Abstract
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.
Highlights
Thin current sheets (TCSs) are widely observed in natural plasma systems from the solar corona to Earth’s magnetotail, especially as a precursor to eruptive episodes of global energy release
TCSs mainly manifest themselves as embedded structures in the central plasma sheet (CPS) during the growth phase of substorm (Sergeev et al, 1993; Pulkkinen et al, 1999)
A new mode of thin current sheet dynamics was investigated in this paper
Summary
Thin current sheets (TCSs) are widely observed in natural plasma systems from the solar corona to Earth’s magnetotail, especially as a precursor to eruptive episodes of global. TCSs are often observed to have a thickness comparable to ion gyroradius (∼1000 km); at this scale, magnetic field can slip through the ion fluid owing to the breakdown of the frozen-in-flux condition. Saito et al (2008a, b) showed that local current disruptions (CDs) are preceded by several minutes of wave activity in the 10–20 mHz range, which they attributed to the ballooning instability. Liang: Electrostatic field and current disruption analysis, Liu et al (2008) and Liang et al (2009) were able to identify that the mode directly responsible for local CD was not the KB mode but a new mode (referred to as the CD mode) occurring at a higher frequency in the 20–100 mHz range. How the theory fits in the larger context and some of its limitations are described in the discussion section
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