A qualitative model of magnetic field reconfiguration as might result from neutral line formation in the central plasma sheet late in a substorm growth phase is considered. It is suggested that magnetic reconnection probably begins before the substorm expansion phase and that cross-tail current is enhanced across the plasma sheet both earthward and tailward of a limited region near the neutral line. Such an enhanced cross-tail current earthward of the original X line region may contribute to thinning the plasma sheet substantially, and this would in turn affect the drift currents in that location, thus enhancing the current even closer toward the Earth. In this way a redistribution and progressive diversion of normal cross-tail current throughout much of the inner portion of the plasma sheet could occur.

The resulting intensified current, localized at the inner edge of the plasma sheet, would lead to a very thin plasma confinement region. This would explain the very tailike field and extreme particle dropouts often seen late in substorm growth phases. When the currents cannot be further enhanced within the inner plasma sheet (and when diffuse electron precipitation has sufficiently increased ionospheric conductivities), an intense (>106 A), very rapid increase of current flow through the auroral ionosphere would be expected. This latter phase, in combination with magnetic reconnection reaching field lines of the plasma sheet boundary layer and the tail lobes, would constitute the substorm expansion phase in this scenario. ¿ American Geophysical Union 1990