RHESSI Hard X‐Ray Imaging Spectroscopy of Extended Sources and the Physical Properties of Electron Acceleration Regions in Solar Flares

Abstract

In this study we present the results of a new approach to studying theacceleration and propagation of bremsstrahlung-producing electrons insolar flares. The method involves an analysis of the size of extendedsolar flare structures as a function of photon energy. Hard X-ray imagesfrom 10 M-class limb events, observed by the Reuven Ramaty High EnergySolar Spectroscopic Imager (RHESSI) to have the general form of a singleextended source, were analyzed by forward fitting to the sourcevisibilities in each energy band. On average the source sizes σincreased slowly with photon energy ɛ asσ~ɛ1/2. This behavior is consistent neitherwith the predictions of a single-loop thermal model nor with a model inwhich nonthermal electrons are injected into a constant-densitystructure from a compact acceleration region. While a nonuniform densitydistribution along the flare loop can in principle reconcile the datawith a nonthermal collisional model with point-source injection, theresulting density profiles are highly questionable. On the other hand,the data are consistent with a nonthermal collisional model thatincorporates an extended acceleration region, perhaps in combinationwith a localized thermal source. We present best-fit results on thedensity and length of this acceleration region. To our knowledge, thisis the first quantitative empirical analysis of the physicalcharacteristics of electron acceleration regions in solar flares.