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

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 Å 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 δ-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 δ-spot in the north, (2) a group of positive-polarity spots, which moved around the δ-spot in the south, moving away from the δ-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, α-spot.

7 pages, 6 Figures, Accepted to be published in Astronomy and Astrophysics

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Cited by in corpus (28)

• Flare-productive active regions
• Eruption of a multi-flux-rope system in solar active region 12673 leading to the two largest flares in Solar Cycle 24
• Rapid buildup of a magnetic flux rope during a confined X2.2 class flare in NOAA AR 12673
• Detection of the strongest magnetic field in a sunspot light bridge
• Magnetic helicity and eruptivity in active region 12673
• Peculiar solar sources and geospace disturbances on 20-26 August 2018
• A Two-Step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673
• Solar Eruptions, Forbush Decreases and Geomagnetic Disturbances from Outstanding Active Region 12673
• Very fast helicity injection leading to critically stable state and large eruptive activity in solar active region NOAA 12673
• Two strong white-light solar flares in AR NOAA 12673 as potential clues for stellar superflares
• Continuous Null-Point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar Active Region~12673
• A comparative study of two X2.2 and X9.3 solar flares observed with HARPS-N: Reconciling Sun-as-a-star spectroscopy and high-spatial resolution solar observations in the context of the solar-stellar connection
• Onset of Photospheric Impacts and Helioseismic Waves in X9.3 Solar Flare of September 6, 2017
• Non-LTE inversions of a confined X2.2 flare: I. Vector magnetic field in the photosphere and chromosphere
• Eruptive-Impulsive Homologous M-class Flares Associated with Double-Decker Flux Rope Configuration in Mini-Sigmoid of NOAA 12673
• Formation of a Magnetic Flux Rope in the Early Emergence Phase of NOAA Active Region 12673
• Can Sub-photospheric Magnetic Reconnection Change the Elemental Composition in the Solar Corona?
• Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 Å and H$\beta$ lines
• High-resolution spectroscopy of a surge in an emerging flux region
• The extremely strong non-neutralized electric currents of the unique solar active region NOAA 13664
• The Link Between Non-Thermal Velocity and Free Magnetic Energy in Solar Flares
• Solar active region evolution and imminent flaring activity through a color-coded visualization of photospheric vector magnetograms
• The formation and decay of sunspot penumbra in Active Region NOAA 12673
• Long-Term Evolution of Magnetic Fields in Flaring Active Region NOAA 12673
• The calm before the storm: High spatial resolution mosaic of active region NOAA 14274 at the onset of an X1.2 flare
• Signatures of Untwisting Magnetic Field in a Small Emerging Bipole in the Solar Photosphere
• Superstrong magnetic fields in sunspot bipolar light bridges
• Energy origination and triggering mechanism of a series of homologous confined flares