Solar Flare Measurements with STIX and MiSolFA

Abstract: Solar flares are the most powerful events in the solar system and the brightest sources of X-rays, often associated with emission of particles reaching the Earth and causing geomagnetic storms, giving problems to communication, airplanes and even black-outs. X-rays emitted by accelerated electrons are the most direct probe of solar flare phenomena. The Micro Solar-Flare Apparatus (MiSolFA) is a proposed compact X-ray detector which will address the two biggest issues in solar flare modeling. Dynamic range limitations prevent simultaneous spectroscopy with a single instrument of all X-ray emitting regions of a flare. In addition, most X-ray observations so far are inconsistent with the high anisotropy predicted by the models usually adopted for solar flares. Operated at the same time as the STIX instrument of the ESA Solar Orbiter mission, at the next solar maximum (2020), they will have the unique opportunity to look at the same flare from two different directions: Solar Orbiter gets very close to the Sun with significant orbital inclination; MiSolFA is in a near-Earth orbit. To solve the cross-calibration problems affecting all previous attempts to combine data from different satellites, MiSolFA will adopt the same photon detectors as STIX, precisely quantifying the anisotropy of the X-ray emission for the first time. By selecting flares whose footpoints (the brightest X-ray sources, at the chromosphere) are occulted by the solar limb for one of the two detectors, the other will be able to study the much fainter coronal emission, obtaining for the first time simultaneous observations of all interesting regions. MiSolFA shall operate on board of a very small satellite, with several launch opportunities, and will rely on moir\'e imaging techniques.