Deducing electron properties from hard X-ray observations

Because hard X-rays represent prompt, optically-thin, radiation from energetic electrons, they are a relatively straightforward, and hence valuable, tool in the diagnostic study of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy,direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship.This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, and Compton back-scatter, using both spectroscopic and imaging techniques.The unprecedented quality of the RHESSI data allows for the first time model-independent inference of not only electron energy spectra, but also the angular distributions of the X-ray-emitting electrons. In addition, RHESSI's imaging spectroscopy capability has revealed hitherto unknown details of solar flare morphology, and hard X-ray polarization measurements have imposed significant new constraints on the degree of anisotropy of theaccelerated electron distribution.