DYNAMICS OF THIN CURRENT SHEETS AND THEIR DISRUPTION BY BALLOONING INSTABILITIES - A MECHANISM FOR MAGNETOSPHERIC SUBSTORMS

Multipoint satellite observations indicate the development of thin cur rent sheets and an impulsive intensification of the cross-tail current density in the growth phase at near-earth distances during a short in terval (< 1 min) just before onset, after a period of sluggish growth (similar to 0.5-1.5 h). These multiple time scales are accounted for b y analysis and two-dimensional magnetohydrodynamic simulation of the;m agnetotail in the high-Lundquist number regime, including the earth's dipole held. Zn the slow growth phase, a thin current sheet develops s panning Y points that stretch from the midtail region (similar to 30R( E)) to the near-earth region (similar to 10R(E)). This is followed by an impulsive enhancement in the current sheet amplitude due to flux pi leup, consistent with observations. The stretched magnetotail with an embedded thin current sheet is found to be unstable to an ideal compre ssible ballooning instability with rapid spatial variation in the dawn -dusk direction. The linear instability is demonstrated by numerical s olutions of the ideal ballooning eigenmode equation for a sequence of two-dimensional magnetotail configurations containing a thin current s heet, realized during the impulsive growth phase. Line-tied boundary c onditions are imposed at the ionosphere, and shown to have a strong in fluence on the linear stability of ballooning modes at near-earth dist ances. It is suggested that the ideal ballooning instability provides a possible mechanism for disrupting the cross-tail current at substorm onset. (C) 1998 American Institute of Physics.