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

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.

Like major flares in general, the flares in regions with small or no spots usually were associated with long-enduring radiation (gradual rise and fall and/or postburst increase) at ∼ 10 cm, and with X-ray enhancements (2–12 Å) at least as great as 4 times the quiet Sun. They were deficient, in the associated occurrence of strong, impulsive, centimetric bursts and of X-ray events > 20 times the quiet Sun. The absence of large spots apparently did not inhibit the occurrence of Type II bursts.

Only 41% of the major flares here studied were accompanied by shortwave fades and of these ionospheric disturbances only a few were great events. In general the flares were not followed by the detection of high energy particles or the onset of geomagnetic storms. However, a few of the flares (including those of 1967 January 11 and February 13) apparently were associated with well observed particle emission and suggest that the presence of a large complex spot is not always necessary for the acceleration of energetic particles or the emission of solar plasma at the time of a large Hα flare.