  We present the evolution of magnetic nonpotentiality associated with an
X-class flare in AR 6919 using a set of MSO (Mees Solar Observatory)
magnetograms. The magnetogram data were obtained before and after the flare,
using the Haleakala Stokes Polarimeter which provides simultaneous Stokes
profiles of the Fe I doublet 6301.5 and 6302.5. A nonlinear least square method
was adopted to derive the magnetic field configuration from the observed Stokes
profiles and a multi-step ambiguity solution method was employed to resolve the
180 degree ambiguity. From the ambiguity-resolved vector magnetograms, we have
derived a set of physical quantities characterizing the field configuration
such as vertical current density, magnetic shear angle, angular shear and
magnetic free energy density. We have examined their changes before and after
the flare occurrence and obtained the following results. 1) There was a flux
decrease in both polarities around a delta-type sunspot region, where an
X-class flare occurred. 2) The vertical current near the delta sunspot region
was strongly enhanced before the flare. 3) The magnetic shear near the delta
sunspot region increased before the flare and then decreased after it. 4) The
sum of magnetic energy density significantly decreased before the flare onset,
implying that magnetic free energy was released through the flaring processes.
