Non potentiality of coronal loops above active regions

Using magnetograms from Kitt Peak and SOHO/MDI we extrapolate the photospheric longitudinal magnetic field of entire active regions in the linear force-free field (lfff) approximation. The method is based on the fast Fourier transform, which imposes a periodicity of the selected magnetogram in the horizontal directions. We concentrate on two observational data sets: July 11, 1991 and Aug. 29 - Sept. 25, 1996. By fitting extrapolated lines to the coronal loops observed either by NIXT or by Yohkoh/SXT, we are able to derive values for the shear parameter alpha in the studied active regions. We obtain the following results for a single AR: The shear is not constant, so that the magnetic field is in fact a non-linear force-free field (nlfff); the shear is higher at low levels of the atmosphere in the vicinity of inversion lines, which is consistent with observations showing the presence of filaments; the shear increases from one rotation to another, leading to accumulation of magnetic energy which is eventually released in DBs and CMEs.