Two-Stage Nature of a Solar Flare with Parallel and Semi-Circular Ribbons

TL;DR

This study analyzes an eruptive M8.2 flare on 2023-09-20 that exhibits both quasi-parallel and semi-circular ribbons. By combining multi-wavelength observations with potential-field extrapolation, it reveals a dome-like magnetic structure with a null point forming a fan-spine topology and two distinct connectivity domains. The authors propose a two-stage reconnection scenario: standard flare reconnection explains the two major HXR peaks at $t_1$ and $t_3$ associated with the quasi-parallel ribbons, while QSL reconnection in the fan-spine topology accounts for the semi-circular ribbon brightening around $t_2$. The interaction between eruptive structures and the dome constrains reconnection and drives the CME, highlighting the role of 3D topology in energy release and guiding future stereoscopic studies and modeling of complex flares.

Abstract

Flare ribbons with parallel and circular morphologies are typically associated with different magnetic reconnection models, and the simultaneous observation of both types in a single event remains rare. Using multi-wavelength observations from a tandem of instruments, we present an M8.2-class flare that occurred on 2023 September 20, which produced quasi-parallel and semi-circular ribbons. The complex evolution of the flare includes two distinct brightening episodes in the quasi-parallel ribbons, corresponding to the two major peaks in the hard X-ray (HXR) light curve. In contrast, the brightening of semi-circular ribbons temporally coincides with the local minimum between the two peaks. Using potential field extrapolation, we reconstruct an incomplete dome-like magnetic structure with a negative polarity embedded within the northwestern part of the semi-circular positive polarity. Consequently, the magnetic configuration comprises two sets of field lines with distinct magnetic connectivities. We suggest that the standard flare reconnection accounts for the two-stage brightening of quasi-parallel ribbons associated with the two HXR peaks. Between the two stages, this process is constrained by the interaction of eruptive structures with the dome. The interaction drives the quasi-separatrix layer reconnection, leading to the brightening of semi-circular ribbons. It also suppresses the standard flare reconnection, resulting in a delayed second HXR peak.

Figures (8)

• Figure 1: (a) GOES soft X-ray fluxes at 1--8 Å (red) and ASO-S/HXI X-ray light curves in 10--20 (blue), 20--30 (olive), and 30--50 (pink) keV. Three vertical dashed lines represent the moments of t1 (14:15:44 UT), t2 (14:16:40 UT), and t3 (14:16:53 UT), corresponding to the two major peaks and the trough of the HXR light curves in 30--50 keV. The two stages of this event (stage 1: 14:14:41--14:16:40 UT; stage 2: 14:16:40--14:18:36 UT) are shaded in gray, separated by t2.
• Figure 2: An overview of the M8.2-class flare on 2023 September 20. (a) SDO/HMI line-of-sight magnetogram of the flaring region, overlaid with red contours at the 90% intensity level from the SDO/AIA 1600 Å emission. (b)--(h) Images observed at $\sim$14:15:44 UT (t1) showing CHASE Fe I (6569.2 Å), H$\alpha$ (6562.8 Å), AIA 1600 Å, SUTRI 465 Å, AIA 304 Å, 171 Å, and 131 Å emissions. R1--R6 refer to the flare ribbons. The yellow arrow in panel (d) points to ribbon S1. Arcades 1 and 2 are marked by red dashed lines in panel (h). (i) Light curves of GOES SXR flux at 1--8 Å (red), SUTRI 465 Å (pink), and AIA 94 Å (atrovirens), 131 Å (blue), 171 Å (brown), 304 Å (deep red), and 1600 Å (green), respectively. The AIA and SUTRI fluxes are integrated over the yellow rectangle indicated in panel (e). The light gray shading denotes the same time interval as that in Figure \ref{['figure01']}. An animation of this figure is available.
• Figure 3: (a)--(h) AIA 1600 Å images from 14:14:14 to 14:17:02 UT. Cyan and red contours in panels (d)--(g) show HXR sources in 15--20 and 30--50 keV, respectively, reconstructed via the CLEAN algorithm and plotted at 40%, 60%, and 90% of the maximum flux. In panel (h), the image is overlaid by the pink and blue contours of the HMI line-of-sight magnetogram being $\pm$ 80 G. Arrows h1 and h2 in panels (e) and (f) mark the hook-shaped extension of ribbons R1 and R2. (i) HMI line-of-sight magnetogram at 14:17:02 UT. Orange contours represent 90% of the maximum intensity from the co-temporal AIA 1600 Å image. An animation of this figure is available.
• Figure 4: (a) HMI line-of-sight magnetogram overlaid with red contours at the 90% intensity level of AIA 1600 Å image at 14:17:02 UT. Cyan rectangles mark ribbon regions R1--R6. Arcade 1 and 2 are delineated by yellow dashed lines. (b) AIA 1600 Å light curves in regions R1 (purple) and R2 (red), and 30--50 keV HXR flux (pink) from 14:13:26 to 14:20:38 UT. The gray shading represents the two-stage time interval, identical to that in Figure \ref{['figure01']}. (c) Same as (b), but for regions R3 (cyan), R4 (olive), R3+R4 (brown), R5 (green), and R6 (orange).
• Figure 5: (a)--(d) Temporal evolution of AIA 131 Å images and HXR 15--20 keV sources (red contours). "Arcade 1" denotes the post-flare loops as outlined by magenta dashed lines. (e)--(h) AIA 131 Å running-difference images from 14:14:42 to 14:17:06 UT. The black box marks the field of view in panel (a). An animation of this figure is available.