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 allow 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 tht 4¿1020 ions drift off a plasma sheet flux tubes as it collapes 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 can be interpreted as either the breakdown of the Birkeland current sheet approximation or the formation of a near-earth neutral line.