Reversal of magnetic field rotation in the reconnection layer due to shear flow effects

We investigate the effects of shear flows on the so-called "component reconnection," in which the guide field B-y not equal 0, by solving a one-dimensional Riemann problem for magnetopause reconnection using a resistive MHD simulation. Specifically, we consider the existence of a shear flow perpendicular to the antiparallel magnetic field B-z, while a finite shear flow tangential to B-z may also be present. In the cases without a magnetosheath flow and having thus a sheared flow across the reconnection layer, two time-dependent intermediate shocks TDIS and TDIS' are present on the magnetosheath side and the magnetospheric side, respectively, and the strength of TDIS is much stronger than that of TDIS'. Nevertheless, the existence of the shear flows modifies the structure and strength of the time-dependent intermediate shocks significantly. (1) The perpendicular shear flow V-y0 can lead to the reversal of the rotation sense of the tangential magnetic field in time-dependent intermediate shocks. The critical shear flow speeds, V-c and V-c', for the reversal of field rotations in TDIS and TDIS', respectively, are calculated. (2) For shear flow speed V-y0 = V-c, the strong TDIS is replaced by a steady intermediate shock (IS), whereas at V-y0 = V-c' an Alfven wave pulse is present in the reconnection layer. (3) The presence of tangential shear flow V-z0 alters not only the strength of TDIS and TDIS' but also the critical speeds V-c and V-c'. The critical shear flow speeds obtained from our simulation are found to agree very well with those from the ideal magnetohydrodynamics (MHD), in which the time-dependent intermediate shocks are replaced by rotational discontinuities.