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Solar active region (AR) 12673 in 2017 September produced two largest flaresin Solar Cycle 24: the X9.3 flare on September 06 and the X8.2 flare onSeptember 10. We attempt to investigate the evolutions of the two great flaresand their associated complex magnetic system in detail. Aided by the NLFFFmodeling, we identify a double-decker flux rope configuration above thepolarity inversion line (PIL) in the AR core region. The north ends of thesetwo flux ropes were rooted in a negative- polarity magnetic patch, which beganto move along the PIL and rotate anticlockwise before the X9.3 flare onSeptember 06. The strong shearing motion and rotation contributed to thedestabilization of the two magnetic flux ropes, of which the upper onesubsequently erupted upward due to the kink-instability. Then another two setsof twisted loop bundles beside these ropes were disturbed and successivelyerupted within 5 minutes like a chain reaction. Similarly, multiple ejectacomponents were detected to consecutively erupt during the X8.2 flare occurringin the same AR on September 10. We examine the evolution of the AR magneticfields from September 03 to 06 and find that five dipoles emerged successivelyat the east of the main sunspot. The interactions between these dipoles tookplace continuously, accompanied by magnetic flux cancellations and strongshearing motions. In AR 12673, significant flux emergence and successiveinteractions between the different emerging dipoles resulted in a complexmagnetic system, accompanied by the formations of multiple flux ropes andtwisted loop bundles. We propose that the eruptions of a multi-flux-rope systemresulted in the two largest flares in Solar Cycle 24.

Eruption of a multi-flux-rope system in solar active region 12673 leading to the two largest flares in Solar Cycle 24