Simulation study of the Riemann problem associated with the magnetotail reconnection

The structure of reconnection layer in the distant magnetotail is studied by solving the Riemann problem for the evolution of an initial current sheet using one-dimensional MHD and hybrid simulations. Initially, the current sheet with total pressure balance separates the plasmas and fields in the two lobes. In the presence of a nonzero normal magnetic field, which is due to the magnetic reconnection, the initial current sheet evolves to form the reconnection layer, which consists of various MHD discontinuities. First, we study the symmetric case with equal magnetic field strengths, equal plasma densities, and exactly antiparallel magnetic fields (By=0) in the two lobes. The Petschek (1964) reconnection layer which contains two switch-off slow shocks, whose intermediate Mach number MI=1, is obtained. In the hybrid simulation, the switch-off shock possesses a large-amplitude, left-hand-polarized rotational wave train of magnetic field in the downstream region. Each slow shock propagates to either lobe in the magnetotail. A strong temperature anisotropy with T∥>T⊥ appears in most region of the reconnection layer. This is due to the interpenetrating of ions between the two lobes and the backstreaming of ions from the downstream of each slow shock to the lobe. Second, the presence of a finite guide field in lobes (By≠0) is considered in the simulation. It is found that the slow shocks becomes nonswitch-off shocks with an intermediate Mach number MI<1, and two rotational discontinuities are also present in the hybrid simulation to bound the reconnection layer from, respectively, the two lobes. In addition, our hybrid simulations show that the presence of the finite By destroys the coherent wave train in the reconnection layer if the lobe By0≥0.08Bx0. Finally, the simulations are extended to the asymmetric cases in which the plasma densities and/or the magnetic fields in the two lobes are unequal. For asymmetric cases with By=0 in the two lobes, it is found that an intermediate shock and a nonswitch-off slow shock are present in the reconnection layer and that the large-amplitude rotational wave structure does not exist in the entire reconnection layer if the density ratio between the two lobes N1/N0 ≥ 1.5, where N0 and N1 are the ion number densities in the two lobes, receptively. For general cases with both density asymmetry and By0≠0, two rotational discontinuities and two slow shocks are present in the reconnection layer, and the critical value of the density ratio, above which the coherent wave train does not exist, is reduced. The absence of a downstream wave train for the slow shocks observed in the magnetotail may be associated with the presence of a finite By0.

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