A magnetic avalanche as the central engine powering a solar flare

TL;DR

Using unprecedented high-resolution, high-cadence Solar Orbiter observations, the study demonstrates that a magnetic avalanche of reconnection within an erupting flux rope powers an M7.7 solar flare. The analysis documents a progression from weak, small-scale reconnection events to rapid, multi-site avalanche reconnection that accelerates particles and heats plasma, producing ribbon-like EUV emission and hard X-ray bursts. It places reconnection within the erupting flux rope at the heart of the impulsive phase, challenging standard models that emphasize reconnection below the rising filament. The work links the observed avalanche dynamics to the universal flare energy distribution and underscores the need for high-resolution, coupled MHD-kinetic simulations to capture the onset and evolution of magnetic energy release.

Abstract

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

Figures (13)

• Figure 1: Coronal overview of the impulsive phase of an M-class flare that started around 23:47 UT, observed by Solar Orbiter. The HRI$_{\rm EUV}$ map of the flaring region is displayed (its time stamp with universal time, UT, measured at Earth is shown at the top right corner). The field of view is roughly 107 Mm$\times$71 Mm. The overlaid colored contours identify the STIX hard X-ray source regions in the respective energy bins as labeled. The STIX images were constructed by integrating the signal over the time frame quoted at the bottom right. The contours are at a level of 60% of the maximum of the corresponding STIX images. An animated version of this figure is shown in movie S1.
• Figure 2: Locations of various slits, loops, and the ribbon region. In the upper panel, a larger field of view of HRI$_{\rm EUV}$ covering the flare is shown. The white square outlines the smaller field of view shown in the lower panels. The four ellipses point to remote ribbon brightenings connected to the flare itself. In the lower panels, Slits 1--5 mark the positions of the artificial slits that we used to construct the five time-distance diagrams shown in Fig. \ref{['fig:avalanche']}. Slits 1--3 (lower left panel) cover specific regions where the unstable filament gets disconnected from the underlying complex loop system. Slits 4--5 (lower right panel) mark regions where the unstable filament exhibits fast unwinding motions. The green box in the lower left panel shows the field of view of the complex loop system in Fig. \ref{['fig:eui_st']}, while in the lower right panel it outlines the flare ribbon maps plotted in Fig. \ref{['fig:eui_ribbon']}. The filled white box (indicated by an arrow) is the location from where we derived the time series of the mean HRI$_{\rm EUV}$ intensity shown in Fig. \ref{['fig:avalanche']}E. An animated version of the upper panel is shown in movie S2.
• Figure 3: Relentless weak magnetic reconnection marking the onset of a weak avalanche during the pre-flare phase. Panels A 1--16 are a sequence of HRI$_{\rm EUV}$ running-difference images at 2 s cadence showing the persistent unwinding of loops and their bidirectional separation near the X-shaped configuration in the complex loop system (indicated by green arrows). Similarly, in panels B 1--16, we show an event exhibiting rapid emission enhancement (green arrows) and subsequent explosive reconnection within the loop system. The location of this loop system within the flaring region is shown in the lower right panel of Fig. \ref{['fig:eui_slit']}. In Panel C we plot the time-distance diagram, covering a duration of about 34 minutes, constructed along the slanted dashed line overlaid on panel A-1. The temporal extent of sequences shown in panels A and B is marked by the respective vertical solid lines. Ridges indicate the reconnection (bi-directional) flows moving along the slit, with their slopes giving the plane-of-sky speed. Representative speeds of such motions are given. An animated version of panels A and B is shown in movies S3 and S4.
• Figure 4: Prominent magnetic activity over the course of two minutes during the pre-flare phase leading up to the flare, which started around 23:47 UT. Similar to Fig. \ref{['fig:eui_st']}C, we plotted the time-distance diagrams to capture the prominent coronal dynamics just before the flare. The positions of slits 1--5 are marked in Fig. \ref{['fig:eui_slit']}. The speeds of reconnection flows along the slit are given. In panel E we plot the time series of EUV emission (gold, HRI$_{\rm EUV}$; blue, SDO/AIA 335 Å) emerging from the foot region of the flaring flux rope. The foot region in the EUI images is outlined by the 25--50 keV contour in Fig. \ref{['fig:eui_stix_map']} (see the white box in Fig. \ref{['fig:eui_slit']} and the yellow circle in Fig. \ref{['fig:sdo']}). The hard X-ray emission time series in the 25--50 keV energy range is also shown (yellow, STIX; dark green, Fermi Gamma Ray Burst Monitor).
• Figure 5: Fine-scale raining plasma blobs forming flare ribbons. Each row shows a sequence of HRI$_{\rm EUV}$ running-difference images, at a 2 s cadence, to highlight the sunward propagating plasma blobs along the individual ribbon threads (indicated by green arrows). The time stamp of the respective first instance of each running-difference sequence is indicated in the leftmost panel. The field of view of this region is further shown in Fig. \ref{['fig:eui_slit']}. An animated version of this figure is shown in movie S5.