Cycle Variation in the Occurrence of Great Soft X-ray Solar Flares

RESUMO

Powerful nonstationary processes such as flares and coronal mass ejections are detected mainly near the solar cycle maxima. However, the analysis of the great flares since 1975, when the GOES monitoring started, leads us to more precise conclusions. We have traced the latitudinal distribution of X-class flares and their dependence on the phase of the cycle throughout four cycles. Besides, we compared directly their positions with the magnetic field structure. It was found that the relatively weak X4--7 flares occur at relatively high latitudes (15 -- 20 degrees), where two waves of activity converge, one moving toward the equator and the other, the wave of the following cycle, directed poleward. Such events are observed almost constantly 1 -- 2 years before the cycle maximum. At the same time, the number of the powerful X-ray flares increases sharply during the maximum phase, but the greatest X10 flares are observed at the beginning of the decline phase, where wave interactions still persist, and throughout the decline phase. Note that they are virtually absent during the cycle growth phase. Thus, we conclude that the greatest X-ray flares begin to appear 1 -- 2 years before the maximum number of sunspots in the overlapping phase, when different kinds of activity waves coexist on the Sun at relatively high latitudes, and, then, continue to appear at the boundary, which separates the wave of local fields and the poleward wave of the following cycle.