Main Objective: To investigate plasma diagnostics during flare precursor events.
Scientific Justification: Despite the standard picture describing solar flares can be considered a satisfactory approximation of reality, more sophisticated interpretations are needed in order to explain the observations. For instance, recently, Hudson et al. (2021) reported an unexpected common temperature behavior during the earliest detectable stages of solar flares. By means of GOES/XRS diagnostics, they reported temperatures between 10 and 15 MK. Moreover, by analyzing SDO/AIA images, they suggested that these early hot onset sources are mainly footpoints and low-lying loops that subsequently become parts of the structure of the flare. However, on the one hand, the limited GOES/XRS response does not allow to detect any potential X-ray emission produced by non-thermal bremsstrahlung during these early stages, i.e., flare accelerated electrons colliding with the chromosphere. On the other hand, SDO/AIA alone, due to the broad temperature response of its passbands, does not allow to properly constrain the origin of these hot sources.
In order to have a better temporal description of both temperature and location of these hot sources, X-ray imaging and spectroscopy in a broader temperature range is needed. The X-ray telescope on board Solar Orbiter, STIX, diagnoses and gives imaging information of thermal and non-thermal emission at high cadence for plasma at temperatures from about 8 MK and above. Moreover, the observations provided by XRT will allow us to constrain the high-temperature end of the SDO/AIA DEM, which, in combination with the STIX measurements, allow to clearly disentangle these hot sources in the EUV images provided by AIA.
Once the location of the hot sources is known, we aim to diagnose other plasma conditions and dynamics during the onset interval. High-resolution IRIS observations will allow us to study the emission at cooler temperatures, in particular at temperatures characteristic of the transition region and the chromosphere in such a way, we can analyze the response of the transition region in case of non-thermal emission detected by STIX (i.e., electron beams colliding and heating the chromosphere). Additionally, the plasmaparameters obtained from EIS and IRIS can be compared.
Therefore, for the motivations previously reported, we believe that such a coordination is of fundamental importance in order to shed some light on this matter. |
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