Saddle-shaped Solar Flare Arcades

The erupting structure was a hot channel (e.g., Zhang et al. 2012) visible in the 94 Å and 131 Å channels of AIA. Before its eruption, the hot channel was embedded in an arcade of coronal loops (Figure 2(a)–(b)) within a bipolar active region NOAA 11193 (Aulanier et al. 2012). The hot channel started to slowly rise after ≈20:30 UT. In 2 minute running-difference (RD) 131 Å images, constructed from images averaged over 1 minute, both the rising hot channel and the first flare loops can be distinguished (panel (b)). During the impulsive phase of the flare at ≈20:45 UT, a completely formed arcade of flare loops is seen in the 131 Å channel (panel (c)). It has a shape of a saddle; the flare loops composing its far ends are relatively higher compared to those in its central part. The high flare loops in the south are also inclined. By analogy using the horse-back riding terminology, we refer to these lifted ends of saddles as "cantles." The saddle is visible for about 15 minutes. During the flare peak (panel (d)), the saddle starts to be obscured by the developing SADs (see Warren et al. 2011). The southern cantle can, however, be discerned later in filters such as 171 Å (panel (g)), or 193 and 211 Å, a consequence of cooling of flare loops.

At this stage, some of the flare loops forming the cooling arcade can also be identified on-disk in the 171 Å channel of EUVI (panel (i)). The lengths of these flare loops vary along the arcade. Notably, one of the footpoints (red circle) of the southermost loop (red dotted line) is located further toward the south below the curved part of the ribbon hook (Section 3.2), and appears more sheared. Its length, measured using circumference of an ellipse passing through its footpoints and apex, was found to be ≈37 Mm, while the typical length of the flare loops elsewhere in the arcade ranges between about 22 and 27 Mm. The cantle loop can easily be identified in AIA, where it composes the cantle at the southern end of the saddle, as it is still the southernmost loop (red circle in panel (g)). In the AIA projection, this cantle loop seen in 171 Å is also relatively higher than the remainder of the flare arcade. The relation of this cantle loop to the flare ribbons is outlined in the following section.

During the eruption, the coronal loops overlying the hot channel moved aside from the erupting hot channel toward the south and north (Figures 2(e)–(f)). While the northern coronal loops remained visible during the eruption, obscuring the northern cantle, most of the southern coronal loops disappeared. EUVI 171 Å observations reveal that these coronal loops were rooted at either side of the J-shaped (hooked) flare ribbons. The straight parts of the ribbons were nearly parallel, separated by a straight PIL (see also Figure 1 in Aulanier et al. 2012). The ribbon hooks, one to the northeast (N-hook) and the other to the southwest (S-hook), are indicated in Figure 2(h).

A magnified view of the coronal loops overlying the S-hook is shown in Figure 3. Panels (a)–(d) contain 2 minute running-difference images produced using AIA 171 Å. Initially, the coronal loops with various inclinations (a) were overlying the hot channel (seen only in 131 Å, Figure 2(b)). At ≈20:35 UT (Figure 3(b)), they moved toward the south. Subsequently, the coronal loops started to vanish (panel (c)) and this process continued until most of them disappeared (d).

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Footpoints of these coronal loops can be identified in EUVI 171 Å observations (Figure 3(e)). After the onset of the eruption at ≈20:35 UT, the S-hook started to become visible (orange dashed line in panel (f)), encircling a core dimming region. Afterwards, the S-hook started to shrink and propagate toward the north (g)–(h). Its tip then elongated toward the north and swept the footpoints of the coronal loops. Meanwhile, these loops were disappearing and the cantle was being formed (Figure 2(c)).

The location of the red circle originally corresponded to the region encircled by the S-hook (Figure 3(f)). Later, as the hook propagated toward the north, it swept through the location of the circle, which subsequently corresponded to a flare loop footpoint (Figure 3(h)). As shown by Zemanová et al. (2019), Lörinčík et al. (2019), and Dudík et al. (2019) for other events, this behavior represents the change from flux rope to flare loop footpoint and is consistent with the ar–rf reconnection between the erupting flux rope and its overlying arcades (see Aulanier & Dudík 2019). Furthermore, as described in Section 3.1, the 171 Å flare loop rooted in the location of the red circle is longer and relatively higher than the remainder of the flare arcade. This behavior is also consistent with the ar–rf reconnection. Comparison of Figures 4(d) and 5(d) of Aulanier & Dudík (2019) reveals that the flare loops originating in the ar–rf geometry can be longer and also higher than the flare loops formed due to the reconnection between overlying arcades, which occurs beneath the erupting structure. A similar situation is also seen in Figure 1(c) therein. The formation of this cantle may therefore be explained by magnetic reconnection between the hot channel and coronal loops rooted nearby.

The 2011 June 7 filament eruption is a well-known event (e.g., van Driel-Gesztelyi et al. 2014; Yardley et al. 2016; Dudík et al. 2019, and references therein). It occurred in an active region NOAA 11226 and was accompanied by a M2.5-class flare.

This eruption started at ≈06:08 UT (Dudík et al. 2019) and the first flare loops were visible roughly after 06:20 UT (first row of Figure 4). In the 94 and 131 Å channels of AIA, flare loops composing the arcade were partially obscured by the erupting filament. Early on, the arcade is, however, well visible in the Be_thick filter of XRT (panel (a)). The flare loops rooted at the southern end of the arcade are longer than others, forming a cantle. About 20 minutes later (panel (b)), a full saddle can be distinguished in AIA 131 Å. Both cantles are visible, one inclined toward the south and the other toward the north. The saddle can be observed for more than an hour; at these later times also in cooler filter channels. The composite image shown in panel (c), produced using the 131 Å (red), 171 Å (blue), and 211 Å (green) channel images, reveals individual flare loops composing the arcade. At many locations, emission is seen in multiple filter channels (white color).

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Hooked ribbons, traced using AIA 1600 Å, are shown in Figure 4(a). The footpoints of flare loops composing the southern cantle, marked using the red circle in the zoomed inclusion, are located at the southern end of the ribbon hook. Dudík et al. (2019) showed that flare loops rooted in this region originated via the ar–rf reconnection geometry.

We next analyze a flare arcade that formed during the 2011 December 26 C5.7-class flare (Qiu et al. 2017). This flare occurred in the active region complex NOAA 11383 and 11384 and was accompanied by a coronal mass ejection (CME; Cheng & Qiu 2016).

The flare morphology is shown in Figure 4, panels (d)–(f). It showed a pair of very straight ribbons studied by Qiu et al. (2017). Nevertheless, we identified a pair of hooked extensions formed at their far ends, shown by arrows in panel (d). The western hook was curved and partially obscured by coronal loops rooted in the southwest at ≈[180'', 360''], toward which the ribbon elongated.

The first flare loops started to appear at around 11:15 UT in AIA 131 Å. In the composite image (panel (d)), they are seen as red-colored emission, located between the ribbons, and extending toward the western hook. As the arcade further developed (panel (e)), a saddle with two cantles started to become visible. The southwestern cantle is more pronounced than the northeastern one. This is because it was composed of long and faint flare loops, extending for more than 100'' toward the western hook in which they are rooted (arrow in panel (e)). On the other hand, the conjugate cantle started to be visible after the saddle cooled, at which it was visible in AIA 131 and 171 Å (arrow in panel (f)).

On 2017 September 6, the large active region NOAA 12673 produced two flares (e.g., Yan et al. 2018a). The first one, a confined X2.2-class one, started at ≈09:00 UT. Roughly 3 hours later, it was followed by an X9.3-class flare, the strongest one in cycle 24 (Zou et al. 2020), and accompanied by a CME.

The eruption started at 11:55 UT. At this time, a pair of flare ribbons started to become visible in AIA 304 Å data, which we used for their tracing. The longer ribbon in the northeast developed a hook in the north, while the conjugate ribbon in the southwest contained a small hook toward the south (orange dashed lines in Figure 5(a)).

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The X9.3 flare occurred in the gradual phase of the X2.2. As it started, it saturated the 131 Å channel. Nevertheless, later on, a saddle can be readily identified at 12:35 UT, see panel (a) of Figure 5. Its northern cantle is composed of bright flare loops, observed to slip along the northern hook. The southern cantle is less bright. Still, some of the loops composing it are rooted near the southern hook. A fully formed saddle is shown in panel (b). Similar to the events described before, the cooling saddle arcade is later visible for many hours in cooler filter channels, such as AIA 171 and 211 Å, see panel (c).

Four days later, when the same active region was behind the limb, it produced another large X8.2 flare (Polito et al. 2018; Warren et al. 2018; Yan et al. 2018b). The erupting flux rope consisted of a filament and a hot channel, studied in detail, e.g., by Yan et al. (2018b). The hot channel was visible in the AIA 94 and 131 Å and consisted of numerous loops located both to the north and to the south of the filament, see, e.g., Figure 3 of Yan et al. (2018b), and Chen et al. (2020).

The first flare loops can be identified at ≈16:00 UT. In panel (e) of Figure 5, a saddle with cantles at both ends is readily identifiable. While the northern cantle is viewed along the axis of the saddle, the higher southern one is viewed from the side. This indicates that the flaring PIL was curved, which is in agreement with the observations of a curved filament residing in these locations before the eruption (Chen et al. 2020). Since the active region was located behind the limb, the flare ribbons could not be observed and it is impossible to relate the cantles with the hooks. However, the location of the southern cantle corresponds to the previous southern extent of the erupting flux rope (southern arrow in panel (d)), while the northern cantle corresponds to the location of the filament eruption.

The fully developed saddle can be seen in panel (f) at 16:47 UT. It remained visible for more than 30 minutes (panel (g)). Even though the highest flare loops composing the southern cantle do not have a counterpart in cooler coronal channels at 17:20 UT (panel (f)), the coronal channels still show that the central part of the flare arcade reaches lower altitudes than both its ends.