On February 24, 2015, around 10:00UT, the Sun put on quite a show with a gorgeous filament eruption. PROBA2 was on the outlook, and its EUV camera (SWAP) recorded a magnificent movie of the event.
Filaments are clouds of plasma which are suspended in the corona, but they are denser and cooler than the surrounding coronal plasma. This plasma is trapped in dips in the magnetic field, such that it can not flow back down to the chromosphere. Filaments generally occur above photospheric neutral lines, which are the dividing line between magnetic fields that are directed into (negative, "black") and out (positive, "white") of the surface of the Sun, as can be seen in the sketch underneath. Filaments can be very long, like reported in previous news items (such as on 11 February 2015), or they can be smaller. Most of the shorter ones occur within active regions. In the images below, from the day of the eruption, you can see both long and short filaments. But you can't see the filament that erupted, because at the time these images were taken it had already erupted!
The eruption that you can see in the movie looks like it begins slightly behind the limb of the Sun, so that from the point of view of Earth we might not be seeing the whole original filament. Unfortunately, the STEREO spacecraft are currently in hibernation on the far side of the sun, so we don't have any more information about the 3D nature of this eruption. But we can clearly see the filament material rise into the solar atmosphere during the eruption.
In the early stages of this eruption, the filament roughly traces a large loop that is expanding. As it expands, the leg closest to the equator appears to "break", as indicated in the top middle image. The filament plasma is a tracer of the magnetic field, it is not showing us the whole picture. While it is possible that there is a magnetic reconnection event where the filament plasma separates, it is not the only explanation. In this case the field might very well be continuously expanding with no abrupt topological changes at that stage.
Later in the event, some of the filament material is seen to fall back down to the surface of the Sun (see image above on the left). These parcels of filament material are trapped on dips in the magnetic field. When the field expands during the eruption, some of the field stretches and the dips disappear. If a magnetic flux tube straightens out, but remains connected to the Sun, the plasma on that tube may slide back to the Sun, and this is what we see in the movie. The filament plasma then causes localized brightenings where it hits the surface, as indicated by an arrow in the image above on the right.
Behind these localized brightenings, bright loops form rapidly near the former location of the filament footpoint shortly after 12:00UT. These loops are known as post eruptive loops, and they are a common signature of reconnection. See the news items of 21 October 2014 and 22 January 2015 for more examples of post eruptive loops.
By looking at SOHO's coronagraphic imagery, however, we do know that this filament eruption caused a gorgeous CME. Unfortunately for us, the direction of the CME is almost perfectly aligned with the support post of the disk that hides the detector from direct sunlight.
This article was written by the PROBA2 Team. If you have any questions, you can contact them at swap_lyra@oma.be
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