Dark Filament This SDO close-up of a filament and active region, taken in extreme UV light, shows a dark and elongated filament hovering above the Sun��s surface (May 18, 2010). The bright regions beneath it, which show where heating is going on in the magnetic field, send up shafts of plasma that trace magnetic field lines emerging from them. Filaments are cooler clouds of gas that are suspended by tenuous magnetic fields. They are often unstable and commonly erupt. This one is estimated to be at least 60 Earth diameters long (about 500,000 miles).