Hard X-ray and radio investigations prior to or during the impulsive phase of solar flares

A comparative study of radiation in the different spectral ranges, including X-ray and radio observations, can establish constraints for the electron acceleration/injection mechanisms. Such investigation, focusing on the activity prior to and during the impulsive phase of solar flares, is presented. Observations provide evidence for electron acceleration prior to the impulsive phase. The association between type-III groups and hard X-ray bursts becomes more apparent with increasing starting frequency of the former observed during the impulsive phase. It is shown that pure type-III burst groups, when they are associated with X-rays, do not correspond to an intense X-ray emission. In contrast, the type-III/V events can be associated with strong X-ray emission. Radioheliograph observations place constraints on the geometry of the injection/acceleration site. 12 references