  We investigate the restructuring of the magnetic field in sunspots associated
with two flares: the X6.5 flare on 6 December 2006 and the X2.2 flare on 15
February 2011. The observed changes were evaluated with respect to the
so-called twist-removal model, in which helicity (twist) is removed from the
corona as the result of an eruption. Since no vector magnetograms were
available for the X6.5 flare, we applied the azimuthal symmetry approach to
line-of-sight magnetograms to reconstruct the pseudo-vector magnetic field and
investigate the changes in average twist and inclination of magnetic field in
the sunspot around the time of the flare. For the X2.2 flare, results from the
full vector magnetograms were compared with the pseudo-vector field data. For
both flares, the data show changes consistent with the twist-removal scenario.
We also evaluate the validity of the azimuthal symmetry approach on simple
isolated round sunspots. In general, the derivations based on the azimuthal
symmetry approach agree with true-vector field data though we find that even
for symmetric sunspots the distribution of the magnetic field may deviate from
an axially symmetric distribution.

Solar and Stellar Astrophysics