Paper#: 133
Poster #:
Session Name: Workshop 4C: Waves and Instabilities
Room: 200ABC
Day: Wednesday
Time: 10:25-10:50 a.m.
Abstract Title: Kinetic Ballooning Instability as a Mechanism for Substorm Onset and Magnetic Field Dipolarization
PresentSurname: Cheng, C.Z.
All Authors: C.Z. Cheng, A.T.Y. Lui
Abstract : The observational features of substorm onset processes, and a kinetic ballooning instability (KBI) theory for substorm onset and its nonlinear consequence for causing plasma transport and magnetic field dipolarization are presented. Based on the AMPTE/CCE observations, a low frequency instability with period of about 1 minute is identified near the end of the late growth phase before the substorm is initiated. The current disruption and the magnetic field dipolarization begin as the amplitude of the low frequency instability becomes large. During the substorm expansion phase, higher frequency instabilities strongly couple with the low frequency instability to form a plasma turbulence state. Based on these observations, we propose a theory of KBI which involves multiscale coupling of different spatial and temporal scales due to kinetic effects of finite ion Larmor radii and fast trapped electron dynamics. We demonstrate how a large amplitude KBI can produce a duskward ion drift and its role in the physical mechanism of the substorm onset. The KBI also has a parallel electric field to accelerate electron and thus a field-aligned current in the auroral zone. Moreover, we propose a plasma transport theory due to turbulence resulting from KBIs and higher frequency instabilities with periods of a few seconds. The turbulence can cause plasma pressure profile (averaged over the turbulent fluctuation scales) to evolve so that the magnetic field dipolarization occurs and is accompanied by a corresponding change in dawn-dusk electric field. Other outstanding issues of substorm mechanisms will also be discussed.