Resolving Interchange Reconnection Dynamics in a Fan-Spine-like Topology Observed by Solar Orbiter

Interchange reconnection is believed to play a significant role in the production of solar jets and solar wind. However, the dynamics of interchange reconnection in the low corona might be more complex than recognized before in higher temporal and spatial resolutions. Using unprecedentedly high-resolution observations from the Extreme Ultraviolet Imager (EUI) onboard the Solar Orbiter, we analyze the dynamics of interchange reconnection in a small-scale fan-spine-like topology. Interchange reconnection that continuously occurs around the multi-null points of the fan-spine-like system exhibits a quasi-periodicity of ~200 s, nearly covering the entire evolution of this system. Continuous evolution and reversal of multiple current sheets are observed over time near the null point. These results reveal that the dynamics of interchange reconnection are likely modulated by the emerging magnetic structures, such as mini-filaments and emerging arcades. Moreover, a curtain-like feature with a width of 1.7 Mm is also observed near the interchange reconnection region and persistently generates outflows, which is similar to the separatrix curtain reported in the pseudo-streamer structure. This study not only demonstrates the complex and variable reconnection dynamics of interchange reconnection within small-scale fan-spine topology but also provides insights into the self-similarity of magnetic field configurations across multiple temporal and spatial scales.

💡 Research Summary

This paper presents a detailed observational study of interchange reconnection occurring within a small‑scale fan‑spine‑like magnetic topology, using unprecedentedly high‑resolution data from the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter. The event took place on 5 April 2024 when Solar Orbiter was positioned 0.29 AU from the Sun at an 83° separation angle from Earth, allowing a clear view of the western limb. The authors focus on the dynamics around the magnetic null point(s) that define the fan‑spine configuration, which consists of a dome‑shaped fan separatrix surface, a central null, and inner and outer spines.

Observational Data and Methodology
The primary dataset consists of High‑Resolution Imager (HRI) EUV 174 Å images with a 16 s cadence and a pixel scale of 0.49″ (≈0.108 Mm). Complementary Full‑Sun Imager (FSI) 174 Å and 304 Å images (10 min cadence, 4.44″ pixel) provide context over a larger field of view. The authors applied cross‑correlation techniques for image alignment and jitter removal, then constructed time–distance diagrams, performed wavelet and Fourier analyses, and produced schematic sketches to interpret the magnetic connectivity.

Evolution of the Fan‑Spine‑Like Structure
Initially, a small dark closed loop (interpreted as a cool filament or arch) emerges to the right of a coronal plume. It expands slowly (~2 km s⁻¹) toward the ambient open field. At 22:13:48 UT an X‑point forms where the emerging loop interacts with the surrounding open field, producing a new closed loop and a thin, sheet‑like feature that the authors identify as a current sheet (CS1). Bright plasma blobs appear within CS1, consistent with tearing‑mode instability and previous observations of plasmoid formation in reconnection layers.

Multiple Current Sheet Transitions
Around 22:17 UT a second current sheet (CS2) appears within the fan‑spine system, accompanied by the upward motion of the first filament (Filament 1). Filament 1 eventually erupts, changing from a closed to an open configuration. A second filament (Filament 2) activates at 22:21 UT, erupts in the direction of CS2, and appears to be triggered by the removal of an overlying arcade by CS2. By 22:24 UT a bright linear feature (CS3) connects the base of the jet spire to the top of a post‑flare arcade, resembling the classic flare current sheet that forms behind an erupting flux rope.

Quasi‑Periodic Reconnection and Outflows
Wavelet analysis of the intensity along a slit crossing CS3 reveals a quasi‑periodic signal with a dominant period of ≈200 s. Time–distance diagrams show continuous plasma outflows associated with this periodicity, with speeds of several hundred km s⁻¹. The authors argue that the quasi‑periodic interchange reconnection is modulated by the emergence and evolution of the mini‑filaments and the associated arcade structures.

Curtain‑Like Feature and Self‑Similarity
A narrow, curtain‑like bright structure of width 1.7 Mm is observed near the reconnection region. This feature persists throughout the event and continuously channels plasma outflows, closely resembling the “separatrix curtain” reported in large‑scale pseudo‑streamer configurations. Its presence suggests that the same topological elements—multiple nulls, separators, and quasi‑separatrix layers (QSLs)—govern reconnection dynamics across a wide range of spatial scales.

Interpretation and Implications
The study demonstrates that interchange reconnection in a fan‑spine topology is not a single, isolated episode but a complex, multi‑stage process involving:

1. Repeated formation and dissolution of current sheets (CS1 → CS2 → CS3).
2. Generation of plasmoids (bright blobs) via tearing instability.
3. Modulation by emerging magnetic structures (mini‑filaments, emerging arcades).
4. A quasi‑periodic reconnection signature (~200 s) that drives continuous high‑speed outflows.
5. A curtain‑like separatrix that guides plasma ejection, mirroring large‑scale pseudo‑streamer behavior.

These findings bridge the gap between small‑scale jet and bright‑point phenomena and the larger‑scale slow‑solar‑wind sources associated with pseudo‑streamers. By revealing magnetic self‑similarity, the work supports the notion that the same fundamental topological ingredients control energy release from the low corona to the heliosphere.

Methodological Contribution
The combination of ultra‑high‑cadence EUV imaging, quantitative time–distance analysis, and spectral (wavelet/Fourier) techniques provides a robust framework for dissecting rapid reconnection dynamics. The ability to resolve current‑sheet evolution on timescales of tens of seconds represents a significant advance over previous studies that relied on lower‑cadence data.

Conclusions
Interchange reconnection within a small‑scale fan‑spine‑like topology exhibits a quasi‑periodic (~200 s) cadence, multiple current‑sheet transitions, and a persistent curtain‑like outflow channel. The dynamics are driven and modulated by the emergence of mini‑filaments and arcades, highlighting the importance of low‑lying magnetic evolution in governing coronal energy release. The observed self‑similarity with pseudo‑streamer structures suggests that insights gained from these high‑resolution, low‑corona observations can be extrapolated to understand larger‑scale solar wind and heliospheric phenomena.