ABRUPT LONGITUDINAL MAGNETIC FIELD CHANGES IN FLARING ACTIVE REGIONS

Abstract

We characterize the changes in the longitudinal photospheric magnetic field\nduring 38 X-class and 39 M-class flares within $65^{\\circ}$ of disk-center\nusing 1-minute GONG magnetograms. In all 77 cases we identify at least one site\nin the flaring active region where clear, permanent, stepwise field changes\noccurred. The median duration of the field changes was about 15 minutes and was\napproximately equal for X-class and for M-class flares. The absolute values of\nthe field changes ranged from the detection limit of $\\sim\\!\\!10$~G to as high\nas $\\sim\\!\\!450$~G in two exceptional cases. The median value was 69~G. Field\nchanges were significantly stronger for X-class than for M-class flares and for\nlimb flares than for disk-center flares. Longitudinal field changes less than\n100~G tended to decrease longitudinal field strengths, both close to\ndisk-center and close to the limb, while field changes greater than 100~G\nshowed no such pattern. Likewise, longitudinal flux strengths tended to\ndecrease during flares. Flux changes, particularly net flux changes near\ndisk-center, correlated better than local field changes with GOES peak X-ray\nflux. The strongest longitudinal field and flux changes occurred in flares\nobserved close to the limb. We estimate the change of Lorentz force associated\nwith each flare and find that this is large enough in some cases to power\nseismic waves. We find that longitudinal field decreases would likely outnumber\nincreases at all parts of the solar disk within $65^{\\circ}$ of disk-center, as\nin our observations, if photospheric field tilts increase during flares as\npredicted by Hudson et al.\n