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Magnetic fields in the solar corona are responsible for a wide range ofphenomena. However, any direct measurements of the coronal magnetic fields arevery difficult due to lack of suitable spectral lines, weak magnetic fields,and high temperatures. Therefore, one extrapolates photospheric fieldmeasurements into the corona. Owing to low coronal plasma $\beta$, we can applya 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 withcoronal plasma tracers, we found that (1) the approximation of potential fieldto coronal structures over large length scales is a reasonable one, 2) linearforce-free (LFF) assumption to AR coronal fields may not be applicable model asit assumes uniform twist over the entire AR, and 3) for modeling fields atsheared, stressed locations where energy release in the form of flares areusually observed, non-linear force free fields (NLFFF) seem to provide a goodapproximation. The maximum available free-energy profile shows step-wisedecrease that is sufficient to power an M-class flare as observed.

Magnetic structure of solar active region NOAA 11158