LOWER-HYBRID-DRIFT INSTABILITY OPERATIVE IN THE GEOMAGNETIC TAIL

Motivated by recent interests in the current-driven instability as a plausible trigger mechanism for current disruption in the magnetotail, an extensive linear analysis of the lower-hybrid-drift instability (LHDI) in a high plasma beta regime is conducted. In the theoretical formulation, the fundamental difference between this instability and the modified-two-stream/ion-Weibel instability is clarified. Numerical solutions of the dispersion equation for the LHDI are obtained for plasma parameters appropriate for the magnetotail. In spite of the tendency for the LHDI to be stabilized at high plasma beta, the near-Earth current sheet environment immediately before the onset of current disruption is found to be just above the unstable threshold of the LHDI. The stability analysis is further extended to a model two-dimensional current sheet of Lembege and Pellat, with parameters chosen to match the near-Earth current sheet at the critical time. It is found that the LHDI is operative over an entire current sheet, thus providing theoretical support for the plausibility that the LHDI may play a significant role in triggering current disruption in the magnetotail.