Reevaluation of the Magnetic Structure and Evolution Associated with the Bastille Day Flare on 2000 July 14

Abstract

The Bastille Day flare on 2000 July 14 was well observed by several space- and ground-based observatories and studied extensively by many researchers. Recently, we discovered that a large fraction of X-class flares are associated with a very interesting evolutionary pattern in δ sunspots: part of the outer δ spot structure decays rapidly after major flares; in the meantime, central umbral and/or penumbral structure becomes darker. These changes take place in about 1 hour and are permanent. We find that the active region NOAA AR 9077 has sunspot structure change similar to that associated with the 2000 July 14 X5.7 flare. We provide additional evidence supporting that we detected the real change in the sunspot structure after the flare. The new evidence presented in this paper include the following: (1) the Evershed velocity of decayed penumbral segments was weakened significantly following the flare, indicating actual weakening of penumbral structure; (2) based on available vector magnetograms before and after the flare, the transverse field strength decreased at the areas of penumbral decay and increased significantly near the flaring neutral line; (3) a new electric current system is found near the flare neutral line after the flare; and (4) the center-of-mass positions of opposite magnetic polarities converged toward magnetic neutral line immediately following the onset of the flare, and magnetic flux of the active region decreased steadily following the flare. There is no flare model capable of interpreting all the aspects of observations. A simple quadrupolar magnetic reconnection model may explain most of our observations: two magnetic dipoles join at the δ configuration before the flare; magnetic reconnection creates two new sets of loops: a compact flare loop and a large-scale expanding loop that might be the source of the CME. The outer penumbral fields become more vertical due to this reconnection, corresponding to the penumbral decay. Following initiation of magnetic reconnection associated with the flare, reconnected fields near the magnetic neutral line are first enhanced, then gradually weakened as it submerges. However, this model is questionable from one aspect of the observations: we failed to identify two far-end footpoints of this quadrupolar magnetic reconnection. We discuss other existing flare models in the context of our observations as well.