On the lower energy cutoff of nonthermal electrons in solar flares

A quantitative method to determine the lower energy cutoff (E,) of power-la w electron beams is established. We apply this method to the 54 hard X-ray events observed with BATSE/Compton Gamma Ray Observatory (CGRO). The result s show that about 75% of the observed broken double power-law spectra of ha rd X-rays can be explained by a lower energy cutoff in the power-law electr on beams. The values of E,, varying among the flares, are all greater than the usually accepted 20 keV! On average, E-c is about 69 keV. So high a low er energy cutoff of nonthermal electrons implies that nonthermal electrons might not be as important in powering solar flares as was previously though t. Further significance of this finding is discussed.