VOL. 83, NO. All JOURNALOF GEOPHYSICALRESEARCH NOVEMBER1, 1978 MULTIPLE-SATELLITE STUDIES OF MAGNETOSPHERIC SUBSTORMS: PLASMA SHEET RECOVERY AND THE POLEWARD LEAP OF AURORAL ZONE ACTIVITY T.H.Pytte, I. West,1R. L. Jr.,- Mc•herron, and E. 2wM.. Hones,G.Kivels•n, Jr.- 2 Abstract. Particle observations from pairs of expansion phase onsets [Kisabeth and Rostoker, satellites (Ogo 5 and Vela 4A and 5A) during 28 1971; Clauer and McPherron, 1974; Wiens and plasma sheet thickening events are examined. Rostoker, 1975; Pytte et al., 1976a, b; Kamide et These data indicate that thickening of the night- al., 1977] and that the westward polar electrojet time plasma sheet during substorms occurs in two sometimes expands westward in an impulsive, step- main stages, one early stage of single or mul- like fashion [Wiens and Rostoker, 1975]. The tiple expansions of the near-earth (geocentric latter feature would indicate a similar steplike distancesr •< 15 RE ) plasmasheet at the onsetof progression of activity also in the geomagnetic substorm expansions (Pi 2 bursts) on the ground tail, as was originally suggested by Rostoker and and another later stage of plasma sheet recovery Camidge [1971]. However, examinations of the that starts near the time of maximum auroral zone plasma sheet dynamics in the near-earth region bay activity and is characterized by a large- scale thickening toward higher latitudes that oc- orbit(r •< 15 (r RE•) 18[Pytte R•.) [Honeset al.,et 1976a]al., 1967,and in 1973, the Vela curs over a broad azimuthal scale and from iono- 1976] have show• no clear evidence of such azi- spheric heights to beyond the Vela orbit muthally localized phenomena in the tail. Thus (r • 18 RE). A detailed analysisof two-satel- the plasma sheet expansion signatures in the near lite observations during eight plasma sheet re- tail during multiple-onset substorms, as observed coveries is presented, showing events that oc- by a single satellite near midnight, are nearly curred within 5 min in widely separated locations identical for each onset; there seems to be no at small(<4 •) distancesfrom the tail's mid- systematic change with time indicative of suc- plane and evenns that occurred concurrently in cessive onsets occurring in separate or in pro- the Vela orbit and at high latitudes in the near- gressively westward moving sectors. In the more distant tail, where the plasma sheet thins during clearearth region.signatures Some of aevents poleward were leapaccompanieldobY (to >7 mag- this early phase of the substom, the most spec- netic latitude) of electron precipitation and the tacular event is the plasma sheet recovery to westward electrojet that probably corresponded to about or greater than presubstorm thickness. an expansion poleward of the ionospheric projec- This recovery too is apparently not localized but tion of the recovering plasma sheet. The eight may cover a significant fraction of the tail's plasma sheet recoveries occurred 10-30 min after breadth [Hones, 1973]. a Pi 2 burst on the ground, and mid-latitude mag- Another apparent discrepancy between current netograms showed no indications of a near-coinci- substorm models based on ground data and models dent formation of a substormcurrent wedge in the based largely on plasma sheet observations is the local time sector of the spacecraft; this indi- absence of any specific reference in almost all cates that the plasma sheet recoveries and the ground models to the 'poleward leap' of auroral poleward leaps were not caused directly by a late zone activity. This phenomenon occurs late in high-latitude substorm expansion. Instead, these substorms and is characterized by a sudden pole- phenomena seem to represent a transition phase ward displacement of bright auroras and of the between a substorm expansion sequence and a sub- westwardelectrojet from auroral zone (650-70ø ) storm recovery. to low polar cap (475ø) magneticlatitudes [e.g., Hones et al., 1970, 1971; Wolcott et al., 1976]. Introduction This same phenomenonis apparently responsible for the recently identified poleward displacement Recent studies of the magnetospheric substorm (to 475ø ) of a detachedregion of field-aligned using ground magnetic and auroral observations currents that seems to occur near the time of have revealed that substorms often have multiple maximumauroral zone bay activity [Iijima and Potemra, 1978]. However, it may be significant that the identification of a poleward leap seems 1Departmentof Physics, University of Bergen, to have been aided by the observed close temporal N-5014 Bergen, Norway. relation of these auroral and ground magnetic signatures with the recovery of the plasma sheet 2Departmentof Earth and Space Sciences, in the Vela orbit [Hones et al., 1973, and refer- University of California, Los Angeles, ences therein]; auroral data and standard mag- California. netograms alone are often not sufficient to dis- tinguish the poleward leap from a high-latitude 3LawrenceLivermore Laboratory, University of substorm expansion. The incorporation of the California, Livermore, California. leap as a separate phenomenon in ground substorm models therefore seems to depend on the existence 4LosAlamos Scientific Laboratory, University of of a set of ground signatures by which these phe- California, Los Alamos, New Mexico. nomena can be separated. If no such signatures exist, it would indicate that the recovery of the Copyright 1978 by the American Geophysical Union. plasma sheet in the Vela orbit is due simply to a Paper number 8A0696. 0148-0227/78/118A-0696501 . 00 5256 Pytte et al.' Plasma Sheet Recovery and the Poleward Leap 5257 -15 • • •OUT Pi 2. Six of these recoveries were observed . "Z 70ø SEPT.6,1969 within a time interval of less than 5 min by two /•. ' i•JULY16,1969 satellites located at relatively small • distances -10 ' •.•/- ' •VELASB from the tail's midplane (dZ <• 4 R•) The ab- senceof a near-coincidentPi 2 bu•s• is a clear ,"• • 12UT __ indication that the plasma sheet recoveries too were not caused by a substorm expansion. Final- xs ••o • _ ly, we show that the plasma sheet recovery is not localized but may occur at widely spaced satel- lites (>15R E east-west)and extend from the Vela orbit to the high-latitude near-earth region of (• .. 02UT the tail. These spatial characteristics, which appear to differ from those observed during sub- Ys• 0BUT SEP• 4, 1968 SEPZ & 1969 storm expansions, do not indicate westward pro- gressing or azimuthally localized plasma sheet phenomena in the tail. A model for plasma sheet Fig. 1. GSM equatorial projections of dynamics during substorms based on th.ese obser- satellite orbits during the intervals studied in vations is therefore proposed. this paper, superimposed upon the projections of magnetic field lines in the Mead and Fairfield [1975] MF73Dmodel for 0ø dipole tilt. Data substom expansion t.hat involves also the plasma The magnetotail measurements used in this pa- sheetat r 4 18 _RE. per were obtained from the Ogo 5 and Vela 4A and The purpose of the present paper is to analyze 5B satellites. Brief descriptions of the rel- these differences in current substorm models and evant instruments are given by West et al. to examine the nature of the poleward leap. To [1973], Hones et al. [1972], and Bame et al. do this, we first show examples of clear signa- [1971]. tures of a poleward leap (at the time of plasma We have selected for detailed presentation sheet recovery) that were not accompanied by eight substorm intervals during periods of con- ground Pi 2 magnetic pulsations; this absence of tinuous tracking of the Ogo and Vela satellites. Pi 2's appears to be an important feature which The geocentric solar magnetospheric (GSM) equa- separates the poleward leap from a substorm ex- torial projections of satellite trajectories are pansion. Next we present a detailed analysis of shown in Figure 1, superimposed upon the pro- eight cases of plasma sheet recovery in different jection of field lines in the Mead and Fairfield local time sectors of the tail that occurred near [1975] magnetospheric field model for magneti- the time of maximumauroral zone bay activity but cally disturbedconditions (Kp • 2). These. field from 10 to 30 min after the last in a series of lines intersect the earth at 70 (solid curves) September •, 1968 Bar I A B C DE 1117 ABC D E FG H 1614 71' 2'helan/c•_ [[ ; , .[ [ , I I ...... III I Ii Ii I i Dixon_ III I I ii I 68* 63 ø I I1• II I Barrow I H I I I I C Che{yuskin[H I i I I I J ß -• ................. 71' D,xonh II[ i-'•,J- DxieH II ; i ;•V/;; "', il I I I I I • 65ø .r..e_Z_ __,,_' ._• 1 t , I 'l I I I II J I ] [ i i Itll I I ,I I I, J ! 08 O0 1000 1200 1•O0 1200 1400 1600 1800 A B C D E Universal Dine SITKA,0915 . 0938 1028 1043,10•1 A B C D E t GUAI•ONAGAWA, !-', ' ' ', JONAGAWAGUAH 134•" •1355 :1410 , -=_ ..... ', • ', • • -:'•= ,,•_ :•-=•.:• __- • 111• /507,1517 ,1548 1614 Fig. 2. Auroral zone riometer recordings (upper panels) and magnetograms (middle panels) and magnetograms (middle panels), together with high-pass-filtered rapid run magnetograms from mid-latitude and low-latitude stations (bottom panels) during two substorm intervals on September 4, 1969. Vertical dashed lines mark substorm expansion onsets as identified by the Pi 2 bursts ; _vertical solid lines mark the start o of a poleward leap of auroral zone activity to •71 corrected magnetic latitude. Local magnetic midnight (eccentric dipole time), indicated by dots, is at 41830 at Dixon and at 41905 UT at Zhelania. 5258 Pytte et al.' Plasma Sheet Recovery and the Poleward Leap and 75ø (dashedcurves) magnetic latitudes and September4, 1968 -II 6 -12 7 serve to approximately map the satellite loca- YS•I I.'1$ 12 7 tions to the ground.