The rate of magnetic reconnection observed in the solar atmosphere

The structure of the solar corona is often observed to evolve in a manner that is generally attributed to the process of magnetic field line reconnection. The spatial and temporal scales for a variety of reconnection events have been examined on the basis of descriptions in the scientific literature. These events tend to fall into two general categories. In the first category, reconnection proceeds as the corona adjusts itself to the emergence of new magnetic flux. In the second category, the field lines have been explosively blown open by a hare or mass ejection, and the field lines eventually return to a more potential field configuration by means of reconnection. Estimates of the density, temperature, and magnetic field strength in the reconnection volume provide a basis on which to calculate the reconnection rate expected for resistive diffusion, tearing mode reconnection, and reconnection at the Alfven speed. The observed temporal and spatial scales of the reconnection events indicate that the rate of reconnection is on the order of 0.001-0.1 of the Alfven rate, the Alfven speed divided by the observed length scale. This indicates that the effective Reynolds number for large-scale structures in the corona is of the order of 10-1000. In none of the cases examined does the reconnection appear to be driven by external flows, but the process is more properly described as the relaxation of nonpotential magnetic field configurations toward a potential configuration.