Filament disappearances during the period of September 1991 through September 1994

Continuous full-disk Hα images recorded by the Big Bear Solar Observatory (BBSO) from 1 September 1991 to 19 September 1994 (the first three years of Yohkoh mission) were digitized and analyzed. The data set consists of nearly 10 000 Hα images, one every half hour for the period when the BBSO was observing. Two statistical studies of the disappearing solar filaments based on this set of data are made: (1) The disk latitude distribution of all larger disappearing filaments with a minimum length of 70 arc sec, including the time of their disappearance. Of the 1095 such filaments, 439 disappeared during our continuous observations, 314 disappeared during the BBSO night gap, 162 disappeared during data gap (more than 94 hours) and 180 rotated beyond the west limb. If we plot latitudes as a function of time for all these disappeared filaments, it shows a uniform distribution in latitude. However, if we plot the distribution of larger disappeared filaments (200 arc sec or above), then the butterfly trend appears - position of filaments tends to drift to lower latitude as solar activity decreases. (2) The disk distribution of all detectable disappearing filaments, large and small, for the 9-months period, January 1994 to September 1994. We find that the size distribution of 351 collected disappeared filaments follows a power law with a power index of -1.40.