Spatiotemporal evolution of UV pulsations and their connection to 3D magnetic reconnection and particle acceleration

¹ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
² University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Bahnhofstrasse 6, 5210 Windisch, Switzerland
³ ETH Zürich, Rämistrasse 101, 8092 Zürich, Switzerland
⁴ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
⁵ Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Kanzelhöhe 19, 9521 Treffen, Austria
⁶ European Space Agency, ESTEC, Noordwijk, The Netherlands
⁷ Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA

^⋆ Corresponding author: stefan.purkhart@uni-graz.at

Received: 11 March 2025
Accepted: 6 May 2025

Abstract

Context. Quasi-periodic pulsations (QPPs) are a common feature of impulsive solar flare emissions, yet their driving mechanism(s) remain unresolved. Observational challenges such as image saturation during large flares, low image cadence, and limited spatial resolution often hinder our ability to study the spatiotemporal evolution of QPPs in detail. The M3.7 solar flare of February 24, 2023 produced long-period (3.4 min) QPPs in hard X-ray (HXR) and ultraviolet (UV) emissions and was associated with a large-scale asymmetric filament eruption.

Aims. This study leverages the unique opportunity presented by the combination of a long pulsation period and the large spatial scale of the eruptive event. Our goal is to pinpoint the location of the observed QPPs within the flare structure, characterize their spatiotemporal evolution, examine their connection to particle acceleration, and understand their relationship to flare ribbon dynamics and the underlying magnetic reconnection geometry.

Methods. The UV emissions of the flare ribbons were analyzed with SDO/AIA 1600 Å base-difference imaging, while the HXR emissions were studied with Solar Orbiter/STIX. The flare region was divided into four subregions focused on the main flare ribbons or filament footpoints in both polarities, from which the integrated light curves were extracted. Time-distance plots were constructed to follow the motion of the UV ribbon kernels and to relate them to the QPPs. A spectral fitting of the HXR observations was performed to characterize the accelerated electron populations and the HXR images were reconstructed to study the X-ray sources.

Results. We find that the strength and characteristics of the UV pulsations varied significantly across subregions, with the strongest correlation to HXR pulsations and the most complex spatiotemporal evolution occurring in the compact southern flare ribbon. In this location, UV pulsations originated from a sequence of UV kernels at an expanding flare ribbon front, accompanied by secondary motions along the front. In contrast, another expanding ribbon front, likely associated with a different subsystem of the reconnecting flux, did not exhibit pulsations. The UV pulsations spatially coincided with HXR sources, indicating non-thermal electrons as a common driver. The HXR spectrum exhibited a soft-hard-soft evolution, suggesting a modulation of the electron acceleration efficiency on the same timescale as the pulsations. Around the eastern filament anchor region, UV pulsations were produced instead by plasma injections into a separate filament channel, highlighting the presence of distinct emission sources beyond flare kernels.

Conclusions. Our results suggest that the spatiotemporal evolution of the UV pulsations is largely driven by the propagation of magnetic reconnection, including slipping reconnection, within an asymmetric magnetic geometry. We further hypothesise that slipping reconnection, combined with the structure of the quasi-separatrix layers (QSLs), plays a key role in the generation and propagation of UV kernels. An additional time-varying reconnection process was potentially required to fully explain the observations. Our findings emphasize the importance of spatially resolved QPP studies on the timescales of energy release to disentangle different emission processes and their connection to magnetic reconnection.