Sigmoid Formation, Filament Destabilization, and Initiation of Weak Flare by Tether-Cutting Reconnection

TL;DR

This study analyzes a B-class solar flare and an associated filament eruption in AR12661 to test the tether-cutting reconnection mechanism for sigmoid formation and eruption initiation. Using comprehensive multiwavelength observations (AIA, XRT, HMI, GONG, H-alpha, IRIS), the authors trace preflare flux cancellation and UV brightenings, the progressive formation of a hot sigmoid, and the subsequent slow-rise and eruption of the filament, along with distant jet-like features. A key result is that the sigmoid forms via early coronal reconnection within the arcade, accompanied by high-temperature plasma exceeding 10 MK, and that the filament’s destabilization follows sigmoid evolution in a manner consistent with TC reconnection, all while no CME is detected. The work highlights the critical role of low-atmosphere reconnection in triggering coronal restructuring and filament eruption, refining our understanding of how weak flares can drive eruptions without CMEs.

Abstract

We have studied a B-class solar flare and an associated filament eruption through multi-wavelength observations. The flare triggers at 16:24~UT on June 7$^{th}$, 2017 from an active region (AR) 12661, and it maximizes at 16:54~UT. The magnetic flux cancellation occurs near the polarity inversion line (PIL) preceding the flare, and ultraviolet (UV) brightenings occur in the pre-flare phase at the flux cancellation sites, suggesting the reconnection occurs in the lower atmosphere, initially. The S-shaped sigmoid forms through successive steps in corona, i.e., small-scale brightenings, helical/twisted field lines, bright patches, and finally, a developed sigmoid. It justifies that runaway reconnection within the coronal arcades forms the sigmoid within the filament. The differential emission-measure (DEM) analysis reveals the existence of the plasma at a temperature of more than 10 MK within the sigmoid. The initial magnetic reconnection reorganizes the field overlying the filament as per the tether-cutting model. Therefore, it enables the filament to rise slowly, and around~16:41~UT, the eruption phase of the filament begins. The filament eruption removes the overlying coronal field, including the sigmoid. During the eruption phase, we have found intersecting/crossing of coronal loops and jet-like structures far away from the sigmoid-filament system. In conclusion, all the observational findings (e.g., magnetic flux convergence, cancellation, UV brightenings, and spatial and temporal correlation between formation/evolution of the sigmoid and rise/eruption of the filament) suggest that the formation of a solar flare and the eruption of the filament are consistent with the tether-cutting model of solar eruption.

Figures (14)

• Figure 1: The panel (a) shows the GOES soft X-ray (SXR; 1.0-8.0 Å) flux profile (red curve) and hard X-ray (HXR; 0.5-4.0 Å) flux profile (blue curve). The SXR flux profile starts to increase at 16:24 UT (vertical black dashed line), and peaks around 16:54 UT. The maximum flux is 7$\times$10$^{-7}$ W/m$^{2}$, therefore, it is a B-class solar flare. Panel (b) shows the flux profiles from AIA 304 Å (red curve) and AIA 94 Å (blue curve). The behaviour in these filters is consistent with the SXR flux profile.
• Figure 2: The top panel shows AIA 304 Å image from the pre-flare phase. The filament is indicated by yellow arrows, and it is located along the polarity inversion line. The overplotted contours are line-of-sight (LOS) magnetic fields obtained from HMI. The white (blue) contours show the positive (negative) polarity. A positive polarity region, located at a far-away location, is outlined by a cyan rectangular box. AIA 171 Å image (bottom-left panel) shows that loops exist between this positive polarity and the negative polarity below the PIL. The filament is also visible in AIA 171 Å, which is indicated by yellows in the bottom-left panel. The bottom-right panel displays the XRT/Be-thin image, which clearly shows the existence of a sigmoid.
• Figure 3: The panel (a) shows the LOS magnetogram at 16:24 UT. The black (white) region corresponds to the negative (positive) polarity. The bigger red-dashed box outlines the flare region; the same box is displayed in the panel (e) of Figure \ref{['fig:fig_94']}. The TD images produced from red and green slits (shown in panel (a)) are displayed in panels (b) and (c), respectively. The blue, cyan, and pink vertical dashed lines are located at the initiation, maximum, and end of the solar flares. In both TD images, it is explicitly visible that opposite polarities are approaching each other. Various paths are drawn on the positive and negative polarity patches (see slanted blue, red, and green dashed paths) to estimate their merging speeds. The panels (d), (e), and (f) show the intensity maps from AIA 1600 Å. The compact brightnings are indicated by cyan arrows, and they are located under the red and green slits (same as shown in panel (a)).
• Figure 4: The figure shows the evolution of the event using H-$\alpha$ observations. Several important features of the event are indicated by arrows, namely, filament (cyan arrows in panel (a)), brightenings around the filament (yellow arrows in panels (b) and (c)), filament rise (blue arrows in panels (d), (e), and (f)), filament eruption (blue arrows in (g), (h), and (i)) and plasma fall back (encircled by the red circle in panels (k) to (n)). The white arrow in each panel indicates the top part of the filament which does not change during the course of this event. The pink arrow in panel (f) indicates the remote brightening, which is located at the positive polarity location outlined by the cyan rectangular box in the top panel of Figure \ref{['fig:ref_fig']}. Animation: The animation shows the temporal evolution of H-$\alpha$ and complements the figure shown here. The animation spans 16:15 UT to 18:15 UT at a cadence of 1 minute (20 frames per second; total duration $=$ 5 s). See animation halpha$\_$movie.mp4.
• Figure 5: Panel (a) shows the H-$\alpha$ image of the filament eruption and an artificial cyan slit is drawn along the path of the filament eruption. Then, the TD map is obtained using the cyan slit, which is shown in panel (b). The blue dotted curve in the TD map is drawn along the filament eruption. Firstly, the filament rises slowly (the slow-rising phase is enclosed within the vertical green lines), and later on, the filament erupts. The up-flow speed of the filament eruption (i.e., after the slow rise phase) is calculated, and it is 63 km/s. At the end of the event, the downfall of the plasma is visible, and a blue-dashed path is drawn along the downfall to estimate the downfall speed. The downfall speed is 58 km/s. The bright region before the filament rise motion is due to the flare ribbons.