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

TL;DR

This study analyzes a 2 April 2022 solar eruption using multi-directional observations from Earth and space to dissect reconnection processes during flux-rope eruption. It identifies signatures of arcade-to-rope external reconnection via hot loops and a bright helical feature, and reveals internal reconnection within the rising rope evidenced by a bright EUV quasi-circular structure and a complex suite of radio bursts (SPDBs, U/U-bursts) linked to beam acceleration. Hard X-ray imaging and spectroscopy from STIX and Fermi corroborate thermal and high-energy electron-beam activity concentrated near the filament leg, while radio timing and spectral analyses yield electron-density estimates at reconnection sites. Overall, the work demonstrates a coherent, multi-wavelength picture of both external and internal reconnection in an erupting flux rope, with implications for energy release and particle acceleration in solar eruptions.

Abstract

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.

Figures (15)

• Figure 1: Overview of filament eruption and accompanying flare. (a): SDO/HMI magnetogram (b): GOES X-ray observations. (c)--(h): SDO/AIA images in 304 Å (c--e) and 131 Å passbands (f--h). The filament and flare loops are indicated. The contour in panel (a) corresponds to 50 DN s$^{-1}$ px$^{-1}$ in 304 Å (panel c), showing the relative location of the erupting filament with respect to the photospheric polarities. Dashed and dotted lines indicate times and FOV of Figs. \ref{['Fig:Hot_loop']}, \ref{['Fig:Hot_loop_304']}, \ref{['Fig:euv_1325UT']}. An animation of the SDO/AIA data is available online, spanning 12:50–13:55 UT.
• Figure 2: Overview of 20--5000 MHz radio spectrum in time interval 13:20-13:32 UT observed during 2 April 2022 flare. Top to bottom: 10--100 MHz Greenland-Callisto spectrum, 150--1000 MHz ORFEES spectrum, 800--2000 and 2000--5000 MHz Ondřejov spectra. Horizontal emission bands are due to terrestrial artificial sources.
• Figure 3: Detail of 800--2000 MHz radio spectrum in time interval 13:21:00--13:22:00 UT at very beginning of flare. Numbers denote bursts (Table \ref{['Table1']}), white horizontal bar denotes the STIX time interval II (Table \ref{['tab:stix_time_intervals']}).
• Figure 4: Details of 800--2000 and 2000--5000 MHz radio spectra in time interval 13:22:40--13:23:40 UT. Numbers denote bursts (Table \ref{['Table1']}).
• Figure 5: Details of 800--2000 and 2000--5000 MHz radio spectra in time interval 13:23:40--13:24:40 UT. Numbers denote bursts (Table \ref{['Table1']}), white horizontal bar denotes the STIX time interval III (Table \ref{['tab:stix_time_intervals']}).