  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.
