STIX observation of chromospheric evaporation

Context. Hard X-rays (HXRs) offer the most direct insight into nonthermal electron populations in solar flares, presenting an essential tool for investigating the physical states within flaring structures. The thick-target model suggests that HXR source altitudes in flare footpoints decrease with rising energy due to increased column density. Nonetheless, this relation not depends only on the density, but it is also shaped by the power-law distribution of nonthermal electrons. Furthermore, during the impulsive phase, significant changes in plasma density and ionization levels within footpoints can occur due to heating and chromospheric evaporation, complicating the interpretation of observed HXR footpoint altitudes. Aims. We aim to investigate the evolution of footpoint source locations for various energy ranges using the most recent measurements collected by the Spectrometer Telescope for Imaging X-rays (STIX). Methods. Images were reconstructed with the MARLIN algorithm for narrow energy and time bins. For each time and energy interval, images have been reconstructed 101 times, varying detector counts according to their uncertainties. Thus, the source positions were estimated with errors. The absolute reference level (photosphere) has been estimated directly from the analysis of the density distribution derived from the energy-altitude relation. Results. We found two velocity fields. In the chromosphere, we observed a plateau in the energy-altitude relation that is related to fast mass flow (chromospheric evaporation) up to 250 km s −1 . However, with estimated errors in the source positions, at times, we can observe the bump instead of the plateau. If this is indeed a real result, it clearly cannot be explained by the density changes only. In the low corona, we observe a steady flow of plasma, with a velocity of up to 100 km s −1 .