Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral zone activity

Particle observations from pairs of satellites (Ogo 5 and Vela 4A and 5A) during 28 plasma sheet thickening events are examined. These data indicate that thickening of the nighttime plasma sheet during substorms occurs in two main stages, one early stage of single or multiple expansions of the near-earth (geocentric distances r ≲15 RE) plasma sheet at the onset of substorm expansions (Pi 2 bursts) on the ground and another later stage of plasma sheet recovery that starts near the time of maximum auroral zone bay activity and is characterized by a large-scale thickening toward higher latitudes that occurs over a broad azimuthal scale and from ionospheric heights to beyond the Vela orbit (r ∼18 RE). A detailed analysis of two-satellite observations during eight plasma sheet recoveries is presented, showing events that occurred within 5 min in widely separated locations at small ( 71° magnetic latitude) of electron precipitation and the westward electrojet that probably corresponded to an expansion poleward of the ionospheric projection of the recovering plasma sheet. The eight plasma sheet recoveries occurred 10–30 min after a Pi 2 burst on the ground, and mid-latitude magnetograms showed no indications of a near-coincident formation of a substorm current wedge in the local time sector of the spacecraft; this indicates that the plasma sheet recoveries and the poleward leaps were not caused directly by a late high-latitude substorm expansion. Instead, these phenomena seem to represent a transition phase between a substorm expansion sequence and a substorm recovery.