Numerical experiments of fast magnetic reconnection driven by the rapidly expanding plasma

Three modes of magnetic reconnection, flux pile-up, Sonnerup, and hybrid, are examined in the context of driven magnetic reconnection via 2D and 2.5D magnetohydrodynamic (MHD) numerical simulations. They result from variances in gas pressure and magnetic field strength in the reconnection inflow region. The simulation demonstrates that the Spitzer diffusion region of magnetic reconnection is not just an X-point; instead, it appears as a slim and elongated current sheet that creates two pairs of the slow-mode shock (SS) on either end. These shocks contribute to forming four boundaries that separate the inflow from the outflow. In the regions far from the Spitzer diffusion region, two sets of rotational discontinuity (RD) stand inside the SSs and form the combination of SS and RD structures. The RDs reverse the magnetic field inside the reconnection outflow region, and create a W-shaped magnetic field in that region. The scenario that the rotation of the magnetic field is not caused by an intermediate wave, and the SS is located outside the RD, is consistent with the inference of Priest (Mon. Not. R. Astron. Soc. 159, 389 (1972)), and is contrary to that of Petschek and Thorne (Astrophys. J. 147, 1157 (1967)) and Vasyliunas (Rev. Geophys. Space Phys. 13, 303 (1975)).