In this paper, we investigate how photospheric material moves below a dark filament, and we study chromospheric gas motions inside the filament. In the photosphere we trace granular motions by means of a local correlation tracking (LCT) technique to derive horizontal velocity field, while inside the filament we obtain the line-of-sight velocity field by subtracting a blueshifted H image from a redshifted H image. We find that a typical value of horizontal photospheric velocity is 1 km s-1, and its divergence map maintains a large-scale pattern during several hours. We also find that photospheric motions around a filament channel are random in space and changeable in time. As for the motion inside the filament, our results show that there is an area of upward motions at one side of the filament axis and an area of downward motions at the other side, which means that filament material has a helical motion inside the filament. We think that these results provide new important information on theories of filament formation.
INTRODUCTION
Filaments are probably one of the most conspicuous things observed in the solar atmosphere. When they are observed in some particular line such as H, filaments have a dark, long, extended body on the solar disk, whereas they are observed to have a bright archlike structure at the solar limb. In the latter case, they are called prominences. Macroscopically, filaments are usually in a quiet state, although they sometimes suddenly enter a dynamical stage, erupting upward and finally disappearing (filament eruption). For solar physicists, it is an interesting and important problem to understand the nature of such filaments, so many studies have been done on this topic.