3-D MAGNETIC CONFIGURATIONS SUPPORTING PROMINENCES - II - THE LATERALFEET AS A PERTURBATION OF A TWISTED FLUX-TUBE

In a previous paper we have shown that a twisted flux-tube is the most probable magnetic configuration supporting prominences. The model int erprets many observations in a natural way (in particular the magnetic measurements in prominences and the chirality properties). Moreover, prominence feet appear as a direct consequence of the parasitic polari ties present in the filament channel. Here we investigate further the link between feet and parasitic polarities by modelling explicitly the se polarities. We show that the prominence lateral feet appear natural ly, above secondary photospheric inversion lines and we describe the m orphological change of feet as parasitic polarities evolve. This appro ach is applied to an observed filament in Her with the MSDP on the Ger man VTT (Tenerife) where SOHO/MDI magnetograms are available. We show that the shape of the prominence is defined by the distribution of the dips in the computed magnetic configuration. Then we analyse the topo logy of the magnetic field using the quasi-separatrix layers (QSLs) me thod. We describe the basic changes in the topology as the parasitic p olarities evolve, in particular how the configuration pass from an OX to an OF topology. We find a correspondance between the computed QSLs and some of the chromospheric brightenings, observed around the feet o f filaments in the y line (Ca II, 8542 Angstrom). It confirms the dedu ced magnetic configuration and shows that energy release is present at a low level in the complex topology of the filament configuration.