Investigating weak flares energetics through nonthermal emission and solar energetic electrons: STIX and EPD observations

Solar flares are energetic and dynamic phenomena in the solar system emitting radiation impulsively, and solar energetic electrons (SEEs). Therefore, we investigate the high-energy X-ray emission and SEEs observed by STIX and EPD onboard the Solar Orbiter mission. During September 18-30, 2021, the Solar Orbiter mission - being closer to the Sun (~0.6 AU) and having a moderate separation angle (~30-400) from the Sun-Earth line provided a unique opportunity for an exhaustive multi-wavelength investigation of several weak flares, associated nonthermal X-ray emission, and SEE’s characteristics. A multiwavelength investigation of spectral and imaging-mode observations of the 20 weak (~B-class), but hard X-ray (HXR)-rich flares, revealed a definitive role of pre-flare plasma density in the loops to be responsible for disparate thermal-nonthermal emission partition during flares. We further investigate remote and in-situ observations of three flares (two B-class, and a C1.6 -class) showing different thermal-nonthermal X-ray emission partitions, and associated SEEs. The timing and spatial correlation of the solar events at source and in-situ SEE enhancements revealed agreement in the 1) onset time of HXR emission and SEE enhancement, and 2) power-law spectral indices of HXR emission and SEEs. Interestingly, we find a very weak HXR burst (B3-class; nonthermal electron spectral index ~ 6) to cause a significant SEE enhancement despite an impulsive C3 flare that occurred a mere 15 minutes before it without any SEE enhancement signatures. Therefore, a comprehensive assessment of energy released during flares is only possible by characterising the observed nonthermal emission as well as particles.