Magnetotail plasma flow during substorms: A survey with Imp 6 and Imp 8 satellites

Abstract

This paper presents the results of a statistical analysis of substorm‐associated plasma flow in the sector of the magnetotail within (−18 RE >XSM > −35 RE;|YSM| ≤ 12 RE; |dZSM| ≤2.5 RE). This ‘substorm analysis’ used data from the Los Alamos Scientific Laboratory plasma instruments on the Imp 6 and Imp 8 spacecraft and was keyed to substorms identified primarily in magnetograms from individual auroral zone stations. We find that a sudden establishment or enhancement of flow in a persisting direction is observed at the onset of about one half of occurring substorms. About 80 percent of these sudden changes produce plasma flow in the tailward direction and only 20 percent produce earthward flow. These results seem more consistent with a neutral line model of substorms (e.g., the tearing theory) that predicts tailward plasma flow tailward of a near‐earth ‘substorm neutral line’ than with a rarefaction wave model that does not envision neutral line formation and that predicts solely earthward plasma flow in the plasma sheet during substorms. We also repeated the ‘thinning analysis’ previously used by other authors, that is keyed to occurrence of plasma sheet thinning during periods when the AE index is enhanced, and for which there is no restriction on dZSM. This analysis did not reveal the preponderance of earthward flow during plasma sheet thinning that the other authors reported. Rather, it showed nearly equal occurrence of earthward and tailward flows, with some preference for tailward flows within |dZSM| < 2.5 RE.