Description
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, robustly determines the
low-energy cut-off in the nonthermal electron distribution, so that the
energetics of nonthermal electrons can be deduced 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.
