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

Abstract

Major Ha 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/~) at least as great as 4 times the quiet Sun. They were deficient, in the associated occurrence of strong, impulsive, centimetfic 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 Hc~ flare.

An Observational Overview of Solar Flares

“…A small fraction of flares do occur in so-called "spotless" regions (Dodson & Hedeman 1970;Martin 1980), and largescale filament eruptions with flare-like properties can happen anywhere on the quiet Sun (e.g., Harvey et al 1986). It is not yet possible to predict the time or location of a solar flare.…”

Global Properties of Solar Flares

“…In some well-studied cases (e.g., Robbrecht et al 2009) virtually no flare-like emissions can be detected at all. Many events of this kind call to mind the flares in spotless active regions (e.g., Dodson and Hedeman 1970;Harvey et al 1986;Hudson et al 1995). For events of this kind one can imagine that the magnetic environment of the disturbance simply cannot confine the energy release, leading to a relatively free expansion (see Sect.…”

On the Origin of the Solar Moreton Wave of 2006 December 6

“…Moreton waves, large-scale waves observed in Hα emanating from the sites of major solar flares, were first reported 50 years ago (Moreton 1960(Moreton , 1961(Moreton , 1964Moreton & Ramsey 1960;Athay & Moreton 1961;Dodson & Hedeman 1964;Ramsey & Smith 1966;Dodson & Hedeman 1968). Such waves have characteristic speeds of ∼1000 km s −1 and tend to be directional, with angular widths typically in the range from 60 • to 150 • (Smith & Harvey 1971;Warmuth et al 2004a;Veronig et al 2006), although cases with fragmented arcs collectively spanning larger angles have been reported (Pick et al 2005;Balasubramaniam et al 2007;Muhr et al 2008Muhr et al , 2010.…”