Disruption of magnetospheric current sheet by quasi-electrostatic field

Abstract

Recent observational evidence has indicated that local current sheet disruptions are excited by an external perturbation likely associated with the kinetic ballooning (KB) instability initiating at the transition region separating the dipole- and tail-like geometries. Specifically a quasi-electrostatic field pointing to the neutral sheet was identified in the interval between the arrival of KB perturbation and local current disruption. How can such a field drive the local current sheet unstable? This question is considered through a fluid treatment of thin current sheet (TCS) where the generalized Ohm's law replaces the frozen-in-flux condition. A perturbation with the wavevector along the current is applied, and eigenmodes with frequency much below the ion gyrofrequency are sought. We show that the second-order derivative of ion drift velocity along the thickness of the current sheet is a critical stability parameter. In an E-field-free Harris sheet in which the drift velocity is constant, the current sheet is stable against this particular mode. As the electrostatic field grows, however, potential for instability arises. The threshold of instability is identified through an approximate analysis of the theory. For a nominal current sheet half-thickness of 1000 km, the estimated instability threshold is E~4 mV/m. Numerical solutions indicate that the two-fluid theory gives growth rate and wave period consistent with observations.