Letter to the Editor

Hard X-rays from the deep solar atmosphere

An unusual UV burst with flare properties^⋆

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: chitta@mps.mpg.de
² School of Physics & Astronomy, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
⁴ Space Sciences Laboratory University of California, Berkeley, CA 94720, USA
⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Northumbria University, Newcastle upon Tyne NE1 8ST, UK
⁷ University of Applied Sciences and Arts Northwestern Switzerland, 5210 Windisch, Switzerland
⁸ Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401a, 79110 Freiburg, Germany

Received: 24 May 2024
Accepted: 6 July 2024

Abstract

Explosive transient events occur throughout the solar atmosphere. The differing manifestations range from coronal mass ejections to Ellermann bombs. The former may have negligible signatures in the lower atmosphere, and the latter may have negligible nonthermal emissions such as hard X-radiation. A solar flare generally involves a broad range of emission signatures. Using a suite of four space-borne telescopes, we report a solar event that combines aspects of simple UV bursts and hard X-ray emitting flares at the same time. The event is a compact C-class flare in active region AR11861, SOL2013-10-12T00:30. By fitting a combined isothermal and nonthermal model to the hard X-ray spectrum, we inferred plasma temperatures in excess of 15 MK and a nonthermal power of about 3 × 10²⁷ erg s⁻¹ in this event. Despite these high temperatures and evidence for nonthermal particles, the flare was mostly confined to the chromosphere. However, the event lacked clear signatures of UV spectral lines, such as the Fe XII 1349 Å and Fe XXI 1354 Å emission lines, which are characteristic of emission from hotter plasma with a temperature over 1 MK. Moreover, the event exhibited very limited signatures in the extreme-UV wavelengths. Our study indicates that a UV burst – hard X-ray flare hybrid phenomenon exists in the low solar atmosphere. Plasma that heats to high temperatures coupled with particle acceleration by magnetic energy that is released directly in the lower atmosphere sheds light on the nature of active region core heating and on inferences of flare signatures.