The Non-Eruptive Reconfiguration of a Quiescent Filament After a Nearby Active Region Emergence

The unpredictability of solar filament eruptions presents major challenges for forecasting space weather, as such eruptions frequently drive coronal mass ejections (CMEs) that impact the heliosphere. While nearby flux emergence is often linked to their destabilisation, the specific characteristics of both the emerging flux and the filament that determine whether an eruption occurs remain unclear. We report observations of a quiescent filament that did not erupt following the nearby emergence of active region NOAA 13270 and a subsequent C-class flare in April 2023. Our analysis combines multi-viewpoint extreme ultraviolet (EUV) imaging and X-ray imaging with EUV spectroscopy, radio imaging and measurements of, and extrapolations from, the photospheric magnetic field. We identify the formation of a coronal null point and fan-spine topology at the interface between the active region and filament which exhibited persistent slow reconnection, indicated by chromospheric brightenings, persistent radio emission, and plasma upflows. Our results indicate that ongoing reconnection and jets can relieve magnetic stress and enable filament stability, even when under strong perturbation. We suggest that the orientation of emerging flux relative to the ambient field is a critical parameter in filament evolution, and provide observational constraints for models of filament stability and eruption.

💡 Research Summary

The paper presents a detailed case study of a quiescent solar filament that remained stable despite the nearby emergence of active region (AR) NOAA 13270 in early April 2023. Using a comprehensive suite of observations—including multi‑viewpoint EUV imaging from SDO/AIA and Solar Orbiter/EUI‑FSI, EUV spectroscopy from Hinode/EIS, photospheric vector magnetograms from SDO/HMI with NLFFF extrapolations, radio imaging from the Nanҫay Radioheliograph, and hard/soft X‑ray imaging from Solar Orbiter/STIX and Hinode/XRT—the authors trace the magnetic and plasma evolution from the first signs of flux emergence on 1 April through the C3.9 flare on 6 April.

The emergence began as a small bipole on the northern edge of the filament’s negative polarity region. By 2 April the flux had intensified, expanding the AR and producing remote brightenings parallel to the filament channel. On 3 April the trailing positive polarity of the AR became parasitic within the surrounding negative field, giving rise to a circular chromospheric ribbon and, crucially, a coronal magnetic null point with a fan‑spine topology. NLFFF extrapolations and Q‑map analysis identified a high‑Q dome (the fan) anchored in the photosphere and inner/outer spine field lines converging at the null.

Signatures of ongoing, slow reconnection at this null were observed repeatedly: (i) persistent chromospheric brightenings and a circular ribbon in 304 Å and 171 Å images, (ii) a long‑lasting Type I noise storm in the 150–445 MHz band, indicating continuous low‑energy electron acceleration, and (iii) steady upflows of 10–30 km s⁻¹ in the Fe XII 195 Å line measured by EIS, spatially coincident with the fan‑spine footpoints. These reconnection signatures lasted for several days, well before and after the C‑class flare.

The flare itself was confined to the fan‑spine structure; hard X‑ray sources and hot (≈10 MK) loops imaged by STIX and XRT were located within the dome and did not intersect the filament’s core field. Consequently, the flare did not remove the overlying “strapping” field that typically stabilizes a filament. Instead, the continuous reconnection acted as a pressure‑release valve, dissipating magnetic stress and allowing the filament to reconfigure without loss of equilibrium.

The authors argue that the orientation of the emerging flux relative to the ambient field—specifically the formation of a parasitic polarity that creates a null‑point fan‑spine system—determines whether the interaction will destabilize or stabilize a filament. This contrasts with the classic magnetic breakout scenario, where reconnection at a null removes overlying flux and triggers eruption. Here, reconnection is slow, persistent, and largely confined to the fan‑spine, thereby protecting the filament.

Key conclusions are: (1) a coronal null point and fan‑spine topology can develop rapidly during AR emergence near a filament; (2) sustained low‑rate reconnection at such a topology can relieve magnetic stress and prevent eruption; (3) the relative orientation of emerging flux is a critical parameter for filament stability; and (4) observable proxies—chromospheric ribbons, Type I radio noise storms, and steady EUV upflows—provide reliable diagnostics of this stabilizing reconnection. The study supplies valuable constraints for filament‑eruption models and highlights the importance of non‑eruptive interactions as tests for theoretical frameworks.