# Measuring X-ray anisotropy in solar flares. Prospective stereoscopic capabilities of STIX and MiSolFA

## TL;DR

This paper evaluates the prospective stereoscopic X-ray observation capabilities of the upcoming STIX and MiSolFA missions to measure electron anisotropy in solar flares, which is crucial for understanding flare acceleration mechanisms.

## Contribution

It presents simulations of instrumental and physical effects for both missions, predicting their ability to detect flare anisotropy during the next solar maximum.

## Key findings

- STIX and MiSolFA can detect low levels of flare anisotropy in M1-class or stronger flares.
- Both instruments will measure the X-ray anisotropy of about 40 solar flares during the next solar maximum.
- Stereoscopic observations will help distinguish between different flare-acceleration mechanisms.

## Abstract

During the next solar maximum, two upcoming space-borne X-ray missions, STIX on board Solar Orbiter and MiSolFA, will perform stereoscopic X-ray observations of solar flares at two different locations: STIX at 0.28 AU (at perihelion) and up to inclinations of $\sim25^{\circ}$, and MiSolFA in a low-Earth orbit. The combined observations from these cross-calibrated detectors will allow us to infer the electron anisotropy of individual flares confidently for the first time. We simulated both instrumental and physical effects for STIX and MiSolFA including thermal shielding, background and X-ray Compton backscattering (albedo effect) in the solar photosphere. We predict the expected number of observable flares available for stereoscopic measurements during the next solar maximum. We also discuss the range of useful spacecraft observation angles for the challenging case of close-to-isotropic flare anisotropy. The simulated results show that STIX and MiSolFA will be capable of detecting low levels of flare anisotropy, for M1-class or stronger flares, even with a relatively small spacecraft angular separation of 20-30{\deg}. Both instruments will directly measure the flare X-ray anisotropy of about 40 M- and X-class solar flares during the next solar maximum. Near-future stereoscopic observations with Solar Orbiter/STIX and MiSolFA will help distinguishing between competing flare-acceleration mechanisms, and provide essential constraints regarding collisional and non-collisional transport processes occurring in the flaring atmosphere for individual solar flares.