Evaluation of Algorithms for Reconstructing Electron Spectra from Their Bremsstrahlung Hard X‐Ray Spectra

The Ramaty High Energy Solar Spectroscopic Imager (RHESSI ) has yielded solar flare hard X-ray spectra with\udunprecedented resolution, enabling reconstruction of mean source electron energy spectra F(E) by deconvolution of\udphoton energy spectra I (). While various algorithms have been proposed, the strengths and weaknesses of each have\udyet to be explored in a systematic fashion. For real data F(E) is unknown, so these various algorithms must instead\udbe tested on simulated data for which the ‘‘true’’ F(E) is known. Accordingly, we devised several forms of F(E)\udwith ‘‘interesting’’ features, generated the corresponding (noise-added) I (), and recovered F(E) using a variety of\udalgorithms, including zero- and first-order Tikhonov regularizations, triangular matrix row elimination, and forward\udfitting using a parametric form consisting of a double power law with low/high cutoffs plus an isothermal component.\udAll inversion methods reconstructed the general magnitude and form of F(E) well, suffering only from (1) blurring\udof sharp features and (2) poor recovery at low electron energies E in cases in which F\ud0\ud(E) was positive and large.\udAddition of a steep thermal component at low E did not prevent recovery of features at higher values of E. Forward\udfitting did recover large-scale forms and features well but, inevitably, failed to recover local features not expressible\udwithin the parametric used. This confirms that inversions are the most dependable way to discover such features.\udHowever, examination of the pattern of I () residuals can suggest feature locations and so help refine the parametric\udform used. Since quite smooth F(E) forms do reproduce the observed I () form with relatively small residuals, it appears\udthat sharp features may be uncommon in actual flares