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Solar active regions (ARs) that produce major flares typically exhibit strongplasma shear flows around photospheric magnetic polarity inversion lines(MPILs). It is therefore important to quantitatively measure such photosphericshear flows in ARs for a better understanding of their relation to flareoccurrence. Photospheric flow fields were determined by applying theDifferential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) methodto a large data set of 2,548 co-aligned pairs of AR vector magnetograms with12-min separation over the period 2012-2016. From each AR flow-field map, threeshear-flow parameters were derived corresponding to the mean (<S>), maximum(S_max) and integral (S_sum) shear-flow speeds along strong-gradient,strong-field MPIL segments. We calculated flaring rates within 24 hr as afunction of each shear-flow parameter, and also investigated the relationbetween 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 foundthat the larger S_sum an AR has, the more likely it is for the AR to produceflares within 24 hr. It is also found that among ARs which produce majorflares, if one has a larger value of S_sum then {\tau} generally gets shorter.These results suggest that large ARs with widespread and/or strong shear flowsalong MPILs tend to not only be more flare productive, but also produce majorflares within 24 hr or less.

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