Horizontal flow below solar filaments

Context. Observations of the internal fine structures of solar filaments indicate that the threads of filaments follow magnetic field lines. The magnetic field inside the filament has a strong axial component. Some models of magnetic fields suggest that the field structure in filaments could be caused by the horizontal plasma velocity field near both sides below the filament, where observable shearing effects from the axial component are expected. Aims. The horizontal velocity field in the vicinity of polarity inversion lines is measured in order to determine, if it exhibits a systematic movement that induces shear along the filament axis and convergence perpendicular to the axis. Methods. The horizontal velocity was obtained from the displacement of supergranules, which were derived from Doppler measurements in the solar photosphere. Dopplergrams corrected for rigid rotation and p-mode oscillations were further analyzed by local correlation tracking. Results. Vector fields of the horizontal velocities were measured in 16 areas during 8 time intervals in the years 2000-2002 on both solar hemispheres, each for a few consecutive days. For 64 selected filaments the nearby horizontal velocity vectors were split up into a component along the filament axis and a perpendicular component. Conclusions. Differences between the axial velocities on both sides of the filaments were calculated. In almost all cases the velocity gradient corresponds to the inclination of the threads observed in H alpha images. The average transverse velocity does not show any preferred tendency towards a divergence or convergence to the filament axis. Testing the horizontal velocity for the creation of the differential rotation profile in the photosphere reveals a strong dependence of the averaging process on the scale of our velocities.