Homologous White Light Solar Flares Driven by Photospheric Shear Motions

We describe the peculiarity of two recurrent white light flares that occurred on 2017 September 06, in the super active region NOAA 12673, with a time interval, between their peaks, of about 3 hr. These events of the X2.2 and X9.3 GOES classes are very important, not only for their high level of emission and for the visible effects on the lower layers of the solar atmosphere, which are discernible as clear white light ribbons, but also for the strong horizontal photospheric motions, which seemed to drive them. In fact, we observed the displacement of a negative umbra located in the main delta spot of the active region for several hours before the flare occurrence. We measured velocities of up to 0.6 km s⁻¹. The strong and persistent shear motion of the photospheric structures, together with the high intensity of the magnetic flux involved in these events, can be considered responsible for the new energy that is continuously supplied to the magnetic system. From the timing of the emissions at different wavelengths, we were able to provide some constraints for the modeling of such events.