  Magnetic fields in the solar corona are responsible for a wide range of
phenomena. However, any direct measurements of the coronal magnetic fields are
very difficult due to lack of suitable spectral lines, weak magnetic fields,
and high temperatures. Therefore, one extrapolates photospheric field
measurements into the corona. Owing to low coronal plasma $\beta$, we can apply
a force-free model in lowest order to study the slow evolution of active region
(AR) magnetic fields. On applying these models to AR 11158 and compared with
coronal plasma tracers, we found that (1) the approximation of potential field
to coronal structures over large length scales is a reasonable one, 2) linear
force-free (LFF) assumption to AR coronal fields may not be applicable model as
it assumes uniform twist over the entire AR, and 3) for modeling fields at
sheared, stressed locations where energy release in the form of flares are
usually observed, non-linear force free fields (NLFFF) seem to provide a good
approximation. The maximum available free-energy profile shows step-wise
decrease that is sufficient to power an M-class flare as observed.
