Photospheric Shear Flows in Solar Active Regions and Their Relation to Flare Occurrence

Abstract

Solar active regions (ARs) that produce major flares typically exhibit strong plasma shear flows around photospheric magnetic polarity inversion lines (MPILs). It is therefore important to quantitatively measure such photospheric shear flows in ARs for a better understanding of their relation to flare occurrence. Photospheric flow fields were determined by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) method to a large data set of 2548 coaligned pairs of AR vector magnetograms with 12-min separation over the period 2012 – 2016. From each AR flow-field map, three shear-flow parameters were derived corresponding to the mean ($\langle S\rangle$), maximum ($S_{\mathrm{max}}$) and integral ($S_{\mathrm{sum}}$) shear-flow speeds along strong-gradient, strong-field MPIL segments. We calculated flaring rates within 24 h as a function of each shear-flow parameter and we investigated the relation between the parameters and the waiting time ($\tau$) until the next major flare (class M1.0 or above) after the parameter observation. In general, it is found that the larger $S_{\mathrm{sum}}$ an AR has, the more likely it is for the AR to produce flares within 24 h. It is also found that among ARs which produce major flares, if one has a larger value of $S_{\mathrm{sum}}$ then $\tau$ generally gets shorter. These results suggest that large ARs with widespread and/or strong shear flows along MPILs tend to not only be more flare productive, but also produce major flares within 24 h or less.