Homologous White Light Solar Flares Driven by Photospheric 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.

Solar flare forecasting using morphological properties of sunspot groups

“…In order to illustrate the application of our method for a specific case, we consider the forecasting for the flare activity of the super active region (SAR) NOAA 12673 that appeared on the Sun during the first two weeks of September 2017 and unleashed more than 40 C-, about 20 M-, and 4 X-GOES class flares during its passage over the solar disc. In particular it hosted the strongest flare of the cycle 24 on September 6, 2017 (Romano et al, 2018(Romano et al, , 2019. The SAR complexity is visible in Figure 3, where we show the continuum filtergram taken by HMI/SDO at 617.3 nm.…”

“…Although their origin is usually located in the higher layers of the solar atmosphere, the magnetic field configuration suitable for the occurrence of these events is mostly driven by the photospheric evolution of the active regions (ARs). The emergence of new magnetic flux from the convection zone into the solar atmosphere and the rearrangement of the coronal field due to the horizontal photospheric displacements of the field line footpoints are two fundamental mechanisms which determine the AR configuration suitable for the flare occurrence (e.g., Romano & Zuccarello (2007), Romano et al (2015) and Romano et al (2018)). In particular, these mechanisms have greater effects when they occur near the magnetic polarity inversion line (PIL).…”

Long-term optical monitoring of the solar atmosphere in Italy

“…The OACT observations, which are also used for studying the chromospheric configuration of active regions (e.g., Guglielmino et al 2017, Romano et al 2018, have been recently included as federated products in the Solar Weather Expert Service Centre maintained by the European Space Agency (ESA).…”

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

“…The unexpected and intense activity of the active region (AR) numbered 12673 by the National Oceanic and Atmospheric Administration (NOAA) led many researchers to describe it as a Super Active Region (see Romano, et al, ). Indeed, the region, which was in a decaying phase, experienced new flux emergence and produced between 4 and 10 September 2017 tens of GOES C‐class, 27 M‐class, and 4 X‐class flares as a result of the greatest magnetic flux emergence ever observed (Sun & Norton, ).…”

“…On 6 September, it produced two X‐class flares: the X2.2 SOL2017‐09‐06T08:57 and the X9.3 SOL2017‐09‐06T11:53 (hereafter SOL2017‐09‐06), the most powerful event of cycle 24. These events are peculiar not only because of their soft X‐ray flux but also because they produced WL emission, being a rare case of homologous WL flares (Romano et al, ). Moreover, SOL2017‐09‐06 produced two coronal mass ejections, one near the peak time and the next 1 hr later (Goryaev et al, ).…”