Abrupt changes in photospheric magnetic and Lorentz-force vectors during neutral-line flares

TL;DR

This study investigates the rapid magnetic and Lorentz-force changes in the photosphere during six major solar flares, revealing consistent patterns of field strengthening, collapse towards potential configurations, and forces indicating loop contraction.

Contribution

It provides detailed observational evidence of magnetic field relaxation and force changes during flares, highlighting the dominant role of tilt relaxation over shear in magnetic stress release.

Findings

Neutral line fields become stronger and more horizontal during flares.

Fields collapse closer to potential tilt angles, indicating stress relaxation.

Vertical Lorentz force shows large downward change, consistent with loop collapse.

Abstract

We analyze the spatial and temporal variations of the abrupt photospheric magnetic changes associated with six major flares using 12-minute, 0.5-arcsec vector magnetograms from NASA's Helioseismic and Magnetic Imager instrument (HMI) on the Solar Dynamics Observatory satellite. The six major flares occurred near the main magnetic neutral lines of four active regions, NOAA 11158, 11166, 11283 and 11429. During all six flares the neutral line field vectors became stronger and more horizontal, in each case almost entirely due to strengthening of the horizontal field components parallel to the neutral line. In all six cases the neutral line pre-flare fields were more vertical than the reference potential fields, and collapsed abruptly and permanently closer to potential field tilt angles during every flare, implying that the relaxation of magnetic stress associated with non-potential tilt angles plays a major role during major flares. The shear angle with respect to the reference potential field did not show such a pattern, demonstrating that flare processes do not generally relieve magnetic stresses associated with photospheric magnetic shear. The horizontal fields became significantly and permanently more aligned with the neutral line during the four largest flares, suggesting that the collapsing field is on average more aligned with the neutral line than the pre-flare neutral line field. The vertical Lorentz force had a large, abrupt, permanent downward change during each of the flares, consistent with loop collapse. The horizontal Lorentz force changes acted mostly parallel to the neutral line in opposite directions on each side, a signature of the fields contracting during the flare, pulling the two sides of the neutral line towards each other. The greater effect of the flares on field tilt than on shear may be explained by photospheric line-tying.