Rapid changes in the longitudinal magnetic field related to the 2001 April 2 X20 flare

Big Bear Solar Observatory observed the X20 are that occurred at approximately 21:50 UT on 2001 April 2 with its standard complement of instruments. In this paper, we discuss the evolution of high-resolution and high-cadence longitudinal magnetograph observations in the region of the are. The data reveal that there was a significant increase in the magnetic field on the limbward side of the neutral line of the active region at the location of the are, while the magnetic field on the side of the neutral line closer to the disk center remained constant. We discuss possible rearrangements in the active region's magnetic field that could lead to the observed changes.