On the Nature of the Photospheric Horizontal Magnetic Field Increase in Major Solar Flares

The rapid increase of the horizontal magnetic field (B ( h )) around the flaring polarity inversion line is the most prominent photospheric field change during flares. It is considered to be caused by the contraction of flare loops, the details behind which is still not fully understood. Here we investigate the B ( h ) increase in 35 major flares using HMI high-cadence vector magnetograms. We find that the B ( h ) increase is always accompanied by the increase of field inclination. It usually initiates near the flare ribbons, showing a step-like change in between the ribbons. In particular, its evolution in the early flare phase shows a close spatiotemporal correlation to flare ribbons. We further find that the B ( h ) increase tends to have similar intensity in confined and eruptive flares but a larger spatial extent in eruptive flares in a statistical sense. Its intensity and timescale have inverse and positive correlations to the initial ribbon separations, respectively. The results altogether are well consistent with a recent proposed scenario that suggests that the reconnection-driven contraction of flare loops enhances the photospheric B ( h ) according to the ideal induction equation, providing statistical evidence of the reconnection-driven origin for the B ( h ) increase for the first time.