A narrow arch or loop above the Sun’s photosphere, visible in hydrogen light as a darker absorption feature against the solar disk, in a similar manner to normal filaments; when seen at the limb, though, they appear bright. Arch filament systems are a characteristic feature of developing active regions on the Sun and connect areas of opposite magnetic polarity (usually sunspots) in the innermost part of young bipolar groups. Individual arches have typical lengths of 30 000 km and lifetimes of some tens of minutes, although the entire arch filament system can last for several days. Their absorption of hydrogen light implies the arches are cool and dense, and so they are believed to be either photospheric plasma that has risen into the corona or coronal plasma that has cooled.