Evolution of the Earth's distant magnetotail: ISEE 3 electron plasma results

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Abstract
Electron plasma measurements made by ISEE 3 from October 18, 1982, through April 19, 1983, were examined for signatures identifying the spatial evolution of the earth's magnetotail out to distances of ∼225 RE. Nearly 91,000 individual electron spectra were combined into a data set in which each point was first labelled as being magnetosheath, lobe, or plasma sheet and then separated into one of four radial‐distance bins. The lobe density distribution shows two peaks near earth (≤ 60 RE), a narrow peak at ∼10−2 cm−3 representing quiet lobe, and a second broader peak above 10−1 cm−3 representing boundary layer phenomena. At larger distances the lower density peak disappears and the remaining density peak (at ∼10−1 cm−3) becomes broader and contains a high density tail which extends well above 1 cm−3. The increase in lobe density at large distances is believed to be due to plasma which crosses the magnetopause on open field lines along its flanks and drifts inward toward the plasma sheet. The plasma sheet density distribution shows only a slight decrease in density at larger radial distances. The temperature of the plasma sheet and lobe electron plasma decreases with increasing distance, with the plasma sheet temperature decreasing at a faster rate. Evolution in the density and temperature of the electron plasma in the magnetotail results in a situation wherein it is often difficult to separate plasma sheet from lobe plasma at large radial distances by these two parameters alone. The bulk flow velocity of the plasma sheet and lobe plasma increases dramatically at larger radial distances. The electron bulk velocity observed in the plasma sheet beyond 180 RE often exceeds the normal solar wind velocity range (300–700 km/s). The most striking result of this study is the observed evolution of the electron bulk velocity flow angle from a bimodal distribution near earth to a single tailward directed peak beyond 180 RE. The decreasing amount of earthward flow with increasing distance directly implies that the location of the distant neutral line is usually within 120 RE of earth, and rarely beyond 180 RE. These observations also imply that the location of the neutral line is variable and dependent upon local interplanetary and geomagnetic conditions.