On the origin of two X-class flares in active region NOAA 12673 - Shear flows and head-on collision of new and pre-existing flux

TL;DR

This study analyzes the magnetic and flow properties of NOAA 12673 during two X-class flares, revealing that shear flows and flux collision played key roles in flare initiation and development.

Contribution

It provides detailed observations linking shear flows and flux collision to the occurrence of consecutive X-class flares in NOAA 12673.

Findings

Shear flows along the polarity inversion line were significant.

Collision of new flux with existing spots triggered flares.

Magnetic and flow changes preceded and accompanied flares.

Abstract

Flare-prolific active region NOAA 12673 produced consecutive X2.2 and X9.3 flares on 06/09/2017. To scrutinize the morphological, magnetic, and horizontal flow properties associated with these flares, a 7-hour time-series was used consisting of continuum images, line-of-sight/vector magnetograms, and 1600 {\AA} UV images. These data were acquired with the SDO HMI and AIA. The white-light flare emission differed for both flares, while the X2.2 flare displayed localized, confined flare kernels, the X9.3 flare exhibited a two-ribbon structure. In contrast, the excess UV emission exhibited a similar structure for both flares, but with larger areal extent for the X9.3 flare. These two flares represented a scenario, where the first confined flare acted as precursor, setting up the stage for the more extended flare. Difference maps for continuum and magnetograms revealed locations of significant changes, i.e., penumbral decay and umbral strengthening. The curved magnetic polarity inversion line in the {\delta}-spot was the fulcrum of most changes. Horizontal proper motions were computed using the DAVE4VM. Persistent flow features included (1) strong shear flows along the polarity inversion line, where the negative, parasitic polarity tried to bypass the majority, positive-polarity part of the {\delta}-spot in the north, (2) a group of positive-polarity spots, which moved around the {\delta}-spot in the south, moving away from the {\delta}-spot with significant horizontal flow speeds, and (3) intense moat flows partially surrounding the penumbra of several sunspots, which became weaker in regions with penumbral decay. The enhanced flare activity has its origin in the head-on collision of newly emerging flux with an already existing regular, {\alpha}-spot.