Abstract
The stability of the Earth's neutral sheet is analyzed. It is found that for conditions just prior to substorm expansion onset, the cross-field current instability (CFCI) can be triggered. Numerical solution of the dispersion equation shows that the CFCI has a growth time comparable to the substorm onset time. The excited waves have a mixed polarization in the lower hybrid frequency range and propagate obliquely to the local magnetic field. The ensuing turbulence of this instability can lead to a reduction of the cross-tail current and the formation of a substorm current wedge as observed. The CFCI is driven by neutral sheet ions becoming unmagnetized and accelerated by the cross-tail electric field. Heating of both ions and electrons is anticipated from this instability. The scenario for substorm development based on this mechanism is proposed which can account for the skew in local time of substorm onsets, the three conditions in the solar wind under which substorm onsets are observed, pseudo-breakups prior to substorm onsets, local time expansion of the substorm current wedge with discrete westward jump for the evening expansion, and tailward spreading of the current reduction region.
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