Height and Energy Evolution of X-ray Double Sources in a Solar Flare

TL;DR

This study analyzes the decay-phase evolution of X-ray coronal sources in an M6.7 limb flare on 2022-08-28 using Solar Orbiter/STIX and SDO data. It introduces a geometry-based method to estimate X-ray source heights and tracks a persistent double coronal source across multiple energy bands, complemented by AIA-based DEM analysis. The results reveal that higher-energy emissions originate at higher altitudes and that the double source consists of two thermally distinct plasmas, implying multiple energy-release sites within a rising post-flare current sheet. Collectively, the findings illuminate the 3D structure and time evolution of energy release in the decay phase of solar flares and provide constraints on reconnection-driven transport in the supra-arcade region.

Abstract

In the standard model, magnetic reconnection at a vertical current sheet above the flare arcade is key to explaining many aspects of solar eruptions. The supra-arcade region is where the vertical current sheet is supposedly located, and X-ray/EUV emission therein reflects underlying energy release and transport processes, therefore providing valuable insight into the structure and evolution of the current sheet. Previous studies have focused primarily on the impulsive phase of flares, but phenomena in the decay phase are also crucial for understanding the complete flaring scenario. In this paper, we investigated an M6.7-class limb flare that occurred on August 28, 2022, combining observations from the Solar Orbiter (SolO) and Solar Dynamics Observatory (SDO). Coronal X-ray sources are continually observed by the Spectrometer/Telescope for Imaging X-rays (STIX) onboard SolO for over two hours, revealing a multi-phase evolution with varying velocities and multiple substructures, with higher-energy components consistently appearing at higher altitudes. Such a height-energy relation is notably observed in a double coronal source during the decay-phase, which is dominated by thermal emission. The energy distribution of the double source distinguish itself from previous studies that showed a symmetric distribution, with the higher-energy components being closer to the center of the double source during the impulsive phase. Obtained from two opposite side-on perspectives on the supra-arcade region, these findings reveal the spatio-temporal complexity of the energy release process in the post-flare current sheet during the decay phase.

Figures (8)

• Figure 1: Overview of the eruption from two different viewpoints of the SDO and SolO spacecrafts. The red arrow in panel (a) indicates the CME detected by SOHO/LASCO C2. Panels (b-c) show the snapshots before and after the associated M7.6 flare by combining the three AIA passbands, 131 (red), 193 (green), and 171 Å (blue), with the white arrow indicating the twisted, tube-like structure. The field of view in panels (b-c) corresponds to the small black rectangle in (a). In panel (d) the blue, orange and yellow dots indicate the relative positions of the satellites SolO, SDO (Earth) and the Sun, respectively, and the green arrow indicates the direction of the eruption. Panel (e) shows the active region in the EUI 174 Å image near the start time of the flare. The time stamps shown in each panel correspond to the original observation times recorded by the respective instruments. The accompanying animation shows the X-ray light curves in the same format as in Figure \ref{['fig:spec_fit_tl']}a, the time-elapsed AIA 131, 193, and 171 Å images, as well as the composite images combining the three narrow passbands, from 15:30 to 19:15 UT on 2022 August 28.
• Figure 2: X-ray light curves and spectral fitting. In panel (a), the black and gray curves show respectively 1--8 Å and 0.5--4 Å soft X-ray flux observed by GOES. The colored curves show the X-ray count fluxes measured by SolO/STIX at different energy ranges from 4 to 50 keV. The original STIX observation times have been added by 115.24 seconds for the light traveling from the SolO to the Earth. The brown vertical lines indicate the corrected times of panels (b--f), in which the STIX time stamps are shown. The gray vertical line denotes the flare peak time. The horizontal long arrow marks the time interval during which SADs are observed. In panels (b--f), the observed photon fluxes are shown in histograms. The green and blue curves denote respectively the thermal and nonthermal component of the fitting scheme; the latter adopts the thick-target model. The sum of all the individual components is indicated by the purple curve.
• Figure 3: Co-alignment of the STIX X-ray source positions. The the background images from the left to right columns are EUI 174 and 304 Å, reprojected AIA 1600 Å and corresponding difference image. The superimposed X-ray sources (16--28 keV) reconstructed by the MEM_GE (1st row) and EM (2nd row) methods are shown as green contours (30%, 50%, 80% of the maximum value). The time stamps are recorded by the instruments.
• Figure 4: Photospheric magnetic field in relation to the STIX X-ray sources. Panles (a1--a2) show the line-of-sight (LoS) magnetic field observed by SolO/PHI. In panels (b1--b2) the LoS magnetic field observed by SDO/HMI is reprojected to the SolO perspective. The original HMI LoS magnetogram is shown in panel (c). Curves A and B denote the main PILs. The orange arrow indicates where the magnetic field polarity is uncertain. The STIX X-ray sources (16--28 keV) obtained using the MEM_GE and EM methods are superimposed as contours (30%, 50%, and 80% of the maximum value). The time stamp shown in each panel is recorded by the respective instrument.
• Figure 5: Method to estimate the height of coronal X-ray sources above the solar surface. Panels (a1–a4) illustrate the basic assumptions and coordinate systems to help determine the height of the loop-top source. The blue dots indicate the the observed X-ray sources (corresponding to the blue plus signs in panel (b)), while the red dot indicates the midpoint between the two footpoint sources (corresponding to the red plus signs in panel b). The unit vector $\mathbf{\hat{v}}$ points from the red to the blue dot above the surface, and its projection onto the observational plane is denoted by the unit vector $\mathbf{\hat{v}}_p$. The $xyz$ axes in the coordinate system 'O' are indicated by the arrows in the bottom left corner of panels (a1–a3), and the unit vector $\mathbf{\hat{n}}$ is perpendicular to the observation plane. The relationship between the coordinate systems 'O' and 'L' (see the text) is illustrated in (a4). Panels (b-c) provide examples of the observed coronal X-ray sources, with (panel (b)) or without (panel (c)) footpoint sources.