Double‐adiabatic MHD theory for motion of a thin magnetic filament and possible implications for bursty bulk flows

Abstract

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 can reasonably be represented in terms of single‐fluid magnetohydrodynamics, 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 counterstreaming 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. An initial simulation is presented of a double‐adiabatic filament that starts out with lower gas pressure than nearby flux tubes. As in earlier calculations for the isotropic case, 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. 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.