Letter to the Editor

One-second variations in non-thermal X-ray source morphologies in an X-class solar flare

¹ University College London, Mullard Space Science Labratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
² University of Applied Sciences and Arts Northwest Switzerland (FHNW), School of Computer Science, Bahnhofstrasse 6, Windisch, 5210, Switzerland
³ Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of informatics, Università della Svizzera italiana, Locarno, Switzerland
⁴ Swiss Federal Institute of Technology in Zurich (ETHZ), Rämistrasse 1, 8001 Zürich, Switzerland
⁵ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin, D15 XR2R, Ireland
⁶ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

^⋆ Corresponding author: daniel.ryan@ucl.ac.uk

Received: 3 March 2025
Accepted: 28 September 2025

Abstract

Context. Modelling in recent decades suggests that electron acceleration in solar flares occurs on timescales of ≤1 s. Timeseries analyses of spatially integrated hard X-ray (HXR) emission from these electrons has shown similarly rapid variations. Further probing the acceleration process requires characterising the spatial and spectral evolution of the electrons with HXR imaging spectroscopy. However, this has not previously been available on such fast timescales.

Aims. We provide the first HXR spectroscopic imaging of accelerated electrons in a solar flare on timescales of 1 s.

Methods. We examined the evolution of non-thermal HXR source morphologies and their spatially averaged spectra in the solar flare SOL2024-05-20T05:14L171C109 (estimated-X16.5) with Solar Orbiter/STIX. The non-thermal component of this flare was one of the most intense ever observed, and STIX was the first solar HXR spectroscopic imager capable of 1 s (and sub-second) imaging. These observations are therefore the best so far for high-cadence non-thermal X-ray imaging spectroscopy.

Results. We observed physical changes in the non-thermal HXR source morphologies on timescales of 1 s, which is consistent with predictions for the acceleration process. The evolution of the spatially averaged electron spectral index was slower, however.

Conclusions. This may suggest a coherent driver between different acceleration events and/or pathways whose spectrum varies on timescales > 1 s. Alternatively, it may be an illusion of the spatial averaging in the spectral analysis. Either way, these results show that sub-second electron acceleration in solar flares can be associated with morphological dynamics on similar timescales. This highlights the importance of high spatial resolution sub-second HXR imaging spectroscopy for elucidating this process.