Solar filaments are clouds of charged particles ("plasma") 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 and as bright blobs when seen near the solar limb (then they are called "prominences"). Special filters are required to observe these features, and one such a filter is the Hydrogen-alpha (H-alpha) line in the red part of the solar spectrum.
A prominence started to round the solar east limb on 07 November. This can be seen in the collage above running from 07 till 09 November (intervals of 12 hours). Initially it seemed to consist of a bunch of isolated chunks of "cold" plasma, but as it turned further onto the solar disk, the full extent of the filament became clear. It appeared to be a single J-shaped filament of about 800.000 km in length, open near its southeast end in H-alpha (in extreme ultraviolet, it appeared nearly closed). It was draped around a spotless region of positive magnetic polarity, and surrounded by areas of negative magnetic polarity. The impressive length, unusual shape, and magnetic configuration of this filament can be seen in the image underneath. It combines an H-alpha picture from GONG with a magnetogram from the Solar Dynamics Observatory on 14 November at 15:55UT. The reddish color indicates positive magnetic polarity (field lines coming out of the solar surface), the bluish color represents negative polarity (magnetic field lines returning to the solar interior).
Long filaments are known to eventually erupt, as the surrounding magnetic fields usually become unstable at some point. In this case though, only a small sunspot region to the north of the filament was visible (NOAA 2453), and negative coronal holes to the northeast and southeast. So it took a while before this filament erupted, first showing some instability to its top (near NOAA 2453), before erupting during the evening of 15 November starting from its southernmost footpoint, possibly the result of the interaction with another nearby filament. The curbed (western) part of the filament got ejected into space, with a part returning to the remaining northeastern part.
This MOVIE shows the eruption in H-alpha and in the extreme ultraviolet (combination of SDO/AIA 193, 171 and 304). The coronal loops and a transient coronal hole to the north in the AIA 193 testify of the material being ejected into space, as can be seen in the before/after eruption comparison underneath. The end of the movie also shows the eruption of the smaller filament to the southwest. It was also associated with a transient coronal hole and post-eruption coronal loops, and contributed considerably to the very complex coronal outlook. Coronal mass ejections related to both filament eruptions were observed.
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