Small-scale and transient extreme ultraviolet kernels in solar flare ribbons

¹ University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
² ETH Zürich, Rämistrasse 101, 8092 Zürich, Switzerland
³ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA
⁴ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
⁵ Solar-Terrestrial Centre of Excellence (SIDC), Royal Observatory of Belgium, Ringlaan 3 Av. Circulaire, 1180 Brussels, Belgium
⁶ University College London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 December 2025
Accepted: 12 March 2026

Abstract

Aims. Flare ribbons form when energy released by coronal magnetic reconnection is deposited in the low layers of the solar atmosphere. Therefore, by studying the dynamics of flare ribbons, one obtains an indirect measurement of reconnection in the corona. The aim of this work is to quantify the spatial and temporal scales of substructures within the extreme ultraviolet (EUV) flare ribbons, known as kernels, as a probe of the spatial extent and duration of energy injection during the impulsive phase of solar flares.

Methods. To do this, unprecedented observations of an M2.5 GOES-class flare from the March 2024 major flare campaign of Solar Orbiter were used. These data were obtained at high cadence in short-exposure mode with the Extreme Ultraviolet Imager’s high-resolution telescope, HRI_EUV. Individual kernels were automatically identified using a classical computer vision algorithm. From this, the size distributions of ribbon kernels were derived, and an average light curve of individual kernels was extracted.

Results. The EUV flare kernels were small (≲60 pixels ≈ 1 Mm²) and a significant fraction were unresolved at a plate scale of 135 km/pix in this flare. Furthermore, we derived surprisingly short EUV kernel heating times of less than a few seconds. The average profile exhibits a sharp rise of 1.7 ± 0.3 s from half maximum, requiring an additional 2.3^+0.7_−0.4 s to return to its reference value.

Conclusions. Our findings indicate that approximately one-half of the kernels were unresolved in this flare, despite the enhanced angular resolution offered by Solar Orbiter’s proximity to the Sun at 0.38 AU. Furthermore, we show that energy was only injected in a localised region (≲1 Mm²) of flare ribbons for less than a few seconds. These results necessitate an in-depth investigation into the implications of such small-scale and transient injections on the energy flux deposited in solar flares, and the resulting response of the solar atmosphere.