Homologous white light solar flares driven by photosperic shear motions

Abstract

We describe the peculiarity of two recurrent white light flares occurred on Sept. 06, 2017, in the super Active Region (SAR) NOAA 12673, with a time interval, between their peaks, of about 3 hours. These events of X2.2 and X9.3 GOES class are very important not only for their high level of emission and for the visible effects on the lower layers of the solar atmosphere, discernible as clear white light ribbons, but as well for the strong horizontal photospheric motions which seemed to drive them. In fact, we observed for several hours before the flare occurrence the displacement of a negative umbra located in the main delta spot of the Active Region. We measured velocities 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 by these events can be considered responsible for the new energy continuously supplied into 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.