Search Results
Identifying the energy release site in a solar microflare with a jet
Context. One of the main science questions of the Solar Orbiter and Parker Solar Probe missions deals with understanding how electrons in the lower solar corona are accelerated and how they subsequently access interplanetary space. Aims. We aim to investigate the electron acceleration and energy release sites as well as the manner in which accelerated electrons access the interplanetary space in the case of the SOL2021-02-18T18:05 event, a GOES A8 class microflare associated with a coronal jet. Methods. This study takes advantage of three different vantage points, Solar Orbiter, STEREO-A, and Earth, with observations drawn from eight different instruments, ranging from radio to X-ray. Multi-wavelength timing analysis combined with UV/EUV imagery and X-ray spectroscopy by Solar Orbiter/STIX (Spectrometer/Telescope for Imaging X-rays) is used to investigate the origin of the observed emission during different flare phases. Results. The event under investigation satisfies the classical picture of the onset time of the acceleration of electrons coinciding with the jet and the radio type III bursts. This microflare features prominent hard X-ray (HXR) nonthermal emission down to at least 10 keV and a spectrum that is much harder than usual for a microflare with γ = 2.9 ± 0.3. From Eartha's vantage point, the microflare is seen near the limb, revealing the coronal energy release site above the flare loop in EUV, which, from STIX spectroscopic analysis, turns out to be hot (i.e., at roughly the same temperature of the flare). Moreover, this region is moving toward higher altitudes over time (∼30akmas-1). During the flare, the same region spatially coincides with the origin of the coronal jet. Three-dimensional (3D) stereoscopic reconstructions of the propagating jet highlight that the ejected plasma moves along a curved trajectory. Conclusions. Within the framework of the interchange reconnection model, we conclude that the energy release site observed above-The-loop corresponds to the electron acceleration site, corroborating that interchange reconnection is a viable candidate for particle acceleration in the low corona on field lines open to interplanetary space.
Detection of Energy Cutoffs in Flare-accelerated Electrons
Energy cutoffs in electron distribution define the lower and upper limits on the energy range of energetic electrons accelerated in solar flares. They are crucial parameters for understanding particle acceleration processes and energy budgets. Their signatures have been reported in studies of flattened flare X-ray spectra, i.e., the impulsive emission of nonthermal bremsstrahlung from energetic electrons impacting ambient, thermal plasma. However, these observations have not provided unambiguous constraints on the cutoffs. Moreover, other processes may result in similar spectral features. Even the existence and necessity of cutoffs as physical parameters of energetic electrons have been under debate. Here we report a search for their signatures in flare-accelerated electrons with two approaches, i.e., in both X-ray spectra and solar energetic particle (SEP) events. These represent two different electron populations, but may contain information of the same acceleration process. By studying a special group of late impulsive flares, and a group of selected SEP events, we found evidence of cutoffs revealed in both X-ray spectra and SEP electron distributions. In particular, we found for the first time consistent low- and high-energy cutoffs in both hard X-ray-producing and escaping electrons in two events. We also showed the importance of high-energy cutoff in studies of spectral shapes. These results provide evidence of cutoffs in flare-accelerated energetic electrons and new clues for constraining electron distribution parameters and particle acceleration models.