Abrupt longitudinal magnetic field changes in flaring active regions

TL;DR

This study analyzes 77 solar flares to characterize abrupt, permanent changes in the photospheric magnetic field, revealing stronger effects in X-class flares and near the solar limb, with implications for solar seismic activity.

Contribution

It provides the first comprehensive statistical analysis of stepwise magnetic field changes during flares, highlighting their magnitude, duration, and correlation with flare intensity and position.

Findings

Field changes ranged from 10 G to 450 G with a median of 69 G.

X-class flares caused significantly stronger magnetic field changes than M-class.

Longitudinal flux generally decreased during flares, especially near disk-center.

Abstract

We characterize the changes in the longitudinal photospheric magnetic field during 38 X-class and 39 M-class flares within $65^{\circ}$ of disk-center using 1-minute GONG magnetograms. In all 77 cases we identify at least one site in the flaring active region where clear, permanent, stepwise field changes occurred. The median duration of the field changes was about 15 minutes and was approximately equal for X-class and for M-class flares. The absolute values of the field changes ranged from the detection limit of $\sim\!\!10$~G to as high as $\sim\!\!450$~G in two exceptional cases. The median value was 69~G. Field changes were significantly stronger for X-class than for M-class flares and for limb flares than for disk-center flares. Longitudinal field changes less than 100~G tended to decrease longitudinal field strengths, both close to disk-center and close to the limb, while field changes greater than 100~G showed no such pattern. Likewise, longitudinal flux strengths tended to decrease during flares. Flux changes, particularly net flux changes near disk-center, correlated better than local field changes with GOES peak X-ray flux. The strongest longitudinal field and flux changes occurred in flares observed close to the limb. We estimate the change of Lorentz force associated with each flare and find that this is large enough in some cases to power seismic waves. We find that longitudinal field decreases would likely outnumber increases at all parts of the solar disk within $65^{\circ}$ of disk-center, as in our observations, if photospheric field tilts increase during flares as predicted by Hudson et al.