Stereoscopic measurements of X-ray anisotropy in solar flares with STIX and MiSolFA [SSA]

During a solar flare, a large fraction of the magnetic energy released goes into the kinetic energy of non-thermal particles, with X-ray observations providing a direct diagnostic tool of keV flare-accelerated electrons. However, the electron angular distribution, a prime diagnostic tool of the acceleration mechanism and transport, is poorly known.
During the next solar maximum, two upcoming space-borne X-ray missions, STIX onboard \emph{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 at low-Earth orbit. The combined observations from these cross-calibrated detectors, will allow us to confidently detect the electron anisotropy of individual flares for the first time.
Both instrumental and physical effects are simulated 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 and stereoscopic measurements during the next solar maximum. The range of useful spacecraft observation angles is estimated, 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, even with a relatively small ($\sim$20–30$^{\circ}$) angular separation of the spacecrafts, and will directly measure the flare electron anisotropy of about 40 M- and X-class solar flares during the next solar maximum.
Near-future stereoscopic observations with \emph{Solar Orbiter}/STIX and MiSolFA will help distinguish between competing flare-acceleration mechanisms, and provide information regarding collisional and non-collisional transport processes occurring in the flaring atmosphere for individual solar flares.

Read this paper on arXiv…

D. Casadei, N. Jeffrey and E. Kontar
Wed, 1 Mar 17

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