The Sun in Hα
This image is taken through a filter centered on a spectral line of Hydrogen (Hα, wavelength λ = 6563Å) that forms above the surface of the Sun, although large sunspots are still visible. Active regions and plages also show up brighter than their surroundings.
Moreover, in most cases the location of bright Hα regions on the disk coincides with the location of sunspots and active regions. Interesting new features seen on this image are filaments, dark string-like structures visible on the disk, and prominences, bright structures extending outward over the limb. Physically, filaments and prominences are one and the same, namely condensations of cooler gas high up in the solar atmosphere. Their distinctive appearances are a geometrical effect: seen against the bright solar disk, dense cool gas absorbs and scatters sunlight away from the line of sight, and so looks dark; seen above the solar limb against the dark sky, dense cool gas scatters sunlight into the line of sight, and appears bright. Filaments can be rather long-lived, and can often be seen moving across the disk, carried by rotation on successive daily Hα images such as this one.
Written By P. Charbonneau and O.R. White–April 18, 1995