THE SUNSPOTLESS FLARE ON APRIL 12, 1991 AND THE EVOLUTION OF THE NEIGHBORING FILAMENTS

On April 12, 1991, a 2N sunspotless flare was observed by using the sp ectra-spectroheliograph at Yunnan Observatory. H-alpha filtergrams and sequence H-alpha and H-gamma spectra were obtained. In this paper, th e morphology of the sunspotless flare and the evolution of the nearby filaments are studied. The results show that the arch-shaped filament which appears on the neutral line to separate the active region from t he solar quiet region is activated several days before the onset of th e sunspotless flare, and disappears several hours before the onset. It is suggested that the sunspotless flare is associated with the filame nt, i.e. its activation, disparition brusque and so forth. The sunspot less flare is of unusual long duration (about 80 min in the H-alpha em ission) and exhibits a slow rise to maximum intensity (about 20 min in the H-alpha emission) i.e. a 'long duration effect' and a 'gradual ri se-and-fall effect' in both the H-alpha emission and the soft X-ray ra diation, and the flash phase is poorly defined or absent. The energy r eleased by the sunspotless flare may come from the magnetic energy sto red in the corresponding plage region. The velocity along the line-of- sight is small for the sunspotless flare (less than several kilometers per second), but rather large for the nearby corridor filament (about a hundred km s(-1)). The horizontal extension velocity of the sunspot less flare is not large. Compared with the general major H-alpha flare of the same importance, the sunspotless flare is relatively gentle.