Context. The Spectrometer Telescope for Imaging X-rays (STIX) on board Solar Orbiter enables exciting multipoint studies of energy release and transport in solar flares by observing the Sun from many different distances and vantage points out of the Sun-Earth line.
Aims. We present a case study of an M4-class flare that occurred on 28 March 2022, near Solar Orbiter’s first science perihelion (0.33 AU from the Sun). Solar Orbiter had a longitudinal separation of 83.5° west of the Sun-Earth line, making the event appear near the eastern limb from its perspective, while Earth-orbiting spacecraft observed it near the disk center. We follow the evolution of the X-ray, extreme-ultraviolet (EUV), UV sources, analyze their relation to plasma dynamics and heating, and relate our observations to magnetic field structures, including the erupting filament.
Methods. The timing and location of the STIX X-ray sources were related to the plasma evolution observed in the EUV by the Extreme Ultraviolet Imager (EUI) on Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory, and to the chromospheric response observed in 1600 Å by AIA. We performed differential emission measure (DEM) analysis to further characterize the flaring plasma at different subvolumes. The pre-flare magnetic field configuration was analyzed using a nonlinear force-free (NLFF) extrapolation.
Results. In addition to the two classical hard X-ray (HXR) footpoints at the ends of the flaring loops, later in the event we observe a nonthermal HXR source at one of the anchor points of the erupting filament. These results are supported by a robustness analysis of the STIX images and the co-temporal chromospheric brightenings observed by AIA. The full evolution of the AIA 1600 Å footpoints indicates that this change in footpoint location represents a discontinuity in an otherwise continuous westward motion of the footpoints throughout the flare. The NLFF extrapolation suggests that strongly sheared field lines close to, or possibly even part of, the erupting filament reconnected with a weakly sheared arcade during the first HXR peak. The remainder of these field lines reconnected later in the event, producing the HXR peak at the southern filament footpoint. Furthermore, we found several thermal X-ray sources during the onset of the impulsive phase and a very low-lying initial thermal loop top source that passes through a double structure during its rise. We were able to relate many of these observations to features of the complex flare geometry involving multiple interacting magnetic flux systems.
Conclusions. The combined STIX and AIA observations, complemented by the NLFF extrapolation, allowed us to successfully constrain and verify the signatures of energy release and transport in the flare under study. Our results show that the reconnection between field lines with very different shear in the early phase of the flare plays a crucial role in understanding the motion of the HXR footpoint during later parts of the flare. This generalizes simpler models, such as whipping reconnection, which only consider reconnection propagating along uniformly sheared arcades.
