched off, and initially open field lines in the lobes earthward-flow intensifications appeared at 5 to 8 min of the tail reconnect at the NEXL. The rapid inclusion intervals. The most severe velocity decreases occurred of newly reconnected flux in the magnetotail causes the during brief excursions into the lobe from the PSBL. closed field line region, and with it the NEXL, to expand Thus, while the flow appears highly variable at the lo- tailward. After this the NEXL becomes the "distant" cation of Geotail, it probably did not turn off between X line until the next substorm cycle. observations of fast flows. The several reversals of Bx For a satellite such as Geotail located in the distant during this period indicate that the tail was flapping plasma sheet, the signatures predicted by the NEXL with the satellite alternately in both the northern and model during a strong, isolated substorm are clear. (1) southern PSBLs. The peaks in Vx,_ occurred when the Some time after substorm onset, tailward plasma con- satellite was crossing the center of the plasma sheet. vection should be detected in a region with Bz > 0 and These flow enhancements continued until 2140 UT when Ey < 0. (2) As the center of a plasmoid approaches the the flow turned tailward for _10 min. This time Bz did spacecraft, the polarities of Bz and E}, should reverse. not reverse, indicating that the tailward plasma ftow oc- (3) For a brief period after the X line passes the sate]- curved on dosed magnetic field lines and was caused by lite, Bz > 0 and Ey < 0 should be detected. (4) As processes happening nearer to the Earth than Geotail. the magnetotail approaches equilibrium, both Bz and Ey should be positive as plasma convects earthward. Discussion Another characteristic of plasma dynamics implicit in the NEXL model relates to the directions of field- aligned flows in the PSBL. Schindler and Birn [1987 I Data presented in the previous sections can be used showed that conservation of momentum requires flows to study interactions of the magnetotail with the pole- of plasma along magnetic field lines near the separa- ward portion of the auroral oval during substorms. In trices associated with magnetic X lines. Thus, if an both cases the development of activity was measured by X line is located earthward/tailward of the observ- magnetometers arrayed along magnetic meridians in the ing spacecraft, plasma flow should be field-aligned and midnight sector. In neither instance, however, did we tailward/earthward in the boundary layer between the observe an isolated substorrn that developed steadily lobes and the plasma sheet [Machida et at, 1994]. from a clear onset, through expansion, to maximum The presence of flux ropes make signatures of 0 line epoch and recovery. Multiple a_'tivations occurred in crossings more complex than in the simple picture of both periods. Given the variability of the IMF on Jan- item 2. The relationship between plasmoids and flux uary 14, 1994, this is not surprising. The activities did ropes in the magnet_tail is a much-discussed subject produce auroraJ substorrn signatures, including rapid [Slavin et al, 1995, and raferences therein}. Flux ropes poleward expansions of aurora in the midnight sector, result from field-aligned (force-free) currents that pro- both within the auroral zone and at its poleward bound- duce helical magnetic structures around high-field cores. ary. To uncover relationships between auroral activa- Slavin et at [1995] found that during ISEE 3 encounters tions and expansions and magnetospheric phenomena, with plasmoids, flux ropes were almost invariably em- it is useful to place the Geotail measurements in appro- bedded within the cores. The only possible encounter priate contexts of substorm-related events in the magne- with the core of a plasmoid in the period studied here totail and then ch_k the consistency of interpretations occurred near 1938 UT when Bz reversed sign. An ex- against auroral signatures. amination of By measurements (not shown) atthis time Most suhstorm models explain the observed growth- indicates that flux-rope signatures are indeed present. phase stretching of field lines as caused by intensified, Flux-rope field lines connected at both ends to the plasma sheet currents and convection resulting from the Earth experience a drag from the ionosphere that might MAYNARD ET AL.: GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES 9565 explain the relatively slow speeds with which some large N-EX_L engages lobe field lines, the motion of the pole- plasmoids are observed to propagate down the magne- ward boundary of the auroral zone is controlled by the totail [Hughes and Sibeck, 1987]. distant X line. (5) Activity at a N'EXL does not neces- Nagai et aL [19941 examined Bz responses in the sarily correlate with the activity of a di_ant X line. (6) magnetotall a_ter 89 well-documented substorm onsets. Onset brightening of the equatorwardmost arc is not They identify six categories. Bipolar responses indicate directly caused by a NF__X_L. clear Oeotail encounters with plasmoids in the plasma Although the second disturbed interval appears phe- sheet. They are the mos_ commonly observed signature nomenologically more complex, it i_ easier to reconcile in the range -70 > XGSM > --I.00 RS. The bipolar re- with predictions of the plasmoid model. We thns con- sponse in Bz has an initial increase, a sharp reversal to sider it before discussing _he significance of measure- a large negative peak, then a recovery to lesser negative ments taken between 0600 and 0730 UT. The relation- values. The average duration between peaks was 4 rain. ship between the observed auroral motions and intensi- The Sz reversals are consistent with Geotail crossing fications with magnetotail dynamics are then explored. 0 lines at the centers of plasmoids. The piasmoid may contain _t flux rope at the center [n which case By and Second Substorm Interval IBI maximize. VVhen Geotail was in the lobe of the magnetotall, the passage of a plasmoid was marked by Observations from the second substorm interval are traveling compression re_ons (type T). Less common quite complex. To help the reader _asp them as a responses included fluctuating Bz (.type F), often as- whole, Figure 6 provides a summary timeline. Begin- sociated with multiple onset substorrns, and situations ning at £858 UT, gTound magnetometers detected an when BZ remained northward (type N) for extended activation and three distinct intensifications of the au- periods after substorm onsets. roral electrojet. At the time of the first observed elec- It is seen that the Ceotail measurements reported trojet activation the LA_NL satellite 1991-080 at geos_a- here do not fit eamly ingo the simple pictures of the _ionary altitude in the midnight sector detected a small Hones [1977] model. We contend that phenomenolog- ion injection (the second in a series of small changes). [rally during the 0600 - 0730 UT interval Geotail en- The etectrojet intensifications at 1925, i950, and 2002 countered a type N _ructure. In the [900 - 2200 UT UT were followed by electron injections detected by the interval, Ceotall detected one _rpe F and two modified LANL satellite. In a previous s_udy a few minutes de- bipolar signatures. In reality, the region of substorm lay was observed between dipo[arization/_ound onset engagement is finite in the YCSM direction [Singer et and the arrival of injected particles at the altitude of aL, [9831. Also, not all substorms are isolated, strong, the Combined Release/Radiation Effects Satellite (CR- or even continuous. Moderate to weak events with mul- KES) (in the vicinity o£ oo_eostationary altitude) [May- tiple activations provide different perspectives for un- nard et aL, [996 t. We suggest that. as conditions pro- derstanding substorm processes. In the following para- gTessed from the initial _ound activity at 1858 UT. a _aphs we arg-ue that the observed signatures reported reconnection site developed. The relaxation of the elec- herein are consistent with the basic concepts of Hones rzojet at -,,1909 UT indicates tha_ reconnection at the [1977] if we expand the picture to three dimensions and newly created NEXL either slowed or _opped at this allow for a significant period of reconnect[on on closed time. As a result of this activation, some field lines magnet;c field lines before the lobes are engaged at the with 0 topologies are introduced into the magnetotai]. NEXL. This implies a thick plasma sheet in the near- These field lines and their associated plasma, plasmoid Earth region, perhaps with an embedded thin current 1, should migrate tailward. This is consistent with the sheet [see Sergeev et aL, 1993 I. It also means _hat detection of _ailward flowing plasma by Geotall with the distant X line can a_ively add flux to the cto_ed Bz > 0 ai%er 1915 UT and the reduction of flow _t field line region until reconnect[on at the NEXL breaks 1932 UT. The second intensification of the electrojet through to the lobes. This concept is implicit in the and particle injection corresponds _o a reactivation of quasi-stagnant plasmoid observations of Nishida et aL the re,connection site. This activity created more O- [1996] and Hoshino et at [1996] where _wo X lines can type field lines and plasmoid 2. Note that ptasmoid 2 be active simultaneously. encompasses plasmoid [ since no lobe field lines have Comparing Geotail data with auroral observations is yet reconnected. Plasmoid 2 is marked by the passage difficult since it is impossible to know exactly which re- of an 0 line at 1938 UT and the reduction in flow at gions are magnetically conjugate. We submit the follow- 1955 UT. Once more, however, reconnect[on then either ing postulates to constrain the interpretation of evenm slowed or stopped. It was only during the third acti- at both locations. (1) Field lines in the nights[de auro- vation, beginning at ---1950 UT, that reconnect[on at a ral oval with optical emissions are closed. (2) Electrons NEXL engaged previously open flux. This was followed are accelerated near active X lines. Those with direct •by the passage of the O Iine of plasmoid .3 at 2006 UT access to the ionosphere produce auroral emissions. (3) and the X line at 2021 UT. We return to this point in Merging of closed flux at a NEXL proceeds for a finite the d_ion of auroral boundary dynamics. interval before engaging lobe field lines. (4) Until a Figure 7 sketches the cycle of event_ as a sequence of 9566 MAYNARDETAL.:GEOTAIL MEASUREMENTS AND AURORAL BOUNDARIES

Variable _ _ _ E:lr|hward

Plasmoid #1 Plasmoid #2. Plasmoid #3 Flows

I Lobe I )lasmaI PSBL Plasmal PSBL Lobe t Plasma I Slleet Sheet Sheet

Begin Bz Tailward O X Flow X 0 X North Line Line Line Line Line GEOTAII. l 2 2 3 3 + ++ ÷ + + _ 1 I l I

Ion Electron / Ion (;coStal ion;iry injections. Injections ++ + ÷ + I I I I I

Sotll hward ;Vlaximtnn tmmm_n IMF B Z > 0 Retreat Expansion Ny Alesund SoIII heril 4 \ Soul hem Optical IMF B Z < 0 l|orizon / \ Ilorizon A _II+01";I / I \\

] Low-Latitude Low Latitude 1 lligh-Latitude I ] Activation lntensii']cations Inlensificalions I,'_I:\(;E

I +,+ ¢, , --, ° _\ [ ngl)¢I ()lllCtc rS ii2/,:;I,In,)| ". , .... "lw+.+ - '. x li+qv,illl" " " ' llllill+l)l.liilli "]i)+lliliil . " E)£1111111iiiHIIIIIllUIlll" 'l+'ll'lillli:Bi¢l.... Itl+++,PitililJi|lttttt)l ll+).ttll+l,ti+jt+l,,,r,I+[ IMP B Z

i l 2 2 2 X 9 0 I 2 !.; '_"

i) -_. 0 It :_

Figure 6. Summary timeline of observations during the second substorm int_srval of January 14, 1994. magnetotail "states" during the four segments of this First Substorm Interval substorm interval. Prior to 1900 UT, convection in the Figure 8 gives a timeline overview of gound and plasma sheet near -96 RE is earthward under the influ- space observatio_ during the first substorm interval. ence of a distant X line. Soon after this time a small Onset at 0629 LIT was marked by an activation of pla.smoid forms and moves tailward. The first sign of the electrojet and a brightening of aurora at 65° in- its existence appears at 96 RE in the form of a reversal variant latitude (ILAT). The LANL satellite 1990-095, of Vx and Ey, with Bz remaining northward. In the _-3 hours to the east of midnight, observed an ener_v- second segment the ion flow remained tailward as the dispersed electron injection beginning at -_0632 UT. enlarged plasmoid 2 approached and moved past Geo- These data indicate that a dipolarization of the Earth's tail. The a_ociated O line crossed the location of Geo- magnetic field occurred in the midnight secvar. _ail at 1938 UT. In segment 3 aU of the initially closed The question arises regarding when a NEXL formed field lines become pinched off, and the reconnection of and a plasmoid propagatmd down the magnetotail. A open flux proceeds. As this third pl_moid, 3, convected schematic representation of a scenario that reconciles downtail, Geotail moved into the PSBL where it could NEXL model predictions with Geot_il observations is not fully encounter the O line typology. It did, however, given in Figure 9. Figure 9a shows an equatorial-plane detect the telltale signature of its passage in the bipolar projection of the quiet time rnagnetotail with a distant responses of Bz and Ev. The reduction in velocity near X line. Convection is earthward/tailwaxd on the earth- 2025 UT marked the passage of the plasmoid. Geotail ward/tailward sides of the X line. Figure 9b indicates then moved into the southern lobe of the tail. When it the changed situation after the Ume of onset when the reentered the plasma sheet, Bz, Ey, and Vx were again NEXL Xj forrn_ Figure 9c shows a plasmoid with a fi- positive, consistent with the X line being rmilward of its nite size in the YcsM direction after it has propagated to location. The enhancements in the earthward flow of the vicinity of Geotail. The model predicra that Geo- energetic ions after this time are consistent with an ac- tail should first detect tailward flowing,, plasma sheet tive X line being located tailward of the satellite. ions with Bz > 0. From about 0638 to 0700 UT _his MAYNARD ET AL.: GEOTA[L MEASUREMENTS AND AURORAL BOUNDARIES 9567

_ P!asmotd # 2

_- ....' / / P!aamo,d i; 3 --" / z-.. < .u / r--

:_.' " . P!asrnoid :I 1 '_S .. '-- '!

P!a_moid i_ :3

:: ?lasmoid :" 2

4A j/1

Figure 7. Schematic representation of magnetotail dynamics during the second substorm interval of January 14, 1994. The various diagrams show meridional cuts of the magnetotail appropriate ['or the different time segments defined in the text.

was true. However, instead of observing a reversal in X line or reentered _he plasmoid on the earthward side the polarity of Bz, Oeo_ail moved into a portion o[ the og its 0 line. Particle measurements sho_'n in Pla_e_ PSBL with earthward ion flow. At 0711 UT, Vx became 2 and 3 suggest tha_ the latter interpretation is more strongly tailward and Bz < 0. At 0718 UT, Bz turned plausible. [n Plate 2 the earthward flowing ions in the northward, but the flow of plasma remained tailward. PSBL (0700 - 0705 UT) had energies centered a_ a few Despite apparent difficulties the Geotail meazure_ hundred eV. During the Bz < 0 interval the central en- ments are not inconsisten_ with NEXL generated plas- exgies of the tailward moving ions were > [0 keY. They mold predictions of the Hones [1977] model. Critical _rongly resemble ehe ion fluxes detected from 1940 to for this interpretation is the observation of earthward [952 UT (Plate 3) in the southward Bz portion of pins- flowing ions in the PSBL after 0700 UT. This can only mold 2, discussed above. For the first interpretation mean that Ceotail was temporarily on field lines whose to be correct it is necessary to po6tulate that the dis- dynamics were controlled by remnants of the distant X cant X line underwent a sudden activation and moved line. Recall that Singer et al. [1983] found that outside earthward. However, Plate [ shows neither enhanced of a limited region in local time, field tines remained emissions nor new magnetic deflections near the auro- taillike after substorm onset. Thus a NEXL and its as- ral oval's poleward boundary _ that time. Rather, the sociated piasmoid are of limited YCSM extent. We also polarity reversal of Bz a_ 0718 UT, with Bx > 0, indi- note that Kivelson et al. [19931 and Angelopouios et aL cates that Geotail returned to the plasmoid earthw£rd [1996] postulated that plasmoids must have limited ex- of the O line. The X line passed the satellite near 0720 tent in the dawn-dusk direction in order to explain the UT when Bz again turned positive. The persistence of lack o[" observation of a plasmoid _rom specific identified tailward flow after the passage of _he X line suggests substorms. This case provides observational evidence of that a second, near-Earth X lineX2 ['orrnednear the those conjectures. time of the second activation(0720 LIT) of the electro- As suggested by the Ceotail "trajectory" in Figure 9c, jet at Churchill. the satellite exited a tailward moving plasmoid across Auroral Boundary Dynamics its dawnside border. At a distance of 96 R.¢ the magne- totail moves by ---[.5 R._ ['or every 1° change in the solar The Geotail measurements provide clues for under- wind direction. When Geotail returned to a region of _anding auroral signatures during substorms just as plasma sheet fl-xxes at 0711 UT, both Bz and Vx were those auroral signatures constrain our interpretation o[ negative. There are only two possible interpretations Oeotail signatures. The observed magnetotail dynamics of this observation. Geotail either crossed the distant seen at Geotail have been interpreted within the context 9568 MAYNARDETAL.:GEOTAIL MEASUREMENTS AND AURORAL BOUNDABIY_S

Plasmoid

I l PSBL Ptasm. PSBL Plasma Sheet Lobe Sheet

Southward Bz GEOTAIL

Tailward Earthward Xl Flow Flow Line ¢ + + i 1 Electron injection Geostationary + I I Maximum Expansion CANOPUS Southern /_"'_outhern Optical Horizon Horizon Aurora : 4

1 1 Electro jet Electro jet CA.NOPUS Initiation Intensifications Magnetometers

0 0 0 5 6 7 UT 3 3 3 0 0 0

Figure 8. Summary timeline of observations during the first substorm interval of January 14, 1994.

of NEXL-generated plasmoids. We have deduced that on closed magnetic field lines, the particles accelerated during the second substorm interval, closed flux did not there produce auroral signatures. Until the last closed completely pinch off at the NEXL until the third elec- field line pinches off, auroral emissions axe confined to trojet activation. Afterward, open flux reconnected at latitudes equatorward of the mapping of the distant X the NF_DCL, activity diminished, and the X line eventu- line. As merging of closed flux at the NEXL proceeds, ally passed tailward of GeotaiL In the first substorm then electrons accelerated in the process acquire access case, Geotail moved outside and then back into a plas- to the auroral ionosphere at progressively higher lati- moid that was spatially limited in the YCSM direction. tudes. During this time, however, the distant X line still The poleward boundary moved equatorward during this controls the motion of the poleward auroral boundary, whole process. The following harmonizes these conclu- and closed flux can be added or not as indicated by the sions with the auroral observations. boundary motion. Rapid poteward movement of bright Consider first some consequences in the auroral iono- aurora inside the poteward boundary indicates rapid re- sphere of merging of lobe flux at an X line. We assume connection at the NEXL. Rapid poleward movement of that the energized electrons responsible for the observed the poleward boundary indicatesrapid reconnection of auroral emissions move along dosed magnetic field fines. lobe flux, most likely at the NEXL. In the midnight sector at high latitudes the electric field Plate 1 shows that auroral emissions brightened at causes the feet of these field lines to oonvect with an 65 ° at the time of onset (0629 UT). In the following equatorward component of drift. A typical magnitude 12 rain the bright aurora expanded to 76 °, a distance for this component is _1 km s-1. In steady state flow of _1200 kin, and approached the poleward boundary the boundary between open and closed field lines in of the oval. We believe that the 0636 UT auroral en- the auroral ionosphere should be stationary. Reconnec- hancement at Churchill (69.5 ° ) was related to the acti- tion proceeds at a rate such that the inflow of open vation of NEXL 1. Note that there are no emissions at field lines to the boundary exactly matches the outflow this latitude prior to that activation, including the time of newly closed field lines. If a NEXL becomes active of substorm onset, which places NEXL activation after MAYNAR/D ET AL.: CEOTAIL LVLEASURE,%fENTS AND AURORAL BOUNDARIES 9569

(B} _t

o

Plasmoid / P1asmoid

Figure 9. Schematic representation of magnetotaii dynamics during the first substorm interval of January [4, [994. Equatorial plane projections of the magnetotail appropriate for (a) presubstorm period, (b) substorm onset, (c) the period near 0700 UT when Ceotail entered the plasma _heet boundary layer, and (d) the la_ etec_rojet activation of the interval are shown. The polarities of Bz in the different re$ons are indicated. onset as determined by ground magnetogran_ and en- of view and only a very small emission at the edge of hanced aurora at the lower border. During the expan- the field of view 100 km to the south (at Long:year- sion phase the poleward boundary steadily retreated byen). The second intensification was accompanied by equatorward while the active aurora expanded pole- --,i kR emissions from near the southern horizon at Ny ward. This poleward expanding auroral acZiv/ty was AJesund. Only after the intensification of 1950 UT did totally on the closed field lines. Note that the pole- the aurora rapidly expand poleward across the sky. We ward auroral boundary retreated equaCorward through- associate the poleward movement _rith rapid reeonnec- out she expansion phase, indicating that She di_ant X tion of lobe field lines at the NEXL. As the ra_e of re- line controlled lobe flux reconnection, but recoanection connection slowed, the NEXL moved down the tail, and was at a slower rate than flux transport from the day- the poleward auroral boundary retreated equatorward. side. On the basis of the poleward migration of the Open flux was being added faster on the dayside than it active aurora to the poleward boundary we conclude was being reconnected on the nightside. That the X line that reconnection progressed to the lobe magnetic field continued to be active after its passage beyond Ceo_ail lines but did not engage the lobe until after 0720 UT is evidenced by enhanced earthward flows. Brighten- when the poleward boundary started moving poleward. ings in the aurora are seen at the poleward boundary The lack of an enhancement in auroral emissions as during the equaLorward retrea_. Some of these produce the poleward boundary moved equatorward near 0711 equatorward moving auroral forms. It is not possible UT ks cormmten_ wiLh our s'uggestion that the di_ant to correlate zhese events directly with individual en- X line did not move earthward; rather, Geotail exited hancemenr_ of earthward flows at Geotail; however, we and reentered a piasmoid that was spa_ial|y restricted believe that these phenomena may be related, although in local time. not on a one-to-one basis. The flow enhancements are The second interval was marked by three electro jet both spatially and temporally variable. intensifications at 1858, 1925, and 1950 UT. The fir_ Our two evenr,s are examples of magnetotaiis with produced no optical emissions in the Ny fldesund field multiple, active X lines. Hoshino et aL [1996] reached 9570 MAYNARDET AL.:GEOTAILMEASUREMENTSANDAURORALBOUNDARIES asimilarconclusionforaneventoccurring 3 days after Conclusions our events. The concept of a quasi-stagnant plasmoid [Nishida et ai, 19861 was originally postulated for slowly Activity observed in these two events provides insight moving plasmoids, and it was described as a preexpan- into the behavior of substorms and the magnetotail un- sion phenomena. The slow moving plasmoid was of- der variably driven conditions. These may be more rep- ten followed by a fast moving plasmoid associated with resentative of day-to-day routine occurrences than com- a substorm expansion phase. Our events occur after monly studied isolated or large events. On the basis of the onset of ionospheric substorm activity. The quasi- the data comparisons and the premises for interpreta- stagnant event seen by Kawano el al. [19961 was also tion stated above, we conclude the following: after a ground onset, and it was also followed by a fast 1. Reconnection of closed flux at a NEXL can occur moving plasmoid associated with a later onset. Our re- either continuously or intermittently for significant time sults, complemented by those of Hoshino et al. [1996] intervals. During the first event near-Earth reconnec- and Kawano et al. [1996], show that moderate and vari- tion reached the lobes in about 15 min. In the second able substorm activity can create slow tailward moving event it took three activations and more than 40 min to plasmoids that are within closed field lines being added engage lobe field fines. by an active distant X line. 2. Plasmoids are three-dimensional structures, which are spatially limited in the YGsM direction. This is nec- Energization from the Reconnection Process essary to explain Geotail magnetic field and ion flow During the first event and the first two activations of measurements during the first event. It is also con- the second event, tailward velocities observed at Geo- sistent with near-Earth observations showing that the tail increased with time. This may reflect the location region of dipolarization is confined to a few hours in of the NEXL relative to the lobe. The outflow from the magnetic local time [Singer et al., 1983]. X line occurs at some fraction of the Alfven velocity 3. Since the poleward boundary of the aurora delin- in the inflow re, on {Vasyliunas, 1975]. The Alfvdn ve- eates the last closed field line between the lobe and the locity is smallest in the center of the plasma sheet and PSBL, its'motions are controlled by balances between increases toward the PSBL. Thus we expect to detect the rates of flux transport from the dayside and recon- increasing velocities outflowing from reconnection sites nection in the magnetotail. Rapid poleward motions of as merging progresses toward the lobes. If merging is that boundary, as observed a2ter the third activation of intermittent, if the distance to lobe magnetic field lines event 2, indicate high rates of reconnection of lobe flux, in the near tail is large, and/or if closed field lines are most likely at a NEXL. being added by the distant X line almost as fast as field 4. Fxluatorward motions of the poleward, auroral lines are merged by the NEXL, that velocivy increase boundary during periods of increased magnetic activ- could be over a significant interval of time (e.g., Fig- ity are consistentwith residuallobe-fluxrecormection ure 5 between 1915 and 1955 UT). Consistent with our by the distant X line. Photometer measuremen_ in interpretation that lobe flux reconnected at the NEXL Plate 1 indicatethat at such times, NEXL activityis during the third activation (-,-1950 UT), the velocity on closedmagnetic fieldlines. measured by Geotail during the encounter with plas- 5. Multiple activationsof a near-Earth reconnection mold 3 remained constant with time. We note a sim- sitecan resultin multiple NEXLs before redonnecting ilar increasing trend in V x in Figure 1 of Hoshino et lobe flux.Reconnection at the distantX linecan con- al. 11996]. They deduced that two X lines were simul- tinue to supply energy and closed fluxto the system taneously a_ive for that event, although they did not and prolong the time requiredbefore the NEXL begins comment on the trend or the cause. reconnecting lobe flux. A related effect has been found by Baumjohann et al. 6. The long duration of NEXL activation on cl_ed [1996] in a superposed epoch analysis of substorm effects magnetic field lines su_ests that the plasma sheet was between "10 and 19 RE inside the NEXL region. For sub- relatively thick. It mav have had embedded within it a szorrns which are not magnetic storm associated, the thin current sheet usually associated with NEXL acti- average earthward ion bulk speed gradually increases vation. for the first 45 min after onset, while for storm associ- 7. Optical and Geotail observations during event 1 ated substorms, it peaks much more quickly. They con- provide evidence that the onset of the reconnection at clude that it is unlikely that lobe magnetic field lines the NEXL occurs after the beginning of activity de- arereconnected in the nonstorm case, and since the av- zected by ground magnetometers. erage ion temperature is similar in each category, the This study suggests that ionospheric variations can heating of plasma sheet ions is governed by reconnec- be used to monitor activity in the magnetotail. The tion on closed plasma sheet lines rather than open lobe motions of the auroral oval's polewaxd boundary in the field lines in the storm-related substorms. Our results midnight sector reflect the dynamic balance between suggest that the dynamics of the night,side aurora, es- flux transport from the dayside and reconnection in the pecially at and near the poleward boundary, provide a magnetotaiL They also serve as proxies for indicating means to monitor if, how quickly, and to what extent when near Earth neutral lines assume control of lobe the reconnection process engages the lobes. flux reconnection. MAYNARD ET A.L,: GEOTAIL MEASUREMENTS AND AUKOKAL BOUNDARIES 9571

AcknowiedgznenCs. This work was a collaborativeef- Fujii, R., R. A. Hoffman, P. C. Anderson, J. D. Craven, fort spawned from the NSF GEM Workshops and com- M. Sugiura, L. A. Frank, and N. C. Maynard, Electrody- mon data collection intervals. A. Nishida was instrumental namic parameters in the aigh_-_ime s_tor dmnng auroral in establishing the odi_nnal collaboration. O. de [a 8eau- substorm.s, J. Geophys. R_., gg, 6093, 1994. jardiere arranged GEM data collection intervals. We thank Greenspan, M. E., P. B. Anderson, and J. M. Petagatti, J. Slavin, G. Siscoe, and W. J. Hughes for helpful discus- Characteristics of _he _hermai plasma monitor (SSIES) sions. We thank R. Smith for providing the for the defense meteorologaca/s_tellite program (DMSP) meridian scan photometer data used for confirmation of the spacecraft $8 through SIO, Rep. AFGL-TR-86-O_7, Air Ny _,lesund results and K. Tsuruda for supplying the Geo- Force Geophys. Lab., Hanscom Air Force Base, MA, [986. tail electric field d.ata. We acknowledge E. Frim-Chri_eusen, Hardy, D. A., L. K. Schmidt, M. S. Gu.._enhoven, F. J. TI Rosenburg, R. Doe, W. J. Hughes, and V. Angelopoulos Marshall, H. C. Yeh, T. L. Shumaker, A. Huber, and J. for submitted data and discussions at the GEM Worksho._ Pantazm, Precipitating electron and ion detectors (SSJ/4) which influenced the research but were sot direc_-ly used in for the block 5D/Flight_ 6-[0 DMSP satellites: Calibra- this study. This work was supported in part by the U. S. tion and data presentation, Rep. flFGL-TR-8$-0317, Air Air Force Office of Scientific Research task 231kPL05, NSF Force Geophys. Lab., Hanscom Air Force Base, MA, i984. g'rant ATM-9401629, and NASA grant NAGW 2627. Hones, E. FL, Jr., Subst.orm processes in the magneto[all: The Editor thanks V. A. Sergeev and M. 8. 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