Measurements of electron anisotropy in solar flares using albedo with RHESSI X-ray data

TL;DR

This study uses RHESSI X-ray data and photospheric albedo effects to infer the electron pitch angle distribution in solar flares, finding mostly isotropic distributions with limited anisotropy.

Contribution

It introduces a method applying regularized inversion to RHESSI data to measure electron anisotropy in solar flares, providing new observational constraints.

Findings

Electron distributions are mostly isotropic during flares.

Anisotropy larger than ~3 is inconsistent with data.

Uncertainties increase above 150-200 keV.

Abstract

The angular distribution of electrons accelerated in solar flares is a key parameter in the understanding of the acceleration and propagation mechanisms that occur there. However, the anisotropy of energetic electrons is still a poorly known quantity, with observational studies producing evidence for an isotropic distribution and theoretical models mainly considering the strongly beamed case. We use the effect of photospheric albedo to infer the pitch angle distribution of X-ray emitting electrons using Hard X-ray data from RHESSI. A bi-directional approximation is applied and a regularized inversion is performed for eight large flare events to deduce the electron spectra in both downward (towards the photosphere) and upward (away from the photosphere) directions. The electron spectra and the electron anisotropy ratios are calculated for broad energy range from about 10 and up to ~ 300 keV near the peak of the flares. The variation of electron anisotropy over short periods of time intervals lasting 4, 8 and 16 seconds near the impulsive peak has been examined. The results show little evidence for strong anisotropy and the mean electron flux spectra are consistent with the isotropic electron distribution. The 3-sigma level uncertainties, although energy and event dependent, are found to suggest that anisotropic distribution with anisotropy larger than ~ 3 are not consistent with the hard X-ray data. At energies above 150-200 keV, the uncertainties are larger and thus the possible electron anisotropies could be larger.