Kinetic Ballooning Instability for Substorm Onset and Current Disruption Observed by AMPTE/CCE
Authors: C.Z. Cheng and A.T.Y. Lui
A new scenario of AMPTE/CCE observation of substorm onset and current disruption and the corresponding physical processes is presented. Toward the end of late growth phase, plasma beta increases to greater than or equal to 50 and a low-frequency instability with a wave period of 50-75 seconds is excited and grows exponentially to a large amplitude at the onset of current disruption. At the current disruption onset, higher-frequency instabilities are excited so that the plasma and electromagnetic magnetic field form a turbulent state. Plasma transport takes place to modify the ambient plasma pressure and velocity profiles so that the ambient magnetic field recovers from a tail-like geometry to a more dipole-like geometry. To understand the excitation of the low-frequency global instability, a new theory of kinetic ballooning instability (KBI) is proposed to explain the high critical beta threshold (the high critical beta threshold is greater than or equal to 50) of the low-frequency global instability observed by the AMPTE/CCE. The stabilization is mainly due to kinetic effects of trapped electrons and finite ion Larmor radii which give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension to the ballooning mode. As a result, the high critical beta threshold for excitation of KBI is greatly increased over the ideal-MHD ballooning instability threshold by greater than or equal to O(102). The wave-ion magnetic drift resonance effect produces a perturbed resonant ion velocity distribution with a duskward velocity roughly equal to the average ion magnetic (gradient B and curvature) drift velocity. Higher-frequency instabilities such as cross-field current instability (CCI) can be excited by the additional velocity space free energy associated with the positive slope in the perturbed resonant ion velocity distribution in the current disruption phase.