Debesh Bhattacharjee, Eduard P. Kontar, Yingjie Luo

A primary characteristic of solar flares is the efficient acceleration of electrons to nonthermal deka-keV energies. While hard X-Ray (HXR) observation of bremsstrahlung emission serves as the key diagnostic of these electrons. In this study, we investigate the time evolution of flare-accelerated electrons using the warm-target model. This model, unlike the commonly used cold-target model, can determine the low-energy cut-off in the nonthermal electron distribution, so that the energetics of nonthermal electrons can be deduced more accurately. Here, we examine the time-evolution of nonthermal electrons in flares well-observed by the RHESSI and the Solar Orbiter (SolO, using the STIX instrument) spacecrafts. Using spectroscopic and imaging HXR observations, the time evolution of the low-energy cut-off of the accelerated electron distribution, the total power of nonthermal electrons, total rate of nonthermal electrons, and excess thermal emission measure from the nonthermal electrons, are investigated. We find that the time profile of the low-energy cut-off of the accelerated electron distribution shows a high-low-high trend around the HXR bursts of flares, while the time evolution of the total rate of injected electrons shows a low-high-low behavior. Although the total power of nonthermal electrons is sensitive to the cut-off energy, the temporal variation of the flare power follows the temporal variation of the acceleration rate. We further find that the highest contribution of the excess thermal emission measure coming from thermalization of injected electrons takes place around the hard X-ray peak.