Statistical Study of the Reconnection Rate in Solar Flares Observed with Yohkoh SXT

摘要

We report a statistical study of flares observed with the Soft X-Ray Telescope (SXT) on board Yohkoh in the year 2000. We measure physical parameters of 77 flares, such as the temporal scale, size, and magnetic flux density, and find that the sizes of flares tend to be distributed more broadly as the GOES class becomes weaker and that there is a lower limit of magnetic flux density that depends on the GOES class. We also examine the relationships among these parameters and find weak correlation between the temporal and spatial scales of the flares. We estimate reconnection inflow velocity, coronal Alfvén velocity, and reconnection rate using the observed values. The inflow velocities are distributed from a few km s-1 to several tens of km s-1, and the Alfvén velocities in the corona are in the range from 103 to 104 km s-1. Hence, the reconnection rate is 10-3 to 10-2. We find that the reconnection rate in a flare tends to decrease as the GOES class of the flare increases. This value is within 1 order of magnitude of the theoretical maximum value predicted by the Petschek model, although the dependence of the reconnection rate on the magnetic Reynolds number tends to be stronger than that in the Petschek model.