Physical mechanism of spontaneous fast reconnection evolution

Large dissipative events, such as solar flares and geomagnetic substorms, r esult from sudden onset of magnetic reconnection, so that it is a long-stan ding problem to find the physical mechanism that makes magnetic reconnectio n explosive. As recognized by Petschek, standing slow shocks enable the eff ective magnetic energy conversion in space plasmas of extremely large magne tic Reynolds number. Hence, a basic question is how the fast reconnection m echanism involving slow shocks can be realized as an eventual solution? We have proposed the spontaneous fast reconnection model, which describes a ne w type of nonlinear instability that grows by the positive feedback between plasma microphysics (current-driven anomalous resistivity) and macrophysic s (global reconnection flow). It is demonstrated that the fast reconnection mechanism explosively grows by the positive feedback in a variety of physi cal situations; for the larger threshold of anomalous resistivity, the fast reconnection evolves more drastically. Also, distinct plasma processes, su ch as large-scale plasmoid and magnetic loop dynamics, result directly from the fast reconnection evolution. Even in general asymmetric situations, th e spontaneous fast reconnection model effectively works, giving rise to dra stic magnetic flux transfer.