Sub-Second Hard X-ray Imaging Spectroscopy with Solar Orbiter/STIX

The standard picture describing solar flares is the sudden explosive release of “free” magnetic energy in the solar corona in favour of the kinetic energy of high-energy particles. Their impulsive phase is typically characterized by Hard X-ray (HXR) emissions produced by collisional bremsstrahlung when accelerated electrons encounter higher density regions, such as at lower atmospheric altitudes. This kind of emission, in the basic picture of solar flares, results in two chromospheric footpoints indicating the anchorage locations of the coronal flare loops. An outstanding but unsolved question is whether the HXR emission from the two footpoints is simultaneous. Yohkoh observations back in the 90s already address this question, but further studies with improved instrument performances are needed.

The Spectrometer/Telescope for Imaging X-rays (STIX) aboard Solar Orbiter is the new indirect imaging telescope designed to observe a wide range of solar flares in the energy range from 4 up to 150 keV. While its diagnostics capabilities resemble the ones of its predecessor, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), one of the advantages of STIX is the capability of performing sub-second HXR imaging spectroscopy. In this work, we report on STIX flares observations for which we reconstructed HXR sources with an integration time of 0.5 s. These compelling images clearly show short lived HXR nonthermal sources that, if integrated with longer time intervals, would be smeared out.