Stereoscopic Observations of Solar X-ray Sources Explained by a Data-Constrained Magnetohydrodynamic Simulation

TL;DR

This work addresses how 3D magnetic topology governs particle acceleration in solar flares by integrating stereoscopic hard X-ray observations from ASO-S/HXI and Solar Orbiter/STIX with a data-constrained MHD model anchored to photospheric fields. The approach demonstrates that the X7.1 flare in NOAA AR 13842 involves episodic energy release within a single quasi-separatrix layer ($QSL$) system along the polarity inversion line, with two HXR peaks linked to different reconnection phases that migrate along the same QSL. The MHD simulation reproduces the observed conjugate footpoints and reveals a vertically extended thermal HXR source, validating the 3D magnetic context for interpreting HXR emission. The results underscore the value of combining multi-perspective HXR data with data-constrained MHD to constrain particle acceleration processes and guide forward modeling of flare emissions.

Abstract

We investigated the three-dimensional (3D) magnetic structures and dynamics responsible for particle acceleration in an X7.1-class flare that occurred on October 1, 2024, in NOAA active region 13842. We combined stereoscopic hard X-ray (HXR) observations from the Advanced Space-based Solar Observatory/Hard X-ray Imager (HXI) and the Solar Orbiter/Spectrometer Telescope for Imaging X-rays (STIX) with a 3D magnetohydrodynamic (MHD) simulation constrained by observed photospheric magnetic fields. During the two main peaks of the impulsive phase, HXR footpoints appeared at different locations, indicating a migration of the primary reconnection site in the corona. Our data-constrained MHD simulation successfully reproduced the reconnected field lines linking the observed conjugate HXR footpoints. Furthermore, the simulation shows that these primary reconnections occur along a single quasi-separatrix layer (QSL) system. Therefore, the two main peaks of HXR can be interpreted as episodic energy release within the single QSL system. This study demonstrates that the data-constrained MHD model provides a realistic 3D magnetic context for interpreting HXR emission. Notably, STIX observations revealed a vertically distributed thermal HXR source, extending from the footpoints to the looptop, with its centroid migrating between the two peaks. This marks a first step toward understanding the particle acceleration processes in solar flares.

Figures (5)

• Figure 1: (a) Temporal evolution of X-ray observed by HXI, STIX, and GOES-18. For both HXI and STIX, the data are summed to a 4 s resolution. The blue, red, light blue, and green curves denote 32--50 keV, 14-16 keV, 0.5-4.0 Å, and 1.0-8.0 Å, respectively. The STIX light curves are shifted by 352.9 s to consider the different travel time of the light. For 14-16 keV in STIX, the background detector with pixels 2 & 5 was used. The two gray shaded regions indicate the time intervals 22:10:30--22:10:50 (Phase 1) and 22:13:40--22:14:40 (Phase 2), which are used for hard X-ray imaging. (b) Stonyhurst heliographic coordinates, showing the positions of HXI, STIX, and flare location.
• Figure 2: (a) HMI magnetogram at 22:08:45 UT, with $B_z=1.0 \times 10^{-2}$ T in red and $B_z=-5.0 \times 10^{-3}$ T in blue. (b) AIA 1600 Å at 22:10:38 UT with HXR sources observed by HXI (14-16 keV in red and 32--50 keV in blue at 22:10:30-22:10:50 UT). The blue contours correspond to 30%, 50%, 68%, and 90% of the peak, and red to 30%, 50%, 70%, and 90%. (c) HMI 6173 Å difference image (22:14:00-22:12:30 UT) showing WL kernels in black at N2 and S2. Contours show HXI HXR sources (14-16 keV in red and 70--100 keV in blue). For HXR image synthesis, we selected the intervals 22:14:00-22:14:20 UT for the red contours and 22:13:40-22:14:40 UT for the blue contours. Contour levels are the same as in (b). (d) AIA 1600 Å at 22:14:14 UT with the same contours as in (c). In panels (b)-(d), yellow contours show the enhanced region in each image.
• Figure 3: Temporal evolution of the MHD simulation to show magnetic field lines traced from three locations (N1, SE1, and S2). All panels are presented from the northward viewing perspective. The top rows (a)-(c) show magnetic field lines traced from N1, the middle row (d)-(f) from SE1, and the bottom row (g)-(i) from S2. The field lines are colored by $V_{z}$, and the vertical cross-sections in each panel show $|{\bm J}|/|{\bm B}|$, allowing the current sheet structure to be identified. In the top and middle rows, the bottom image in each panel corresponds to the AIA 1600 Å image in Phase 1 (Figure \ref{['fig:fig2']}(b)), while the bottom row corresponds to Phase 2 (Figure \ref{['fig:fig2']}(d)). Red, green, and blue arrows in each panel show the $x$, $y$, and $z$ axes in the simulation box. $t$ is the time normalized by Alfvén time in the simulation, as mentioned in Section \ref{['sec:mhd']}. An animation of the temporal evolution of this figure is available. The animation proceeds from t = 0.00 to 2.40 and shows magnetic field lines traced from N1 (top row), SE1 (middle row), and S2 (bottom row).
• Figure 4: (a)-(d) Reprojected AIA, HXR, and HMI images from Figure \ref{['fig:fig2']}(b) and (c) to the STIX viewpoint. (a) AIA 1600 Å at 22:10:38 UT with HXR sources from STIX (14-16 keV in red, 32--50 keV in blue, 22:10:30-22:10:50 UT). Blue contours show 15%, 30%, 50%, and 70% of the peak intensity. Red contours show 11%, 20%, 35%, 60%, and 80%. (b) HXI HXR in 32--50 keV as the background in Figure \ref{['fig:fig2']}(b) and the same STIX HXR contours in panel (a). (c) HMI 6173 Å difference image (22:14:00-22:12:30 UT) to see WL kernels with STIX HXR contours (14-16 keV in red at 22:14:00-22:14:20 UT and 70--100 keV in blue at 22:13:40-22:14:40 UT). Contour levels are as in (a). (d) HXI HXR in 70--100 keV as the background in Figure \ref{['fig:fig2']} (c) and the same STIX HXR contours as in panel (c). In all panels, yellow contours show the enhanced region in each image.
• Figure 5: (a) Logarithmic $Q$ at $t=0.00$. Red and blue contours mark $B_z=1.0\times10^{-2}$ T and $B_z=-5.0\times10^{-3}$ T at 20:36 UT. (b) Logarithmic $Q$ at $t=1.92$ with the same $B_z$ contours as in (a). (c) AIA 1600 Å image at 22:10:38 UT with logarithmic $Q$ at $t=1.92$ overlaid, where regions with $\log Q \ge 6$ are highlighted in green for better visualization with the AIA image. The inset shows only the AIA image in the same field of view as (c). (d) Magnetic field lines at $t=1.92$ traced from QSL associated with the flare ribbons. (e) Magnetic field lines at $t=1.92$ traced from QSL associated with the remote brightening. Time $t$ is normalized to the Alfvén time (see Section \ref{['sec:mhd']}). An animation of the temporal evolution of panels (a) and (b) is available. The animation proceeds from t = 0.00 to 2.40.