Reconnection in semicollisional, low- plasmas

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Reconnection of semicollisional, low- plasmas is studied numerically for two model problems using a two-field description of the plasma including electron pressure effects (and hence kinetic Alfvén-wave dynamics). The tearing unstable Harris sheet, with the global parameters of the Geospace Environment Modeling-challenge case, shows a linear growth of the peak reconnection rate with the drift parameter _s when this scale is significantly larger than the resistive skin depth, and the island is smaller than the Harris sheet current layer width. As exemplary for a driven, rather than a spontaneous reconnection situation we study as second model system two coalescing islands, starting from a nonequilibrium situation. The peak reconnection rate again increases initially linearly with _s but saturates and becomes _s independent for larger values. In this saturated regime, no flux pileup occurs, and the reconnection is limited by the rate of approach of the two coalescing islands. The qualitative differences between spontaneous and driven reconnection cases and their scaling behavior are best understood by considering the reconnection rate as a triple product of outflow Mach number, outflow to inflow channel width ratio, and magnetic energy density at a height _s above the X point. ©2009 American Institute of Physics