The impulsivity of solar flares: concepts and applications

Abstract

During the impulsive phase of solar flares a distribution of nonthermal electrons propagates from the site of energy release towards the lower layers of the solar atmosphere. Depending on their pitch angles, these electrons can get trapped inside coronal loops or precipitate to the chromosphere or photosphere. The whole picture of this scenario is given by the trapping-plusprecipitation model. The relevance of both processes, precipitation and trapping, was estimated via two dimensionless quantities, namely, the impulsivity parameter (IP) and the trapping indicator (TI). They were calculated from HXR and microwave emissions, respectively. The IP and TI were computed in a work sample of 228 flares. HXR and microwaves data were provided by RHESSI and NoRP, respectively. These events were classified according to IP into three impulsivity types: high, medium, and low. This alternative classification of solar flares turned out to be independent of the HXR lightcurve used to measure the IP. On the other hand, the work sample was also classified into three trapping types, according to the values of TI. Such types were: short, average, and prolonged trapping. For events having a single peak in both HXR and microwaves, the trapping types define the regimes where precipitation or trapping dominates. Lastly, it was shown that some active seismically flares can be explained as events where the impulsivity is high (large IP) and magnetic trapping is poorly efficient (small TI). Therefore, this support the hypothesis that sunquakes can be generated by direct impact of energetic electrons