Prolonged tailward flow of plasma in the thinned plasma sheet observed atr≈ 18REduring substorms

Abstract

Measurements of plasma and energetic electrons made during substorms by Vela satellites within ≲1 RE of the magnetotail neutral sheet are reported. Rapid antisunward flow of plasma is observed to start at the onset of the expansion phase of a substorm (i.e., at breakup) and to continue until the expansion phase ends and substorm recovery begins, as is indicated by auroral zone magnetograms. When substorm recovery begins, the plasma flow reverses direction, and rapid sunward flow begins. Fluxes of energetic electrons are low, and the temperature of the plasma is reduced during the interval of antisunward flow. The lowest fluxes (i.e., complete dropouts) of energetic electrons are observed just before the reversal of plasma flow to the sunward direction. After the reversal the energetic electron flux intensity rises rapidly to high values as the plasma sheet recovers. These observations are regarded as added evidence that a neutral line forms between the earth and the Vela orbit (r ≈ 18 RE) at substorm breakup and then moves suddenly to distances beyond the Vela orbit when substorm recovery starts. Throughout the expansive phase there exists a thin residual plasma sheet beyond the neutral line in which plasma continually flows rapidly downstream into the more distant tail. The particle dropout just preceding the flow reversal and plasma sheet recovery is thought to be the signature of the passage of the neutral line near the Vela satellite as it moves rapidly outward in the tail. Spin modulation of the proton flux observed continuously for about 1 hour during very quiet conditions preceding one substorm may constitute a direct measurement of the proton component of the cross-tail electric current that maintains the taillike configuration of the magnetotail field.