Kinetic structure of the post plasmoid plasma sheet during magnetotail reconnection

摘要

Ion dynamics during magnetotail reconnection is studied by means of a two-dimensional, large-scale hybrid simulation (macroparticle ions, inertialess electron fluid). The initial setup is a realistic two-dimensional equilibrium with a normal magnetic field component through the current sheet and a Raring lobe field. Reconnection is initiated by a localized resistivity in the near-Earth region. As in MHD, a plasmoid develops and is ejected downtail. The ion kinetic structure in the post plasmoid plasma sheet is studied in detail. In a region of about 10 R-E from the neutral line the ions are demagnetized and are picked up by the electron fluid ejected from the X line. Although the magnetic field in this region is reminiscent of slow mode shocks, no shocks occur, and the structure can be described by a standing large-amplitude whistler. The Hall current leads to a cross-tail magnetic field up to 40% of the lobe field. This may have important; consequences for mapping of low-altitude features into the tail. Further away from the neutral line over a distance up to 30 R-E a thin current sheet develops in which the lobe ions perform quasi-adiabatic orbits. This current, sheet becomes instable and disrupts; the instability is driven by the free energy contained in the non-Maxwellian velocity distributions in the current sheet. In the region of current sheet, breakup the post plasmoid plasma sheet is hot and moves with a bulk speed close to the local Alfven speed. In order to delineate the acceleration and heating process, individual ions are followed during the simulation. There is Iio indication for slow mode shocks in the simulation system within the simulation time. This suggests that slow mode shocks are not to be expected in the geomagnetic tail within several tens of R-E from the X line. [References: 47]