Successive flux rope eruptions from $\delta$-Sunspots region of NOAA 12673 and associated X-class eruptive flares on 2017 September 6

Abstract

In this paper, we present a multi-wavelength analysis of two X-class solar eruptive flares of classes X2.2 and X9.3 that occurred in the sigmoidal active region NOAA 12673 on 2017 September 6, by combining observations of Atmospheric Imaging Assembly and Helioseismic Magnetic Imager instruments on board the Solar Dynamics Observatory. On the day of the reported activity, the photospheric structure of the active region displayed a very complex network of $\delta$-sunspots that gave rise to the formation of a coronal sigmoid observed in the hot EUV channels. Both X-class flares initiated from the core of the sigmoid sequentially within an interval of $\sim$3 hours and progressed as a single "sigmoid--to--arcade" event. Differential emission measure analysis reveals strong heating of plasma at the core of the active region right from the pre-flare phase which further intensified and spatially expanded during each event. The identification of a pre-existing magnetic null by non-force-free-field modeling of the coronal magnetic fields at the location of early flare brightenings and remote faint ribbon-like structures during the pre-flare phase, which were magnetically connected with the core region, provide support for the breakout model of solar eruption. The magnetic extrapolations also reveal flux rope structures prior to both flares which are subsequently supported by the observations of the eruption of hot EUV channels. The second X-class flare diverged from the standard flare scenario in the evolution of two sets of flare ribbons, that are spatially well separated, providing firm evidence of magnetic reconnections at two coronal heights.

24 pages, 13 figures. Accepted for publication by The Astrophysical Journal (ApJ)

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