Statistical studies of the energy-altitude relation in the footpoints of solar flares observed by STIX

Solar flares are efficient accelerators of energetic particles, mainly electrons, which transport energy from reconnection site to the chromosphere. Energetic electrons are thermalized in the chromosphere and produce hard X-ray emission (HXR) following the thick-target bremsstrahlung mechanism. The thick-target model predicts that altitude of the HXR sources in the footpoints of solar flare decreases with increasing energy. The relation was registered for the solar flares observed with Yohkoh/HXT and RHESSI. In our research, we investigated the energy-altitude relation in flare footpoints in a group of strong (M and X GOES class) events that showed emission up to 84 keV recorded by the Spectrometer/Telescope for Imaging X-rays (STIX) onboard the Solar Orbiter. Here we present the results of analysis of the relation, e.g. temporal evolution and density distributions on selected examples. One of the interesting effects visible in some cases is bump in higher energies, when the sources appeared roughly on higher altitudes, which was very rare observed by previous X-ray instruments, but in STIX observations it’s very common. Thanks to unprecedented high temporal and spatial resolutions of STIX data, we can trace changes of plasma dynamics in footpoints like never before.