Observations of a Twin Pair of Atypical Solar Flares and a Magnetic-reconnection Scenario

Abstract

We present observations and a magnetic-reconnection scenario of a twin pair of "atypical flares" that occurred on 2022 April 22 in a quadrupolar magnetic configuration formed by two active regions. The spatio-temporal evolution of the two flares is examined using images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), and from the ground-based Multi-Application Solar Telescope (MAST) in Udaipur, India. Characteristic of atypical flares and indicative of slipping reconnection, the ribbons of each flare (1) do not spread apart and (2) grow longer by sequential brightening of new flare kernels. The two atypical flares are homologous and plausibly have homologous triggers. There are four additional pairs of flare ribbons, each from a different flaring event that releases much less energy than the atypical flares. Two of these four pairs are produced by precursors, each possibly triggering one of the two atypical flares. The remaining two pairs accompany a filament activation, occurring twice within the span of the two atypical flares. Using a nonlinear force-free field (NLFFF) extrapolation model, we approximate the coronal magnetic field and find two quasi-separatrix layers (QSLs) that are nearly rooted in the flare ribbons. The observations and the extrapolated field together suggest a scenario in which the nearly simultaneous occurrence of many reconnections between magnetic field lines crossing at small angles (slipping reconnection) within each of the two QSLs produces the observed pair of atypical flares.

Figures (11)

• Figure 1: Temporal evolution of X-ray irradiance measured by the X-Ray sensor of the GOES-17 satellite in the $0.1-0.8$ nm bandpass at 1-second cadence for a duration of 8 hours. The dashed black lines mark the start times of the GOES C7.7 and GOES M1.1 flares, while the dashed blue and red lines mark their respective peak times. The $y$-axis uses log scaling. The texts in blue and red are the peak X-ray fluxs in units of $\rm{W\,m^{-2}}$ for the first and second atypical flares, respectively.
• Figure 2: A cutout of a full-disk line-of-sight magnetogram showing the two active regions, scaled at $\pm 1000$ G, with red and yellow contours at the level of $+1000\,\mathrm{G}$ and $-1000\,\mathrm{G}$, respectively. The positive ($\rm{P_{1}}$, $\rm{P_{2}}$) and negative ($\rm{N_{1}}$, $\rm{N_{2}}$) polarities at 03:00 UT on 2022 April 22 are pointed to by the arrows. The green paths are polarity inversion lines, three of which are pointed to and labeled as $\mathrm{PIL_1}$, $\mathrm{PIL_2}$, and $\mathrm{PIL_3}$. North is upward and west is to the right in this image, and in all other solar images in this paper.
• Figure 3: Evolution of the positive (red) and negative (blue) magnetic flux in the field of view (FOV) of \ref{['mag']} for a duration of 24 hours, starting from 2022 April 21, 07:00 UT. The dashed and dot-dashed lines mark the starting times of the two solar flares.
• Figure 4: Sequence of SDO/AIA 304 Å images having the FOV of \ref{['mag']}: $\mathrm{R_{1}}$ and $\mathrm{R_{2}}$ are the two ribbons of the twin flares; $\mathrm{C_1}$ and $\mathrm{C_2}$ are brightenings along $\mathrm{PIL_2}$ at 04:18 UT and 05:14 UT; and $\mathrm{T_1}$ and $\mathrm{T_2}$ are brightenings bracketing a northern segment of $\mathrm{PIL_1}$ at 03:15 UT and 04:39 UT. $\mathrm{E_1}$, $\mathrm{E_2}$ and $\mathrm{W_1}$, $\mathrm{W_2}$ are two early segments of $\mathrm{R_1}$ and $\mathrm{R_2}$, respectively. The green contours and the dashed white arrow in panel (b) trace the PILs and point to the clump of positive-polarity flux separating $\mathrm{W_1}$ and $\mathrm{W_2}$. The filament is pointed to in panels (a) and (h). A closed magnetic loop rooted in $\mathrm{R_1}$ and $\mathrm{R_2}$ is pointed to in panel (f). The white (yellow) contours are for $\mathrm{B}_z$=$\mathrm{+1000\,(-1000)\,G}$ at 03:00 UT, while the blue contours trace brightness intensity level 100 DN. Animation: An animation showing the SDO/AIA 304 Å images from 02:00 UT to 06:00 UT at a cadence of 12 seconds is available in the HTML version of this article.
• Figure 5: Sequence of SDO/AIA 131 Å images of the twin flares. Panel (a) shows the apparent X-shape, and panel (b) shows the two loop systems $\mathrm{L_1}$ and $\mathrm{L_2}$, outlined by the gold contours. Panels (c) and (d) show each flare near its peak time. The white (yellow) contours trace $\mathrm{B}_z$=$\mathrm{+1000\,(-1000)\,G}$ at 03:00 UT, while the blue contours outline the SDO/AIA 304 Å flare ribbons at emission intensity level 100 DN. Animation: An animation showing the SDO/AIA 131 Å images from 02:00 UT to 06:00 UT at a cadence of 1 minute is available in the HTML version of this article.