On the nature of photospheric horizontal magnetic field increase in major solar flares

TL;DR

This study analyzes 35 major solar flares and finds that increases in horizontal magnetic field are closely linked to flare ribbons and are consistent with a reconnection-driven contraction model, providing new statistical evidence.

Contribution

It offers the first statistical evidence supporting the reconnection-driven contraction model for photospheric horizontal magnetic field increase during major solar flares.

Findings

$B_h$-increase correlates with flare ribbons and inclination changes.

Larger spatial extent of $B_h$-increase in eruptive flares.

Inverse correlation between $B_h$-increase intensity and initial ribbon separation.

Abstract

Rapid increase of 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 $B_h$-increase is always accompanied by the increase of field inclination. It usually initiates near the flare ribbons, showing step-like change in between the ribbons. In particular, its evolution in early flare phase shows close spatio-temporal correlation to flare ribbons. We further find that $B_h$-increase tends to have similar intensity in confined and eruptive flares, but 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 which suggests that the reconnection-driven contraction of flare loops enhances photospheric $B_h$ according to the ideal induction equation, providing statistical evidence to the reconnection-driven origin for $B_h$-increase for the first time.