Distinguishing between thermal and non-thermal electron populations in solar flares using RHESSI

Solar flare plasmas contain thermal and non-thermal electron populations, both of which emit characteristic X-ray bremsstrahlung spectra. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes X-rays above 3~keV with a spectral resolution of ~1~keV FWHM. RHESSI spectra typically show a thermal continuum dominant below ~10~keV and a non-thermal continuum dominant above ~30~keV. Distinguishing between the two electron populations is crucial in determining the total energy contained in non-thermal electrons. High-temperature plasmas also emit characteristic line spectra, including emission in the Fe and Fe/Ni "line complexes" at ~6.7 and ~8~keV. Recent predictions and subsequent empirical confirmation have shown that the fluxes and equivalent widths of these complexes are strongly temperature-dependent (Phillips 2004, Caspi et al. 2004). We use the Fe and Fe/Ni line complexes to obtain constraints on the thermal electrons for a variety of large (GOES class M and X) solar flares, with emphasis on the X4.8 event on 23 July 2002, to determine the time-varying characteristics of the thermal and non-thermal electron populations. We estimate and compare the energy contained in these electrons, and discuss the implications for acceleration and heating of electrons in solar flares.