A filament erupts
posted: February 2, 2016

A movie of this event can be found here.

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.

Long filaments are known to eventually erupt, as the surrounding magnetic fields usually become unstable at some point. This is certainly the case when the filament borders active regions, where dynamic magnetic fields often are at work. No surprize then that space weather forecasters had their eyes on a 30-40 degrees long filament once it had rounded the solar southeast limb on 19 January. Finally, during the afternoon of 26 January, it erupted quite spectacularly. This "Disappearing Solar Filament" (also called "Disparition Brusque") can be seen in the H-alpha images above from the GONG H-alpha Network, showing that the filament took only about 1.5 hours to erupt. Many observers who had followed the source filament for nearly a week, had the impression as if something was missing on the Sun as its southern hemisphere suddenly looked particularly empty in H-alpha.

The movie shows the eruption of the filament in H-alpha. The next clip combines extreme ultraviolet (EUV) images from SDO/AIA 171 and PROBA2/SWAP of the eruption, with SWAP's wider field-of-view allowing to trace the ejected material as it leaves the Sun (annotated by the green ellipse in the above picture, taken at 19:06UT).

The next clip then shows another EUV combination, this time from SDO/AIA filters showing the eruption in "cool" (80.000 degrees; red), "medium" (700.000 degrees; green), and "hot" (several million degrees; blue) temperatures. The eruption was accompanied by a minor C1-flare only, and no proton event was associated with it. The image underneath shows the eruption in full progress at 17:32UT. The clip also shows that a significant part is falling back to the solar surface, with some cold (darkish) plasma floating in the solar atmosphere and to the west.

The last clip shows a combination of SDO EUV images with coronagraphic imagery from SOHO/LASCO. There seems to be two coronal mass ejections (CMEs). The second CME (CME2) first seen at about 19:53UT seems definitely to belong to the eruption of the filament. It is however not clear if the faint CME first seen at 18:42UT (CME1) belongs also to this filament eruption or if it was related to a small eruption in NOAA 2488 (northern hemisphere) around 15:20UT, or even a backside event (Stereo-A images lacking). However, careful analysis revealed that both CMEs were not directed to Earth.

Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, SOHO/LASCO, PROBA2 and (J)Helioviewer.