VOL. 81, NO. 34 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 1, 1976 Multiple-Satellite Studies of Magnetospheric Substorms' Radial Dynamics of the PlasmaSheet T. PYTTE, • R. L. MCPHERRON, •' AND M. G. KIVELSON •' Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024 H. I. WEST, JR. Lawrence Livermore Laboratory, University of California, Livermore, California 94550 E. W. HONES, JR. Los AlamosScientific Laboratory, University of California, Los Alamos, New Mexico 87545 Multiple-satellite measurements of magnetic field and/or energetic particles during four magneto- sphericsubstorms are presented. The data were obtained from Ogo 5 and Vela 4A, located near local midnight at about the same distance from the neutral sheet but separated by 3-10 Rs in the radial direction.The substorm expansion phases all had multiple onsets, typically observed as multiple bursts of Pi 2 magnetic pulsations (which we use for accurate timing of each onset), auroral zone bay in- tensifications, and low-latitude positive bay increases. A minimum in the plasma sheet thickness was probably formed at •¾sm '•' -- 15 RE in association with the formation of an X-type neutral line. Earthward of this line, successive substorm onset signatures were observed in an almostone-to-one relationship with ground Pi 2 bursts: a short (•5 min) burst of magnetic fluctuations confined to the plasma sheet,a plasma sheet expansion,and a field vector rotation toward a more dipolar orientation. These onsets, which occurred at 10- to 15-minintervals,may have beencaused by impulsive enhancements of field line reconnection. Sincethe plasma sheetwas thinning before each new onset, it appearsthat these enhance- mentswere triggeredin the tail each time the plasma sheet became very thin, causing a disruption of the cross-tail electric currents. In the tailward part of the plasmasheet (Xsm • -18 RE) the plasma sheet was thinning down or remained thin until well after the last near-earth expansion, when a recovery occurred. The near-tail observations are consistent with a model in which the associated tailward motion of the neutral line started when the reconnectionrate exceeded the earthward flux return rate, producing a tailward pressure which forced the neutral line to movetailward. A comparison with what appears to be a 'contracted oval substorm'shows that, even though this substorm apparentlytook place within a more limited local time sector, it had all the usual substorm features in the tail. INTRODUCTION The magnetotail playsan important part in magnetospheric substormprocesses, acting as an energy reservoir and as a source and acceleration regionfor particles. There is, in gen- eral, a close correlation between simultaneous large-scale sub- storm phenomena in the tail and on the ground, but observa- tions made in both regions are often very dependent on location. A densenetwork of observatories has made it pos- sibleto studythe complexity and variability of substorm phe- nomena on the ground, whereas only a very few casesof simultaneous measurements in more than one location in space have been studied. Thus our knowledge of substorm phenomena and their relation to ground observations is based upon a great number of independent satellite measurements taken from different regions in space and during different substorms.When combining these measurements, one then has to assume that there is a basicsubstorm pattern and that eachset of observations provides a piece of information to the total picture. In spiteof the limitation setby single-satellite measurements our knowledge of the dynamicsof the magnetotail has been rapidly increasingon the basis of such measurements (see • Permanent address: Department of Physics, University of Bergen, Bergen, Norway. •'Also at the Department of Geophysics and Space Physics, Univer- sity of California, Los Angeles,California 90024. Copyright ¸ 1976 by theAmerican Geophysical Union. reviewsby Hones et al. [1973a] and Russelland McPherron [1973]). The once apparently contradictory results obtained earthward and tailward of r • 18 Re during substorms now appearto be successfully explained by invoking the formation of an X-type neutral line across the tail at bay onset [Russell, 1972; McPherron, 1972; Hones et al., 1973a;McPherron et al., 1973]. This neutral line then separates the plasma sheetinto two radial regions: the earthward region experiencing an ex- pansion and the tailward region a thinning following bay onset. In the regionwherethe neutral line is formedthe plasma sheetis less than 1 Re thick [Honeset al., 1973a; Buck et al., 1973], though Lui et al. [1975] state that such thin plasma sheets are seldom observed at Vela orbit. Experimentalevi- dence for the formation of a neutral line at bay onset has recently been found by Nishidaand Nagayarna [1973, 1975] and Nishida and Hones [1974]. By comparing ground and magnetotail measurements, Hones et al. [1973a] concluded that the neutral line apparentlystaysearthward of r • 18 Re until the beginning of the recovery of auroral zone bays and then moves tailward. There is, however, some disagreement asto the exact nature and significance of the tail processes leading up to the neutral line formation,in particular, the presubstorm thinning of the near-earthplasma sheet [Aubry et al., 1972; Hoffman and Burch, 1973; Kivelson et al., 1973; Buck et al., 1973; A. Nishida and K. Fujii, unpublished report, 1976]. Also, since the as- sumed plasma sheetbehavior in the above substorm modelsis inferred from independent measurements earthward and tail- 5921