Substorm variations of the magnetotail plasma sheet fromXSM≈ −6REtoXSM≈ −60RE

Abstract

We report a study of the substorm-related variations of the magnetotail plasma sheet. The study uses data, much of it previously published, obtained by Vela and Imp satellites in the range −6 RE > XSM > −60 RE, where XSM is geocentric distance measured along the solar magnetospheric x axis. Evidence is presented that the thickening or recovery of the plasma sheet, which has been shown in Vela satellite measurements (at γ ≈ 18 RE) to occur late in substorms, is causally related to a rapid poleward shift or ‘leap’ of the principal current of the auroral electrojet evidenced by recovery of magnetic bays at auroral latitudes (65° ≲ λm ≲ 70°) and their onset at low polar cap latitudes (e.g., λm ≈ 74°). That is, it appears to be inaccurate to regard the thickening of the plasma sheet in the far magnetotail as a process that commences near the earth at the onset of the expansive phase of a substorm and moves more or less uniformly out into the tail as the expansive phase of the substorm evolves. A schematic description of the responses of the plasma sheet to a substorm, based on this study and previous studies, is presented.