Major H alpha flares in centers of activity with very small or no spots

TLDR: In the years 1956-1968 these flares represented ∼ 7% of all confirmed flares of importance ⪖ 2 as mentioned in this paper, in which the flares were of unusually long duration and rose to maximum intensity slowly.

Abstract: Major Hα flares (importance ⪖ 2) in plages with only small or no spots constitute a rare but well observed aspect of solar activity. Information relating to 83 such flares has been assembled and studied. In the years 1956–1968 these flares represented ∼ 7% of all confirmed flares of importance ⪖ 2. In general, the flares were of unusually long duration and rose to maximum intensity slowly. A flash phase was often absent or poorly defined. In a number of cases, the flare emission included two bright filaments more or less parallel. The flares usually occurred during the late, flare-poor phase of a center of activity, and their outbreak did not presage a resurgence of activity in subsequent rotations. The flares were frequently associated with the position of dark filaments.