Multiple Extreme Ultraviolet Peaks Attributed to Three-dimensional Magnetic Reconnection in a Long-duration Solar Flare

Abstract

Solar flares are a major driver of hazardous space weather, whose intense electromagnetic emissions and energetic particles can significantly disturb the near-Earth space environment. Therefore, understanding the physical processes during a solar flare and predicting its radiation profiles are of great importance. In this study, we analyze and model an M1.4 two-ribbon long-duration flare, whose multiple extreme-ultraviolet (EUV) emission peaks are found to correspond to different three-dimensional (3D) magnetic reconnections driven by the continuous evolution of a flux rope. In particular, the second and third peaks in the 335 Å EUV channel originate from longer and higher flare loops with extended cooling times, formed by reconnection between flux-rope field lines and ambient sheared-arcade field lines ($ar\text{--}rf$) and between flux-rope field lines themselves ($rr\text{--}rf$). These results are supported by the drifting of the flux-rope footpoint (and flare ribbon) and the decrease in toroidal flux of the flux rope, as well as by the connectivity transfer of representative field lines in the magnetohydrodynamic (MHD) simulation. This work points out, for the first time, new manifestations of the 3D flare scenario in EUV light curves. On the one hand, it provides an explanation for two-ribbon late-phase flares. On the other hand, the conclusions presented here help bridge the gap between imaging observations, EUV light-curve diagnostics, and the magnetic structures of the associated coronal mass ejections.

Figures (10)

• Figure 1: General evolution of the flare. Left: Time profiles of the GOES 1--8 Å flux (a), and light curves from AIA 131 Å (b), AIA 171 Å (c) and AIA 335 Å (d) integrated over the entire active region (gray) as well as over Region 1, Region 2, and Region 3 (colored). The background is subtracted from each light curve using the average intensity during a quiescent period. Vertical dashed lines indicate prominent peaks in the GOES flux and AIA intensities. Right: snapshots of the flare evolution in AIA 131 Å (e1)--(e4), 335 Å (f1)--(f4), and 171 Å (g1)--(g4). The colored areas in panels (f2), (f3) and (f4) mark the three regions of interest. Several characteristic features are also highlighted; see the text for details. Note that the AIA subregion covers a field of view of $200^{\prime\prime} \times 200^{\prime\prime}$, enclosing the flare-hosting active region.
• Figure 2: Flare ribbon observations in AIA filter channels. Panel (a) presents a zoomed-in view of the flare-hosting active region in AIA 94 Å, with Region 2 and Region 3 overlaid as in Figure \ref{['figure1']}. Panel (b) displays the traced flare ribbons at 06:02, 06:08, 06:14, and 06:24 UT. Colored dots denote the locations shown in panels (c1)--(c4) (purple dots). Selected characteristic structures are highlighted; see the text for further details. A zoomed-in view of the area within the dashed boxes in panels (a) and (b) is shown in panels (c) and (d). Panels (d1)--(d4) illustrate the temporal evolution of the flare ribbons in AIA 1600 Å, corresponding to the times shown in panels (c1)--(c4). Red contours in panels (c) outline the positive footpoint, labeled as "FP+". The animation illustrates the drifting of the flare ribbon and the associated formation of flare loops near the footpoint across its 6-second duration. (An animation of this figure is available.)
• Figure 3: Evolution of the reconnection flux and flux-rope footpoints. Panel (a) presents the soft X-ray light curve (black), the magnetic reconnection flux (pink; shaded area indicating uncertainties), and the time derivative of the reconnection flux (gray). Panel (b) shows the magnetic fluxes of the northern positive (blue) and southern negative (black) dimming regions. The blue and black dashed lines mark the onset of flux decrease in the northern positive and southern negative dimming regions, respectively.
• Figure 4: DEM inversion results for the flare. Top: EM maps of the active region at two selected times when Region 2 and Region 3 (delineated by the colored areas in panels (a) and (b), respectively) are clearly defined. Panel (c) represents the temporal evolution of the volume emission measure (EM) for Region 2 and Region 3 (distinguished by color). Vertical lines mark the respective times of peak volume EM for each region. Bottom: DEM-weighted temperature maps of the active region at two selected times (panels (d) and (e), respectively). Panel (f) represents the temporal evolution of the DEM-weighted temperature for Region 2 and Region 3, with vertical lines marking the peak times for each region.
• Figure 5: Top-view snapshots of the data-constrained numerical simulation and their comparison with AIA imaging observations for the 2011 August 2 flare. The three rows from the top to bottom display the observations and simulations at 05:24, 05:44, and 06:00 UT. The first column shows the top views of the 3D magnetic field configurations revealed from the simulation, where the pink and cyan field lines correspond to overlaying arcades at the eruption onset, and the yellow and green field lines show the flux rope with different topological connectivity. The second column presents the QSLs and twist distributions on the bottom plane. The third and fourth column present AIA 131 Å and 1600 Å images, respectively. The yellow (second column) and white (fourth column) boxes in QSLs and 1600 Å images outline the regions of flare ribbons. An animation showing the evolution of the simulation from 05:24 to 06:14 UT is available. The real-time duration of the animation is 5.3 s. (An animation of this figure is available.)