Thermal-nonthermal energy partition in weak flares observed by STIX, XSM, and SDO

The disparate nature of thermal-nonthermal energy partition during weak flares compared to that during the large flares is still an open issue and quantifying the relative productivity of multi-wavelength emission during weak flares can enable inferring the underlying energy release mechanism. Therefore, we analyze multi-wavelength emission from ∼150 flares during September 20-25, 2021, commonly observed from Spectrometer Telescope for Imaging X-rays (STIX) (at ∼0.6 AU), STEREO-A, GOES, and SDO observatories (significant overlap of observing field-of-view). The ratio (Qf ) of HXR (>12 keV) fluence (Fhxr ) and SXR (4-10 keV) flux (Fsxr), at the maximum of Fsxr (tp), is derived to quantify the relative productivity of HXR and SXR emission during flares. The variation of Qf with Fsxr enabled us to quantitatively identify the cases of strongly non-thermal (cold) and highly thermal (hot) flares. The identification of the source active region using the EUV images (from AIA) revealed the uniform behavior of different active regions in producing cold and hot flares. Besides, thermal-nonthermal plasma parameters as estimated by spectral-fit of STIX and XSM observations indicate a possible role of pre-flare density in flare loops to be resulting in disparate thermal-nonthermal emission partition. Therefore, we conduct case studies of flares of the aforementioned types by – synthesizing the X-ray images from STIX observations – analyzing the E/UV images, and magnetograms, and — performing the hydrodynamical simulations using the 1D Palermo-Harvard code. With such a multi-wavelength analysis of an ensemble of weak flares, we probe the energy release mechanism and evaluate the same in the framework of the standard model of energy release during large flares.