The EVE plus RHESSI DEM for Solar Flares, and Implications for Residual Non-Thermal Soft X-Ray Emission

We combine observations of solar flares from the EUV Variability Experiment (EVE) on-board the Solar Dynamics Observatory (SDO) with X-ray data from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) to calculate the Differential Emission Measure (DEM). This improvement over the isothermal approximation is intended to help to resolve ambiguities in the range where thermal and non-thermal emission overlap. For this current project we are interested in constraining cutoffs in the "residual" non-thermal spectrum; i.e., the RHESSI spectrum that is left over after the DEM has accounted for the bulk of the soft X-ray emission. (Previous work by Caspi et.al. 2014ApJ...788L..31C concentrated on obtaining DEM models that fit both instruments' observations well). Solar flare spectra are typically dominated by thermal bremsstrahlung emission in the soft X-ray (< 10 keV) energy range; at higher hard X-ray energies (> 30 keV) the emission is non-thermal from beams of electrons. The low energy extent of non-thermal emission can typically only be loosely quantified. In particular, it is difficult to obtain a lower limit for any possible non-thermal cutoff energy due to the larger amount of thermal emission. In this model, thermal emission is due to a DEM that is parametrized as multiple gaussians in Log(T). Non-thermal emission is modeled as a photon spectrum obtained using thin and thick-target emission models. Spectra for both instruments are fit simultaneously in a self-consistent manner. Preliminary results have been obtained using a sample of 102 large (GOES X and M class) solar flares observed between February 2011 and February 2013. These results show that it is possible to determine low energy cutoffs and breaks early during large flares, and to get good values for the low energy limit to the non-thermal cutoff.