filament (on the Sun)
A filament is a strand of relatively cool gas suspended by magnetic
fields over the solar photosphere so that it appears as a dark line over the Sun's disk. A filament on the
limb of the Sun seen in emission against the
dark sky is called a prominence. Filaments
often mark areas of magnetic shearing and can be seen only in the centers
of strong spectral lines, such as H-alpha or the H and K lines of calcium.
A filament channel is a broad pattern of fibrils in the chromosphere, marking where
a filament may soon form or where a filament recently disappeared.