Effects of Compton Scattering on the Gamma-Ray Spectra of Solar Flares

AbstractUsing fully relativistic GEANT4 simulation tool kit, the transport of ener-getic electrons generated in solar flares was Monte-Carlo simulated, and resultantbremsstrahlung gamma-ray spectra were calculated. The solar atmosphere was ap-proximated by 10 vertically-stacked zones. The simulation took into account twoimportant physical processes,that the bremsstrahlung photons emitted by precipitat-ing relativistic electrons are strongly forward beamed toward the photosphere, andthat the majority of these gamma-rays must be Compton back-scattered by the solaratmosphere in order to reach the observer. Then, the Compton degradation was foundto make the observable gamma-ray spectra much softer than is predicted by simpleanalytic calculations. The gamma-ray signals were found to be enhanced by severalconditions, including a broad pitch-angle distribution of the electrons, a near-limbflare longitude, and a significant tilt in the magnetic field lines if the flare longitude israther small. These results successfully explain several important flare properties ob-served in the hard X-ray to gamma-ray range, including in particular those obtainedwith Yohkoh. A comparison of the Yohkoh spectrum from a GOES X3.7 class limbflare on 1998 November 22, with a simulation assuming a broad electron pitch-angledistribution, suggests that gamma-rays from this particular solar flare were a mixtureof direct bremsstrahlung photons and their Comptonization.Key words: solar flare: gamma-ray: Compton scattering1