Origin of extreme solar eruptive activity from the active region NOAA 12673 and the largest flare of solar cycle 24

The active region NOAA 12673 appeared on the eastern limb of the Sun as a simple unipolar sunspot. Subsequently, the active region exhibited the constant emergence of magnetic flux of both the polarities as well as photospheric motions including shearing and rotational motions (Figure 1). This led to the quick evolution of the region into the most complex configuration, namely the $\delta$-type, resulting a series of large eruptive flares. Notably, the AR produced 4 X-class and 27 M-class flares along with numerous smaller events. In Table 1, a summary of the flares of GOES class M and above is given. From Table 1, we note that the first M class flare from NOAA 12673 originated on 4 September 2017 following which 12 further M-class flares occurred within 2 days. After producing 2 homologous X-class flares on 6 September 2017 (see Section 2.2), the AR produced another X-class flare (X1.3) on the next day. Subsequently, on 10 September, a large X8.2 flare occurred while the AR was transiting toward the far side of the Sun.

The first X-class flare from NOAA 12673 (GOES class X2.2) originated on 6 September and within $\approx$3 hours after its initiation, the AR produced the largest flare of the solar cycle 24, having GOES class X9.3. Figure 2 represents a few representative 94 Å and 304 Å images showing the evolution of X2.2 flare. The EUV images prior to the onset of the flare (Figures 2(a) and (d)) readily confirms that the coronal configuration associated with the AR took the shape of an impressive ‘inverse-S’ i.e., a prominent coronal sigmoid (the sigmoidal structure is highlighted by the dotted curve in Figure 2(a)). Notably, the sigmoid encompassed the entire stretch of the AR and axis of the sigmoid was associated with the PIL between the positive and negative flux regions. The intense emission observed in the 94 Å images along the coronal sigmoid is suggestive of the presence of a hot corona channel i.e., an activated flux rope. During the impulsive phase of the flare, this flux rope underwent partial eruption with the formation of chromospheric flare ribbons.

After the X2.2 flare, the sigmoidal structure underwent a gradual expansion and rise in the intensity of emission (cf. Figures 2c and  3a). Notably, the X9.3 flare initiated with a localized spot-like brightening at the northern footpoint of the hot channel (indicated by the white arrow in Figure 3(b)). Almost simultaneously we observed prominent signatures of filament eruption which initially ascended with the hot channel (indicated by the arrow in the inset of Figure 3(f)). Following the eruption, we observed formation of two prominent flare ribbons which were situated close to each other (shown within the inset of Figure 3(g). The narrow post-reconnection arcade connecting these flare ribbons is indicated by the yellow arrow in Figure 3(c). Importantly, the gradual phase of the flare was characterized by the formation of a large-scale loop arcade and flare ribbons that encompassed the entire stretch of the AR (indicated by the blue arrows in Figures 3(d) and (h)). A set of extended flare ribbons at the footpoints of the large-scale loop arcade was identified in the AIA 304 Å images (indicated by the green arrows in Figure 3(h)).

To understand the magnetic nature of the $\delta$-spots of NOAA 12673 prior to X-class flares, in Figure 4, we plot co-temporal white light image and magnetogram of the AR during its most flaring phase and calculate magnetic gradient across the PIL associated with the bipolar region. The white light image suggests the presence of a few segmented umbrae within the central sunspot area which were surrounded by a single penumbra (Figure 4(a) and (b)). Comparison of the white light image and the magnetogram suggests a few of these fragmented umbrae to have negative polarity and a few to be associated with positive polarity. Our analysis readily suggests that LOS magnetic strength sharply changed from $\approx$1500 G to $\approx$-1000 G within a distance of $\approx$1 arcsec across the PIL. Consequently, the PIL region was associated with the highest magnetic gradient with a value $\approx$ 2.4$\times$10³ G Mm^-1. This confirms NOAA 12673 to be associated with all the characteristic properties of typical $\delta$-type sunspots prior to the onset of the X-class flares.

In order to understand the coronal magnetic configuration leading to the onset of the two X-class flares, we carried out NLFFF extrapolation at 08:48 UT and 11:48 UT (prior to the onset of the X2.2 and X9.3 flares, respectively). The modeled magnetic field configuration prior to the onset of the X2.2 flare (Figures 5(a) and (b)) readily indicates the presence of a flux rope extending over the PIL (shown by the green lines) with northern and southern footpoints fixed at negative and positive polarity regions of the central sunspot region, respectively. Notably, the appearance of the hot channel, observed just prior to the onset of the X2.2 flare, was almost precisely the same as the shape of the flux rope (see Figure 2(b)). The location next to the northern footpoints of the flux rope was characterized by a complex coronal structure similar to the fan-spine configuration that involved a magnetic null point. The two sets of fan-spine lines are depicted by the blue and pink lines in Figures 5(a) and (b) and the coronal null point is indicated by a black arrow in Figure 5(b).

The overall model magnetic configuration prior to the onset of the X9.3 flare at the core region of NOAA 12673 (Figures 5(c)-(d)) was similar to that corresponding to the X2.2 flare. However, the flux rope prior to the X9.3 flare (shown by the green lines in Figures 5(c)-(d)) appeared to be magnetically more twisted than that of the X2.2 flare. The overall fan-spine configuration prior to the X9.3 flare is shown by the blue and yellow lines in Figures 5(c) and (d). Notably, the northern footpoint of the flux rope itself appeared to be involved in the fan-spine configuration. In Figure 5(d), the null point involved in the fan-spine configuration is indicated by a black arrow. Here we remember that, the X9.3 initiated with a point-like brightening from the northern footpoint location of the hot channel (Figure 5(b)).