Double-adiabatic MHD theory of a thin filament in the geotail and possible applications to bursty bulk flows and substorms.

摘要

During fast fluid flows in Earth's magnetotail, the plasma distribution function often takes the form of one beam flowing through another, which raises the question of whether Bursty Bulk Flows (BBF's) can reasonably be represented in terms of single fluid magnetohydrodynamics (MHD), either in global MHD codes or in thin-filament theory. An exact kinetic solution is compared with exact fluid solutions for a simplified case of cold, collisionless particles in a pipe, under conditions where there are counter-streaming beams similar to the ones that often occur in Earth's magnetotail. The results from kinetic theory differ from standard fluid theory but are exactly consistent with Chew-Goldberger-Low double-adiabatic fluid theory. Double-adiabatic MHD equations are derived for the motion of a thin filament through a medium. Simulation results are presented for a double-adiabatic filament that starts out with lower gas pressure than nearby flux tubes and also for plasma ejected earthward from a patch of reconnection at X ∼ -25 RE. As in earlier calculations for the isotropic case, in both cases the near-equatorial part of the filament moves rapidly earthward. A compressional shock wave forms in the filament near the equatorial plane and propagates earthward. The near-equatorial region of the filament exhibits characteristics similar to a flow burst, while the behavior far from the equatorial plane resembles that of earthward-streaming plasma-sheet boundary layer. In both cases, the double-adiabatic filament becomes firehose unstable after the shock wave reflects from the earthward boundary of the simulation and propagates back into the tail. The tailward-propagating compressional wave, which brakes the earthward flow in the filament, is thus characterized by strong magnetic fluctuations. Within the context of the Near-Earth-Neutral-Line model of substorms, we suggest that firehose instability might cause the intense magnetic-field fluctuations that are observed in the inner plasma sheet at substorm onset. Additional simulations have been carried out to confirm the robustness of our principal conclusion that fast earthward flows in the Earth's plasma sheet should lead to firehose instability.