Energy Shortage of Nonthermal Electrons in Powering a Solar Flare

Open Access
Publisher Website
Google Scholar
Abstract
Within a deka-keV energy range, the power-law electron beams interacting with the solar atmosphere also result in the power-law bremsstrahlung of hard X-rays. The energy spectrum of electrons can thus be deduced from the observed hard X-ray spectrum, and the total energy carried by accelerated electrons can then be estimated. For quite a long time, one has always assumed the lower energy cutoff (Ec) of the power-law electron beams to be around 20 keV, an assumption that constitutes a main ingredient of the so-called standard picture of a solar flare, since the nonthermal electrons are substantial in powering a solar flare. However, there is in fact no solid observational basis for Ec = 20 keV. Here we present a quantitative method to determine Ec and its application to 14 BATSE/Compton Gamma Ray Observatory hard X-ray events. We find that Ec, varying from 47 to 141 keV in our samples, is on average 76.4 keV. The total energy carried by nonthermal electrons is therefore shown to be at least 1 order of magnitude lower than that derived by taking Ec = 20 keV. This energy shortage of nonthermal electrons in our sample hard X-ray events conflicts with the widely accepted scenario of a solar flare.