So, even the Sun struggles to lift heavy objects?

Published: Wednesday, 10 April 2019

Filaments/ prominences are massive clouds of cool plasma suspended above the Sun's surface (actually, whether we call it one way or another depends on our point of view as observers). Studying both the magnetic forces behind them and their mass is important to determine the timing of related coronal mass ejections. Jack Jenkins, from University College London (UK) explains it in this post.

Same feature observed from the two different perspectives: on the right, captured from a Sun's orbiter satellite (so it looks at it from the side, and we see it as a prominence);

and on the left, captured from an Earth orbiter satellite (so wee see it top-down, hence, as a filament).


Filaments / prominences are clouds of cool plasma suspended above the surface of the Sun and embedded within the much hotter (million degree) corona. In fact, the reason that the filament / prominence plasma remains cool, despite being surrounded by hot, coronal material, and doesn’t simply fall back to the surface of the Sun due to gravity, is due to the magnetic field of the Sun. In particular, a magnetic structure with the topology that resembles a rope, called a magnetic flux rope (imaginative, right?), is believed to be responsible. These structures that protrude from the solar surface, arc into the corona, and return to the solar interior a short distance away (forming half of a toroid) contain concave-up dips relative to the solar surface. These dips are then capable of holding and insulating the plasma above the solar surface.

Over the last few decades it has been shown that the destabilisation of these magnetic flux ropes can lead to the large-scale eruptions that are frequently observed to originate within the solar atmosphere. After erupting, these bundles of magnetic field and plasma are called coronal mass ejections (CMEs) and are able to travel through space, possibly interacting with the Earth. When this happens, we can be treated to the wonderful light shows that are the Northern Lights, or it can have more negative effects on our modern global infrastructure, such as disabling the satellites that control the GPS in your phone. This is why it’s so important to understand these solar eruptions.

Despite the fact that these structures are clearly massive, previous work has focused on magnetic forces alone. But, we have recently discovered that mass is very important in determining the timing and hence prediction, of the eruption. To understand how important it is, we need more observations to be able to characterise this delicate balance between magnetic field and mass on both short and long time-scales. The European Solar Telescope will have the ability to simultaneously measure the magnetic field and properties of the plasma using Integrated Field Units (IFUs). This new technology will yield new observations that will greatly improve our understanding of how the last piece of the puzzle, mass, influences solar eruptions.

In this movie we see an observation of the same filament / prominence from two different perspectives. The right side of the movie is from a satellite that is orbiting the Earth and is in this case looking top-down onto the dark plasma structure and so we see it as a filament. The left side of the movie is from a satellite that is orbiting the Sun and is elsewhere in the Heliosphere such that it is looking at the same plasma structure from the side and we see it as a prominence. Focusing on the right side of the movie we can see that the filament is angled diagonally from the top-left to the bottom-right. Just before the eruption you will notice a large, dark blob propagate away from the middle of the filament towards the top-left - this material is in fact leaving the flux rope, a process referred to as mass-draining. This observation provided conclusive evidence for mass-draining being able to trigger solar eruptions, and motivated the study of the influence of mass on solar eruptions.


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