Fast magnetic reconnection supported by sporadic small-scale Petschek- type shocks

Magnetic reconnection is a process of changing connectivity of magnetic field lines, and thought to play a core role in explosive magnetic energy conversion events observed in magnetospheric substorms, solar flares, and tokamak disruptions. According to the classic Sweet-Parker theory, it is, however, problematic to conduct magnetic reconnection efficiently enough in a highly conductive plasma such as in the solar corona. Therefore, Petschek proposed another reconnection mechanism, in which small magnetic diffusion region enables fast reconnection while the energy conversion itself occurs in slow mode MHD shocks. However, recent numerical simulations indicate that Petschek reconnection is not stable in a system with spatially uniform resistivity. Some mechanism such as anomalous resistivity or kinetic physics is needed to sustain the localized diffusion region. It is, therefore, not yet clear how fast reconnection realizes in the reality. In order to address this problem, we performed 2-D resistive MHD simulation with a very high spatial resolution. It is found that small-scale slow mode MHD shocks predicted by Petschek spontaneously form (even under a uniform plasma resistivity) as a result of the secondary tearing (plasmoid) instability of the reconnecting current sheet. In this process, fast motion of large plasmoids in the current sheet play a role of the required localization in the diffusion region, so that slow mode shocks can form in front of the moving plasmoids. Thus, the rate of reconnection is intermittently and repeatedly enhanced up to 0.02 of the Alfven speed, which is sufficient to explain, for example, the time-scale of solar flares. Furthermore, our simulation suggests that the effective reconnection rate doesn't depend on the Lundquist number of a system. Therefore, this is quite a universal mechanism of fast magnetic reconnection. A part of this study is already published in Shibayama et al., Physics of Plasmas, 22, 100706, 2015.