Time variations of the magnetotail plasma sheet at 18REDetermined from concurrent observations by a pair of Vela satellites

Abstract

Energy spectral measurements of electrons (∼100 ev≲Ee⪝18 kev) taken concurrently by hemispherical-plate electrostatic analyzers on Vela satellites 3A and 4A on several passes through the magnetotail plasma sheet during August and September 1967 are reported. The satellites were only ∼6 earth radii (RE) apart in their orbits at that time; thus they were within the plasma sheet simultaneously for several hours during each pass. The measurements showed that the plasma sheet is quite symmetrical about a midplane whose location usually matches, within ∼1 RE, the location of the neutral sheet estimated by the formula of C. T. Russell and K. I. Brody. Both satellites detected a ‘plasma decrease’ (i.e., a pronounced decrease of the energy density of the plasma electrons), associated with nearly every one of the magnetospheric substorms that occurred during the passes. It is shown that these substorm-related ‘plasma decreases’ result from thinning of the plasma sheet, as we have suggested earlier, and not from a flapping motion of it. The thinning process, in which the plasma sheet's thickness is reduced by a factor of about 3 (or possibly more), seems to involve (at 18 RE) not a squeezing or compression of the plasma into a more intense plasma sheet, but a net loss of plasma from that region of the tail. The data suggest that the plasma sheet (at 18 RE) never disappears completely during these events. During quiet periods between substorms, the plasma sheet's boundaries are rather diffuse, the energy density of the plasma falling gradually by a factor of ∼10 between 3 RE and 6 RE from the neutral sheet. However, during the plasma sheet's expansion after a substorm, its boundary is only 2000 to 3000 km thick. A phenomenon that is interpreted as a long-period (∼4 hours) large-amplitude (several RE) flapping motion of the plasma sheet was observed in a pass during a magnetically very quiet period. The flapping motion ceased when a magnetospheric substorm started about midway through the pass.