Identifying the energy release site in a solar microflare with a jet

¹ Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
e-mail: andrea.battaglia@fhnw.ch
² Institute for Particle Physics and Astrophysics (IPA), Swiss Federal Institute of Technology in Zurich (ETHZ), Wolfgang-Pauli-Strasse 27, 8039 Zurich, Switzerland
³ School of Earth and Space Sciences, Peking University, Beijing 100871, PR China
⁴ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁵ Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205 Moscow, Russia
⁶ Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Kanzelhöhe 19, 9521 Treffen, Austria
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ IRSOL Istituto Ricerche Solari “Aldo e Cele Daccó”, Università della Svizzera italiana, via Patocchi 57, 6605 Locarno, Switzerland
⁹ Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, 7260 Davos Dorf, Switzerland
¹⁰ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

Received: 16 September 2022
Accepted: 12 December 2022

Abstract

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 Earth’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 (∼30 km s⁻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.