Measuring X-ray anisotropy in solar flares. Prospective stereoscopic capabilities of STIX and MiSolFA
1 University of Applied Sciences & Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
2 School of Physics & Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
Received: 15 February 2017
Accepted: 28 July 2017
Context. During a solar flare, a large percentage of the magnetic energy released goes into the kinetic energy of non-thermal particles, with X-ray observations providing a direct connection to keV flare-accelerated electrons. However, the electron angular distribution, a prime diagnostic tool of the acceleration mechanism and transport, is poorly known.
Aims. 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 ~ 25°, 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.
Methods. 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.
Results. 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°. Both instruments will directly measure the flare X-ray anisotropy of about 40 M- and X-class solar flares during the next solar maximum.
Conclusions. 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.
Key words: Sun: flares / Sun: X-rays / Sun: atmosphere / space vehicles: instruments
© ESO, 2017
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.