Invited review abstract

The magnetic reconnection is considered to be the main engine of the energy release in solar flares. The structure of the diffusion region is, however, not still understood under the circumstances with enormously large magnetic Reynolds number as the solar corona. In this project, temporal evolution of a current sheet with initial perturbations is studied by using the three-dimensional resistive magnetohydrodynamic (MHD) simulations. Our results are as follows: (1) In the early phase of the evolution, large-wavenumber modes in the z-direction are excited and grow. (2) Many "X"-type neutral points (lines) are generated along the magnetic neutral line (plane) in the current sheet. When they evolve into the non-linear phase, three-dimensional structures in the z-direction also evolve. The spatial scale in the z-direction seems to be almost comparable with that in the xy-plane. (3) The dependence on the magnetic Reynolds number of the reconnection rate was M_A propto Rm^{-1/2}, namely the Sweet-Parker scaling. (4) By adding guide fields, significant change in the process occurred, which causes several times enhancement in the energy release rate. This is interpreted as due to mutual driving effect on the reconnection flow by magnetic islands in the current sheet.