Observations and Modeling of a Filament on the Sun

Halpha observations of a filament were obtained at the Swedish Vacuum Solar Telescope in 1998 June. The U-shaped filament has a prominent barb that exhibits interesting fine structure and internal motions. A three-dimensional magnetic model of the filament is presented. The model is based on a National Solar Observatory (Kitt Peak) magnetogram and is constructed by inserting a twisted flux rope into a potential field representing the overlying coronal arcade; the flux rope has an axial flux of 3.4 x 10 super(19) Mx and poloidal flux of 3.7 x 10 super(9) Mx cm super(-1). Magnetofrictional relaxation is used to drive the configuration to a nonlinear force-free field. The shape of the resulting flux rope is distorted by neighboring network elements. The dips in the helical field lines reproduce the observed filament barb, which is caused by a local distortion of the flux rope resulting from a weak-field extension ([approx]4 G) of a neighboring network element. The pitch of the helical field lines is larger than expected on the basis of a model of flux rope formation. I suggest that this is due to magnetic diffusion within the flux rope. A simple model of magnetic diffusion in a cylindrical flux rope is presented.