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

We analyse 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 solar orbit (Solar Orbiter, STEREO-A), which provide a multi-directional view on the same event. 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 helical structure of the erupting rope. We explained co-temporal radio slowly positively drifting bursts as a result of beam acceleration within the magnetic rope and propagation along the helical structure. Corresponding X-ray spectra were consistent with a thermal origin. The filament rising was accompanied by co-temporal normal and reverse drift type III radio bursts. We interpreted them as a signature of a reconnection event and estimated 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 proposed 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. 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.

💡 Research Summary

This paper presents a comprehensive multi‑instrument analysis of the eruptive M3.9 flare that occurred on 2 April 2022, focusing on the interplay between external (arcade‑to‑rope) and internal reconnection processes within a magnetic flux rope. The event was observed from several viewpoints: Earth‑orbit assets (Fermi/GBM, Hinode/XRT, SDO/AIA) and solar‑orbit platforms (Solar Orbiter/STIX, Solar Orbiter/EUI, STEREO‑A/EUVI). Radio spectra covering 10 MHz–5 GHz were obtained from four ground‑based spectrographs (Greenland‑Callisto, ORFEES, and two Ondřejov receivers).

External reconnection (arcade‑to‑rope).
Prior to the filament lift‑off, AIA 131 Å and Hinode/XRT revealed hot (≥10 MK) coronal loops adjacent to the filament. As the filament rose, these loops brightened and appeared to merge with the filament, a signature interpreted as arcade‑to‑rope reconnection. This geometry matches the “Aulanier effect” predicted by 3‑D standard flare models, where the footpoints of the erupting rope drift due to a change in magnetic connectivity. The reconnection injects hot plasma into the rope, producing the observed EUV brightening and soft X‑ray emission.

Internal reconnection and helical structure.
Simultaneous brightening in AIA 304 Å and 131 Å within the filament revealed a clear helical morphology, indicating that the rope itself undergoes internal reconnection while it expands. Radio observations in the 800 MHz–5 GHz band show several rare phenomena:

• Slowly positively drifting bursts (SPDB, bursts 1‑2) with drift rates of +70 to +100 MHz s⁻¹, previously linked to electron beams propagating along a helical magnetic structure.
• High‑frequency type III bursts (negative drift, hf‑III) and reverse‑drift type III bursts (RS) that trace upward‑ and downward‑propagating electron beams, respectively.

The SPDBs are interpreted as electron beams accelerated inside the rope and guided along its twisted field lines, consistent with earlier case studies (Fárník & Karlický 2007; Zemanová et al. 2020).

Complex burst tangle (U‑ and inverse‑U bursts).
A particularly striking feature appears after 13:24 UT: a dense “tangle” of U‑bursts and inverse‑U bursts (bursts 20 and 22). Imaging shows a quasi‑circular EUV structure, visible both from Earth and STEREO‑A, formed as the rope untwists. The authors propose that multiple electron beams are trapped within this closed magnetic loop, each following a different curved trajectory, thereby generating the overlapping U‑type signatures. Frequency drift measurements (−2000 to +2100 MHz s⁻¹) allow an estimate of the ambient electron density (~10⁹ cm⁻³) and the characteristic length of the acceleration region.

Hard X‑ray signatures.
Hard X‑ray (HXR) data from Fermi/GBM and Solar Orbiter/STIX show that the earliest non‑thermal emission is confined to the filament leg (footpoint) region. Spectral fitting indicates a weak non‑thermal component at the onset, followed by a dominant thermal component (10–20 keV) as the flare progresses. This spatial confinement suggests that accelerated electrons preferentially travel toward the footpoint, while the upper part of the rope mainly exhibits thermal emission.

Synthesis and implications.
By aligning the timing and locations of EUV/X‑ray brightenings, radio burst types, and HXR sources, the study demonstrates that external arcade‑to‑rope reconnection and internal rope reconnection occur simultaneously during the early phase of the eruption. The internal reconnection not only heats the rope plasma but also creates a helical magnetic channel that guides electron beams, producing the observed SPDBs. The subsequent untwisting of the rope generates a large, closed magnetic loop that traps beams, giving rise to the unprecedented U‑burst tangle.

These observations provide the first direct, multi‑wavelength evidence that a single eruptive event can host both external and internal reconnection, each leaving distinct signatures across the electromagnetic spectrum. The detection of rare radio phenomena (slowly drifting SPDBs, U‑burst tangle) offers new diagnostic tools for probing the magnetic topology and particle acceleration sites within erupting flux ropes. The authors suggest that future studies combine high‑resolution radio imaging (e.g., LOFAR, EOVSA) with 3‑D magnetohydrodynamic simulations to quantify the energy partition between heating and particle acceleration during internal rope reconnection.