A feature in the low chromosphere of the Sun. Fibrilles are fine dark lines arranged in a near-spiral pattern around sunspots as seen in Ηα‎ light. They are particularly prominent around large, mature spots, from which they extend horizontally for about 10 000 km, forming a superpenumbra. The whole pattern lasts as long as the active region containing the sunspots, but individual fibrilles live for only 20 min or so. Fibrilles are thought to delineate the magnetic field in the chromosphere around sunspots.