Electron Energy Spectra and e-e Bremsstrahlung from Anisotropic Electron Distributions in Extreme Solar Flares

TL;DR

This study analyzes electron energy spectra and bremsstrahlung contributions in extreme solar flares using RHESSI data, revealing spectral breaks, anisotropic electron distributions, and asymmetries in HXR sources.

Contribution

It introduces detailed modeling of electron spectra considering anisotropic pitch-angle distributions and quantifies the impact of e-e bremsstrahlung in high-energy X-ray emissions from solar flares.

Findings

e-e bremsstrahlung significantly affects X-ray flux above 500 keV

spectral breaks occur around 100 keV with anisotropic electron distributions

electron spectra follow a power-law with index ~5 after including bremsstrahlung

Abstract

We study the electron energy spectra of two powerful solar flares SOL2003-10-28T 11:06:16 (GOES class X17.2) and SOL2002-07-23T00:18:16 (X4.8) on the basis of HXR and gamma-ray spectra obtained from RHESSI data. Electron-electron (e-e) bremsstrahlung makes a significant contribution to the X-ray flux at energies above 500 keV. In X-ray flux calculations different electron pitch-angle distributions were considered: quasi-transverse and quasi-longitudinal. It was shown that breaks in hard X-ray spectra occur on energies $\epsilon\approx100$ keV with photon spectral-index difference $\Delta\gamma\ge 1$ in the case of quasi-longitudinal anisotropy and hard power-law electron energy spectra. We deduce the parameters of non-thermal electrons energy spectra from RHESSI data for the extreme solar flares of 2003 October 28 and 2002 July 23. After including e-e bremsstrahlung, the non-thermal electrons energy spectra obtained in the thick-target model can be fitted using power-law with spectral index $\delta\approx 5$. For the 2002 July 23 flare, there is a pronounced asymmetry towards the southern footpoint, starting from energies above 100 keV. The separation of HXR sources at energies above 100 keV may be related to a feature of relativistic electron transport in flaring loops.