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.
Highlights
Solar flares are the most powerful explosive events in the solar system, and large flares can release up to 1032 erg of energy in a few minutes (Benz 2008; Holman et al 2011)
Current X-ray observations are performed by the spaceborne Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al 2002), using nine rotation modulation collimators and bulk germanium (Ge) detectors to perform indirect imaging and spectroscopy from 3 keV to 17 MeV with an angular resolution of few arcsec (Hurford et al 2002)
Where the Micro Solar-Flare Apparatus (MiSolFA) background distribution is taken to be similar to the RHESSI background at the beginning of the mission, and for Spectrometer Telescope for Imaging X-rays (STIX) a flat component is superimposed with a bump that mimics the background X-ray radiation (Marshall et al 1980)
Summary
Solar flares are the most powerful explosive events in the solar system, and large flares can release up to 1032 erg of energy in a few minutes (Benz 2008; Holman et al 2011). A large fraction of this energy, stored in coronal magnetic fields and released by magnetic reconnection, goes into the acceleration of particles. Hard X-rays (HXR, 20 keV) are a direct link to flare-accelerated electrons and a vital probe of the flare physical processes occurring at the Sun The brightest X-ray sites are often found at the footpoints of newly reconnected magnetic loops that link coronal acceleration regions with the much denser chromosphere. Flare spectra are rapidly falling energy distributions, while the background counts have a more uniform distribution. At high energies (e.g. well above 100 keV) the measurement will be background dominated. The useful energy range for directivity studies extends from the end of the thermal region (∼20 keV), up to the energy bins in which the background counts are of the same order of magnitude as the actual flare
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