Time‐dependent flows in the renovated model of the magnetosphere

Abstract

A revised empirical model of the magnetosphere, based on observations of Birkeland currents, is presented with earthward flow in the plasma sheet confined to the duskside. Convection (E × B drift) carries tail‐associated features in the sunward direction. This competes with Birkeland‐current‐associated plasma depletion (divergence of ion drifts), which allows flux tubes to collapse to a more dipolar form, thereby moving tail‐associated features in the antisunward direction. In substorm intensifications, depletion dominates, and the taillike region recedes from the earth. The recession appears to be unstable. A quantitative relationship between Birkeland currents and convection is developed and is used on published results. It indicates that 4×1020 ions drift off a plasma sheet flux tube as it collapses from taillike to dipolar. Application of the quantitative relationship to a published set of presubstorm and expansion data raises the following points: (1) Consistency is high, suggesting that the assumptions of hot protons, cold electrons, and a pressure‐dominated equilibrium apply in the plasma sheet. (2) The breakup instability condition is that the taillike region is just beginning to recede. (3) The changes at breakup can be interpreted as either the breakdown of the Birkeland current sheet approximation or the formation of a near‐earth neutral line.