Solar flare X-ray spectra are typically dominated by thermal bremsstrahlung emission in the soft X-ray ($\lesssim$10 keV) energy range; for hard X-ray energies ($\gtrsim$30 keV), emission is typically non-thermal from beams of electrons. The low-energy extent of non-thermal emission has only been loosely quantified. It has been difficult to obtain a lower limit for a possible non-thermal cutoff energy due to the significantly dominant thermal emission. 
Here we use solar flare data from the EUV Variability Experiment (EVE) on-board the Solar Dynamics Observatory (SDO) and 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 and any single-instrument DEM helps to resolve ambiguities in the range where thermal and non-thermal emission overlap, and to provide constraints on the low-energy cutoff. 
In the model, thermal emission is from a DEM that is parametrized as multiple gaussians in $Log(T)$. Non-thermal 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 (GOES X- and M-class) solar flares observed between February 2011 and February 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 are typically low, less than 10 keV, with most values of the lower limits in the 5--7 keV range.

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Ionization Equilibria for High Ions of Fe and Ni

The Multi-Instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Non-Thermal Emission

This report has been prepared by the President in collaboration with the Secretary of the Commission. It is based on (1) material supplied by those members and associates of the Commission who responded to requests made in our circulars, (2) a bibliographical search intended to supplement the above material, (3) discussions with other interested persons, and (4) our own knowledge and impressions of the activity within the province of the Commission over the last three years. The report is divided into three parts. Part A is a scientific review aimed not at a comprehensive summary of all work but at a selective review of that which has, in our opinion, introduced some significant change in our concept of a stellar atmosphere or in the likely direction of future developments. Part B is a bibliography. It is this part of the report that is intended to provide an exhaustive summary of work done in the field over the last three years. To authors of those papers we have inadvertently omitted, we offer our apologies: neither were they sent to us nor did we locate them ourselves. Finally, Part C is an outline of the aims and activities of the Commission.

Theory of Stellar Atmospheres

10 – Solar Physics

During the Skylab Mission, the NRL slitless spectrograph photographed a number of flares in the 170–600 A region with a spatial resolution approaching 2″. At flare maximum the 2s 2 S 1/2— —2p 2 P 1/2, 3/2 transitions of Fe xxiv are present, and show the location of the (approx.) 20 × 106 deg plasma with respect to the surface magnetic field and chromospheric (He ii) emissions. Three examples are discussed (two only briefly).

/pdf/fe-xxiv-emission-in-solar-flares-observed-with-the-nrl-atm-43g96jvwor.pdf

Fe xxiv Emission in Solar Flares Observed with the NRL/ATM XUV Slitless Spectrograph

The rate coefficients for radiative recombination of Rb into individual ground and excited nl states (n<or=20, l<or=10), summed over l<n(n<or=10) and over all states, have been computed for the electron temperature range 500<or=T<or=6000K. Using the principle of detailed balance, photoionisation cross sections of Rb calculated in the framework of a single-electron model have been used. The evolution with n and l for given T, with T for given n and various l and with T of the total radiative recombination is analysed. The significant non-hydrogenic features for recombination are outlined. Comparisons are made with the results of the hydrogenic and kramers' models and with other results available for alkali atoms.

Ionization Equilibria for High Ions of Fe and Ni

Citations

The Multi-Instrument (EVE-RHESSI) DEM for Solar Flares, and Implications for Non-Thermal Emission

Theory of Stellar Atmospheres

10 – Solar Physics

Fe xxiv Emission in Solar Flares Observed with the NRL/ATM XUV Slitless Spectrograph

Radiative recombination in rubidium

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