The Multi-instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Nonthermal Emission

Abstract

Abstract Solar flare X-ray spectra are typically dominated by thermal bremsstrahlung emission in the soft X-ray (≲10 keV) energy range; for hard X-ray energies (≳30 keV), emission is typically nonthermal from beams of electrons. The low-energy extent of nonthermal emission has only been loosely quantified. It has been difficult to obtain a lower limit for a possible nonthermal cutoff energy due to the significantly dominant thermal emission. Here we use solar flare data from the extreme ultraviolet Variability Experiment on board the Solar Dynamics Observatory and X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager to calculate the Differential Emission Measure (DEM). This improvement over the isothermal approximation and any single-instrument DEM helps to resolve ambiguities in the range where thermal and nonthermal emission overlap, and to provide constraints on the low-energy cutoff. In the model, thermal emission is from a DEM that is parameterized as multiple Gaussians in Log(T). Nonthermal emission results from a photon spectrum obtained using a thick-target emission model. Spectra for both instruments are fit simultaneously in a self-consistent manner. Our results have been obtained using a sample of 52 large (Geostationary Operational Environmental Satellite X- and M-class) solar flares observed between 2011 and 2013. It turns out that it is often possible to determine low-energy cutoffs early (in the first two minutes) during large flares. Cutoff energies at these times are typically low, less than 10 keV, when assuming coronal abundances. With photospheric abundances, cutoff energies are typically ∼10 keV higher, in the ∼17–25 keV range.