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

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

Problem Statement
Interchange reconnection is a fundamental process in the solar atmosphere, facilitating the transfer of mass and energy between closed and open magnetic fields, driving solar jets, and contributing to the solar wind. While large-scale pseudostreamer structures are known to host gradual interchange reconnection via separatrix curtains and multiple null points, the dynamics of this process in small-scale fan-spine topologies remain less understood. Traditional models often assume fan-spine structures contain a single magnetic null point, yet recent theoretical and observational work suggests the potential for multiple null points, quasi-separatrix layers (QSLs), and separators. Furthermore, the high temporal and spatial resolution required to resolve the complex, continuous dynamics of interchange reconnection in these small-scale systems has historically been a limiting factor. This study addresses the need to characterize the dynamics of interchange reconnection within a small-scale fan-spine-like system with unprecedented resolution to determine if its behavior mirrors the complexity of large-scale pseudostreamers.

Methodology
The authors utilized high-resolution observational data acquired on April 5, 2024, by the Extreme Ultraviolet Imager (EUI) aboard the Solar Orbiter. The event occurred at the Sun's western limb, rendering it invisible from Earth, thus necessitating the exclusive use of Solar Orbiter data.

• Instrumentation: The study primarily employed the High-Resolution Imager (HRIEUV) operating at 174 Å, which provided a time cadence of 16 seconds and a pixel scale of 0.49 arcseconds (0.108 × 0.108 Mm² on the Sun). Complementary data from the Full Sun Imager (FSI) in 174 Å and 304 Å channels were used to study plasma motions over a larger field of view.
• Data Processing: Cross-correlation techniques were applied for data alignment and jitter elimination.
• Analysis Techniques: The authors performed time-distance analysis to track plasma outflows and current sheet evolution. Wavelet and Fourier analyses were applied to light curves extracted from specific regions to identify periodicities in plasma outflows. The study also estimated reconnection rates based on the dimensions of observed current sheets, referencing the Sweet-Parker mechanism.

Key Results
The observations revealed a complex, evolving fan-spine-like topology driven by continuous interchange reconnection around multinull points:

1. Evolution of Current Sheets: Three distinct current sheets (CS1, CS2, and CS3) were identified near the null point.
   • CS1 formed from the interaction between emerging magnetic loops and the ambient field, exhibiting an X-point and plasma blobs indicative of tearing instability.
   • CS2 appeared following the eruption of a filament on the side of the structure, lasting approximately 6 minutes with a width of 0.6–0.8 Mm and a length growing from 3 to 5 Mm.
   • CS3 formed in the wake of a second filament eruption, resembling a flare current sheet. It lasted 17 minutes, with a width of 0.4–0.6 Mm and a length varying up to 7.4 Mm.
   • Reconnection Rates: The estimated reconnection rates were 0.12–0.2 for CS2 and 0.12–0.17 for CS3.
2. Curtain-like Feature: A persistent curtain-like feature with a width of 1.7 Mm was observed near the interchange reconnection region. This feature continuously generated plasma outflows with an average velocity of ~80 km s⁻¹.
3. Quasi-Periodicity: Wavelet and Fourier analyses of the plasma outflows along both the jet spire and the curtain feature revealed a quasi-periodicity of approximately 200 seconds. This periodicity nearly covered the entire evolution of the system.
4. Magnetic Topology: The dynamics, including the reversal of current sheets and the presence of multiple interacting structures, suggest the system involves multiple null points or separators rather than a single null point. The observed "curtain" is analogous to the separatrix curtain found in large-scale pseudostreamers.

Key Contributions

• Direct Imaging of Complex Dynamics: The study provides direct imaging evidence of continuous, quasi-periodic interchange reconnection occurring around multinull points in a small-scale fan-spine topology, a level of detail previously unattainable.
• Identification of a Small-Scale Separatrix Curtain: The authors identify a curtain-like feature in a small-scale system that persists and generates outflows, demonstrating a structural similarity to the separatrix curtains of large-scale pseudostreamers.
• Evidence of Modulation: The results indicate that interchange reconnection dynamics are modulated by emerging magnetic structures, such as mini-filaments and emerging arcades, leading to complex current sheet reversals and transitions.

Significance and Claims
The paper claims that the dynamics of interchange reconnection in small-scale fan-spine topologies are likely more complex than previously recognized, potentially involving multiple null points and separators rather than a single null point. The observation of a curtain-like feature and quasi-periodic reconnection in a small-scale system provides evidence for the self-similarity of magnetic field configurations across different temporal and spatial scales. Specifically, the study suggests that small-scale fan-spine structures may act as scaled-down versions of large-scale pseudostreamer structures, sharing similar reconnection mechanisms (such as separatrix curtains) that contribute to the continuous injection of mass and energy into the high corona and the solar wind. The authors emphasize that these findings offer new insights into the triggering mechanisms of solar jets and the origin of the slow solar wind, highlighting the need for higher-resolution observations to fully understand the variable nature of reconnection in the low corona.