A rapid change in magnetic connectivity observed before filament eruption and its associated flare

To gain insight to the cause of filament eruptions and flares on the Sun, we observed a filament that erupted in active region NOAA 8597. The observations consisted of Ha filtergrams at three wavelengths (line center and 0.5 ?) and line-of-sight magnetograms. All were taken on 1999 June 24 at Big Bear Solar Observatory. We found from the time sequence of Ha images that the filament eruption was preceded by a rapid change in connectivity in a bundle of filament threads. The thread bundle was initially sharply curved near its one end of the filament and suddenly flipped and then became straight in the new orientation. The flipped segment of the thread bundle swept over a 100 # 50 area on the solar surface in about half an hour. At the latter stage of the connectivity change, we observed a downward draining of material along the thread bundle that had a transverse component of 50 km s¹. After that, the filament body split into two parallel parts, one part erupted while the other part remained, and the two-ribbon flare occurred. We also found canceling magnetic features in the vicinity of the initial location of the thread end, which displayed a flux decrease during the Ha connectivity change. Our results show clear and direct evidence that magnetic reconnection takes place in the low atmosphere prior to eruption. This preeruption reconnection seems to be very different from a posteruption coronal reconnection, which is believed to lead to a two-ribbon flare.