(a) Reconstructed filament with flux rope field lines (red lines) overlaid by arcades (green lines). These field lines are obtained from an LMHD extrapolation of an observed photospheric magnetogram for a different prominence. (b) Dips in the magnetic field lines (green lines; adapted from Aulanier & Schmieder 2002). Bottom panels: magnified view of the tornado-like structure showing an accumulation of fine horizontal structures, similarly to the green lines in (b).