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Determination of Low-Energy Cutoffs and Total Energy of Nonthermal Electrons in a Solar Flare on 2002 April 15

The determination of the low-energy cutoff to the spectrum of accelerated electrons is decisive for the estimation of the total nonthermal energy in solar flares. Because thermal bremsstrahlung dominates the low-energy part of flare X-ray spectra, this cutoff energy is difficult to determine with spectral fitting alone. We have used a new method that combines spatial, spectral, and temporal analysis to determine the cutoff energy for the M1.2 flare observed with RHESSI on 2002 April 15. A low-energy cutoff of 24 ± 2 keV is required to ensure that the assumed thermal emissions always dominate over nonthermal emissions at low energies (<20 keV) and that the spectral fitting results are consistent with the RHESSI light curves and images. With this cutoff energy, we obtain a total nonthermal energy in electrons of (1.6 ± 1) × 1030 ergs that is comparable to the peak energy in the thermal plasma, estimated from RHESSI observations to be (6 ± 0.6) × 1029 ergs assuming a filling factor of 1.