Abrupt Changes in the Photospheric Magnetic Field, Lorentz Force, and Magnetic Shear during 15 X-class Flares

During major solar flares, the photospheric magnetic field of the flaring active region is often observed to change abruptly, permanently, and significantly. Here we analyze vector magnetograms covering 15 X-class flares observed in 11 active regions by the Solar Dynamic Observatory Helioseismic and Magnetic Imager. Resolving magnetic changes using a reference potential field, more complex magnetic field, Lorentz force, and magnetic shear changes could be analyzed than previously. In each case, physical and coherent patterns of change were found. Generally the dominant change was in the horizontal field component that strengthened in the central structure, accompanied there by a downward Lorentz force change and strengthening of horizontal magnetic shear consistent with magnetic implosion, with weaker changes of opposite sign in neighboring and peripheral regions including sunspots. In most cases this central structure was a strong, sheared photospheric magnetic neutral line, but in one case it was an isolated unipolar sunspot. Magnetic relaxation in the vertical direction was typical throughout the flaring regions. Unique in this study was the behavior at bald-patch structures during the X-class flares of 2017 September 6 in NOAA. Active Region 12673. During the X9.3 flare, the horizontal and vertical field components weakened and the shear relaxed in horizontal and vertical directions, indicating a large free magnetic energy source for the flare. The magnetic changes at the bald patches exhibited evidence of organized vertical magnetic flux reduction during three X-class flares, not found at other structures, suggesting the greater possibilities of magnetic reconfiguration and energy release at bald patches.