Solar Orbiter Reveals Ultra-fine Magnetic Reconnection Processes in Filament Eruptions.

(Solar Orbiter Nugget #78 S. Tan¹, A. Warmuth¹, F. Schuller¹, J. A. J. Mitchell¹, Y. Shen³, D. Calchetti⁴, J. Hirzberger⁴, T. Oba⁴, A. Ulyanov⁴, Gherardo Valori⁴, D. F. Ryan⁵ and Fanpeng Shi⁶)

Introduction

Solar filaments are cool and dense magnetized plasma structures in the corona. Their destabilization and eruption are key processes that trigger coronal mass ejections (CMEs) and flares. Decades of observations and theoretical and numerical simulation studies have revealed the physical nature of filament eruptions: it is a process of magnetic disruption and energy release. This process occurs through two main mechanisms: one is magnetic reconnection leading to magnetic topology reconfiguration, including tether-cutting reconnection [1,2], breakout reconnection [3], and magnetic flux emergence [4]; the other involves ideal magnetohydrodynamic instabilities, such as kink instability [5,6] and torus instability [7]. Therefore, accurately identifying and evaluating the role of magnetic reconnection through high-resolution observations is crucial for understanding filament evolution.

The Solar Orbiter spacecraft, with its perihelion observations at 0.28-0.29 AU and the ultra-high spatial resolution of the Extreme Ultraviolet Imager (EUI) (105 km/pixel), provides breakthrough opportunities for studying fine structures in filament eruptions.
We utilized two flare observation campaigns by Solar Orbiter in 2024, integrating data from the EUI, STIX, and PHI instruments, to study  frequent small-scale magnetic reconnection events and their cumulative effects during filament eruption processes, providing new insights into solar eruption mechanisms. 

Results

Our results are summarized in two separate studies, highlighted below. 

Study 1: "Persistent Magnetic Cutting" Phenomenon in a Medium-scale Failed Filament Eruption.

On April 5, 2024, Solar Orbiter/EUI conducted continuous 4-hour observations of a double-decker filament (defined as two filaments at different heights that share the same polarity inversion line [8]) approximately 40 Mm long at the western limb, completely recording its failed eruption process, including the pre-eruption disturbances, the reconnection of the surrounding magnetic field, and the sequential eruptions of the two layers.
 

Figure 1. Full-disk image of EUI (inverted grayscale) superposed with HRI, with the relative positions of Solar Orbiter. (b): Image of HRI (rotated to facilitate the analysis), with the centrally positioned white box (also the FOV of panel (d) and its enlargement labeled with the target filament position). (c): Multiple jets throughout the filament eruption. (d1)-(d6): Filament evolution in HRI images. Different features are annotated. The dashed blue line in panel (d5) indicates the slice position. (e): Intensity profiles (blue) along the slice; the red and yellow curves fitted with a Gaussian indicate the UF and LF position, respectively, and the double-decker filament centers are indicated with dashed lines. (f): Time-space plot of the filament eruption; the UF and LF eruption speeds are labeled, and the time corresponds to panel (d5).

Study 1 - Observations.
For this event, we could measure the double-decker filament structure with exquisite level of detail: the upper filament had a thickness of only 2-3 Mm, with a projected height difference of 6.8 Mm between the two layers (Fig. 1). The observations revealed a series of complex magnetic reconnection events: emerging magnetic flux at the base triggered multiple jets; reconnection between the filament and adjacent open magnetic fields produced collimated jets; reconnection between the upper filament and the surrounding magnetic field led to mass drainage (rising velocity of 4.9 km/s); although the lower filament rose at 11.5 km/s, it ultimately underwent reconnection with the overlying magnetic field, leading to a failed eruption.

Figure 2. (a): Reconnection process of the double-decker filament, the nearby open field, and the emerging flux loop at the bottom. Orange and green curves represent the UF and LF, and blue the nearby open field. (b): Small post-flare loop formed by the filament lifting stretching the lower closed field and the large loop formed by the falling filament material. The bottom space is divided into three ranges to indicate the magnetic field around the filament.

Two types of post-flare loops formed after the failed eruption: small-scale compact loops and large-scale loops, with the latter's brightness peak occurring approximately 70 minutes later than the former, indicating different formation mechanisms (Fig. 2).

Study 1 – key results:
Based on the observations discussed above, we introduced the concept of "persistent magnetic cutting," emphasizing the frequently occurring small-scale magnetic reconnection (10-100 Mm) and its cumulative effects during filament evolution. This process has four core characteristics: temporal persistence (throughout the entire evolution), small spatial scales, configurational complexity (multiple locations, various magnetic field structures), and cumulative effects (significantly affecting filament stability). This differs from the traditional theoretical picture of a single catastrophic magnetic reconnection event, providing a new explanation for the relatively short lifetimes of small- to medium-scale filaments.

Study 2: Extremely Dynamic Coronal Jets Surrounding an Erupting Filament

Solar jets are collimated plasma ejections driven by magnetic reconnection, classified into various morphological types including standard and blowout jets [9]. Solar Orbiter's EUI instrument, particularly HRIEUV with its unprecedented spatial resolution, has enabled discoveries of small-scale dynamic features like "campfires" in quiet regions and coronal holes [10]. However, high-resolution HRIEUV observations of coronal jets associated with filament eruptions remain lacking. On September 30, 2024, Solar Orbiter observed an eruption of a filament approximately 80 Mm long, accompanied by transient coronal jets with highly diverse morphologies (see the nine representative jets in Fig. 3.)

Figure 3. (a) HRI image (inverted color scale) showing the erupting filament marked by a black rectangular box, with a length of approximately 80 Mm. (b) Magnified view of the erupting filament in HRI, with gray rectangular boxes marking the locations of jets. (c) Most of the prominent frames of the nine jets, corresponding to the locations marked in (b). (d) STIX light curves in 4--10 keV and 25--50 keV bands (the background intensity was not subtracted), with the occurrence times of the nine jets indicated. It is worth clarifying that due to the intervention of the attenuator, the 4--10 keV light curve shows an abrupt decay from 23:44 UT. For this segment, we use the BKG detector data instead.

Study 2 – Observations.
STIX observations revealed two distinct energy release phases before and during the eruptions, tracing energy release and particle acceleration associated with magnetic reconnection. The median lifetime of nine jets was only 22 seconds, significantly shorter than typical coronal jets (several minutes to tens of minutes). They exhibited highly diverse morphologies and showed systematic spatiotemporal distributions during three evolutionary phases of the filament eruption (initiation phase, rising phase, and peak phase) (Fig. 4). 
 

Figure 4. Group 1 jet representative: Jet 1 exhibiting standard jet morphology, with its key properties labeled (a1 and a2). The blue curve shows intensity distribution along the jet. Group 2 jet representative: Jet 4 displaying standard jet morphology with its key properties labeled (b1 and b2). Group 3 jet representative: Jet 8 showing blowout jet characteristics with its key properties labeled (c1and c2). The blue curve shows intensity distribution across the jet. Cartoon illustration explaining the jets produced during different phases of filament eruption (initiation, rising, peak), with annotations showing the viewing angles from Solar Orbiter and SDO (d).

Study 2 -Key results.
These jets represent a new class of phenomena—they are not produced by independent mini-filament eruptions but are direct products of dynamic magnetic reconnection between the erupting filament and overlying magnetic fields. The dynamic evolution of the jets also corresponds to the filament eruption process. This discovery extends the classification of coronal jets and provides direct observational evidence for the "persistent magnetic cutting" concept proposed in the first study.

Discussion

We highlighted two complementary studies, constructing a complete picture of the complexity of magnetic reconnection during filament eruption processes. The first study demonstrated a multi-stage reconnection sequence throughout a failed filament eruption, while the second study proved frequent and rapid dynamic reconnection through extremely short-lived jets.
These observations of medium-scale filaments (40-80 Mm) bridge the gap between mini-filaments (~10 Mm) and large-scale filaments (100-1000 Mm), indicating that magnetic reconnection plays a dominant role across different scales. We also compared coronal "persistent magnetic cutting" with interplanetary "magnetic flux rope erosion" processes, revealing the multi-scale and multi-mode characteristics of magnetic reconnection in the heliospheric system.
Solar Orbiter's unique advantages—close-distance observations, ultra-high resolution (100 km/pixel), continuous observation capability (several hours), and multi-instrument coordination of EUI/PHI/STIX—made these discoveries possible. Future research needs to combine richer observational data to deeply explore the quantitative characteristics and energy conversion mechanisms of "persistent magnetic cutting," providing more precise physical constraints for understanding complex eruption processes in the solar atmosphere.
 

Both studies are published in Astronomy & Astrophysics.
1): Tan et al., "Solar Orbiter reveals persistent magnetic reconnection in medium-scale filament eruptions", A&A, 702, A88 (2025) DOI: 10.1051/0004-6361/202555300
2): Tan et al., "Extremely diverse coronal jets accompanying an erupting filament captured by Solar Orbiter", A&A, 702, A189 (2025)
DOI: 10.1051/0004-6361/202555297

Affiliations

(1) Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
(2) Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Straße 24/25, 14476 Potsdam, Germany
(3) State Key Laboratory of Solar Activity and Space Weather, School of Aerospace, Harbin Institute of Technology, Shenzhen 518055, China
(4) Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
(5) University College London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
(6) Key Laboratory of Dark Matter and Space Science, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China

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