Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, they appear as dark lines when seen on the solar disk using special filters, such as Hydrogen-alpha that shows the "cold" inner atmosphere of the Sun ("chromosphere").
Early September, a rather long filament transited the solar disk, measuring about 260,000 km in length or more than 6 times the Earth's circumference. Around 6 September, the filament started to rotate over the western solar limb (movie from 2 till 8 September). No longer was it visible as a dark line. Instead, it showed itself as a brilliant stretched bulge, as it was now seen from aside (in profile). Though it is still the same solar feature, its appearance is so different that scientist in the past gave it a different name: prominence, a name that is still used today. The height of the prominence was measured to be about 35,000 km, or nearly 3 Earth diameters.
The long filament was squeezed between 2 (quiet) sunspot groups and a (small) coronal hole, as can be seen in EUV images made by SDO. Both as a filament and as a prominence, the feature was highly dynamical, but did not catastrophically erupt.
For STEREO-A, a spacecraft monitoring the Sun's backside, this blob of ionized gas became first visible as a prominence at the east limb. Then, as the solar rotation gradually moved it onto the solar surface visible for STEREO-A, it became a filament (dark line) once again.
The filament remained very active, but there has not been any eruption yet. So, it will be interesting to see whether or not the filament survives its trip over the Sun's backside, and -for viewers from Earth- will become visible once again as a prominence, this time at the eastern limb of the Sun.
Credits - Imagery for the movie clips were taken from the GONG H-alpha network, SDO and STEREO.
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