Probing solar flare accelerated electron distributions with prospective X-ray polarimetry missions

TL;DR

This paper demonstrates that X-ray polarization measurements, including from non-imaging spectro-polarimeters, can effectively constrain electron anisotropy and energy cutoffs in solar flares, advancing understanding of acceleration mechanisms.

Contribution

It shows that prospective X-ray polarimetry missions, even without imaging, can determine key electron acceleration parameters in solar flares.

Findings

X-ray polarization constrains electron anisotropy.

Spectro-polarimeters can determine high energy cutoff.

Non-imaging missions provide valuable data.

Abstract

Solar flare electron acceleration is an extremely efficient process, but the method of acceleration is not well constrained. Two of the essential diagnostics: electron anisotropy (velocity angle to the guiding magnetic field) and the high energy cutoff (highest energy electrons produced by the acceleration conditions: mechanism, spatial extent, time), are important quantities that can help to constrain electron acceleration at the Sun but both are poorly determined. Here, using electron and X-ray transport simulations that account for both collisional and non-collisional transport processes such as turbulent scattering, and X-ray albedo, we show that X-ray polarization can be used to constrain the anisotropy of the accelerated electron distribution and the most energetic accelerated electrons together. Moreover, we show that prospective missions, e.g. CubeSat missions without imaging information, can be used alongside such simulations to determine these parameters. We conclude that a fuller understanding of flare acceleration processes will come from missions capable of both X-ray flux and polarization spectral measurements together. Although imaging polarimetry is highly desired, we demonstrate that spectro-polarimeters without imaging can also provide strong constraints on electron anisotropy and the high energy cutoff.