Radio, X-ray, and extreme ultraviolet signatures of internal and external reconnection of an erupting flux rope

¹ Astronomical Institute of the Czech Academy of Sciences Fričova 298 25165 Ondřejov, Czech Republic
² Institute for Data Science, University of Applied Sciences and Arts Northwestern Switzerland Bahnhofstrasse 6 Windisch 5120, Switzerland
³ Space Sciences Laboratory, University of California Berkeley 7 Gauss Way Berkeley CA 94720, USA
⁴ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
⁵ Astronomical Institute, Slovak Academy of Sciences Tatranská Lomnica, Slovakia

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

Received: 26 September 2025
Accepted: 28 November 2025

Abstract

Aims. We aim to interpret and relate features observed in gigahertz radio, extreme UV (EUV), and X-ray emissions during a filament eruption at the start of a solar flare on 2 April 2022.

Methods. We analysed imaging (EUV, X-ray) and spectral (radio, X-ray) data obtained by ground-based and space instruments on board space missions both on Earth (Fermi, Hinode, Solar Dynamics Observatory) and in solar orbits (Solar Orbiter, STEREO-A), thus providing a multi-directional view on the same event.

Results. The combination of EUV and X-ray images and X-ray spectra allowed us to identify hot loops in the vicinity of the filament before its eruption. We interpreted their interaction with the rising filament as a signature of an arcade-to-rope reconnection geometry. The subsequent EUV brightening within the filament revealed a helical structure of the erupting rope. We explain co-temporal radio slowly positively drifting bursts as being a result of beam acceleration within the magnetic rope and propagation along the helical structure. The corresponding X-ray spectra are consistent with a thermal origin. The filament rising was accompanied by co-temporal normal and reverse drift type III radio bursts. We interpret them as a signature of a reconnection event and estimated the electron density at the reconnection site. Further untwisting of the helical structure led to formation of a quasi-circular EUV structure seen from Earth and STEREO-A. Its occurrence was co-temporal with a unique tangle of radio U- and inverse U-bursts. We propose that several accelerated beams propagate within that complex structure and generate the burst tangle. During the start of the flare, hard X-ray emission was concentrated near the filament leg only, suggesting predominant propagation of the beams towards its rooting.

Conclusions. We collected multi-wavelength observations indicating interaction of the erupting magnetic flux rope with the overlying arcade and internal magnetic reconnection inside the rising flux rope.