Remote flare brightenings and type III reverse slope bursts

Abstract

We present two large flares which were exceptional in that each produced an extensive chain of Hα emission patches in remote quiet regions more than 10⁵ km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.

The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote Hα emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote Hα brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote Hα brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote Hα chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote Hα brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT∼1 keV) which immediately follow the RS burst electrons along the same loops.