Onset of Photospheric Impacts and Helioseismic Waves in X9.3 Solar Flare of 2017 September 6

The X9.3 flare of 2017 September 6, was the most powerful flare of Solar Cycle 24. It generated strong white-light emission and multiple helioseismic waves (sunquakes). By using data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well as hard X-ray (HXR) data from the KONUS instrument on board the WIND spacecraft, and Anti-Coincidence System on board the INTERGRAL space observatory, we investigate spatio-temporal dynamics of photospheric emission sources, identify sources of helioseismic waves, and compare the flare photospheric dynamics with the HXR temporal profiles. The results show that the photospheric flare impacts started to develop in compact regions in close vicinity of the magnetic polarity inversion line (PIL) in the preimpulsive phase before detection of the HXR emission. The initial photospheric disturbances were localized in the region of strong horizontal magnetic field of the PIL, and, thus, are likely associated with a compact sheared magnetic structure elongated along the PIL. The acoustic egression power maps revealed two primary sources of generation of sunquakes, which were associated with places of the strongest photospheric impacts in the preimpulsive phase and the early impulsive phase. This can explain the two types of helioseismic waves observed in this flare. Analysis of the high-cadence HMI filtergrams suggests that the flare energy release developed in the form of sequential involvement of compact low-lying magnetic loops that were sheared along the PIL.