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Discussion on Evolution and Physics

As mentioned previously, the classification scheme diagrams weren’t designed around evolution. However, it is certainly possible to see how some of the links between prominence types could be interpreted as an evolutionary sequence. From this, we can consider the underlying physics that dictates how prominence evolution occurs, which is of course heavily related to magnetic fields. Every prominence is comprised of material travelling along magnetic fields lines, and so it is the shape and evolution of these lines that we are actually seeing when we monitor prominence evolution. Please note that the caveats and limitations of the scheme discussed above need to be borne in mind for this discussion.

Prominences are of course known to evolve, so it is possible that any given prominence could evolve from one type into at least one other. For example, it’s easy to see how an unconnected arch could evolve into a singe arch, or a pyramid could evolve into a broken pyramid. It is also possible to see how a single type could evolve into two different types, for example:

Diagram showing evolution of prominence arches
Diagram showing evolution of prominence pyramids

These secondary stages could then evolve into their own types, and so on.

During our observing we witnessed real prominence evolution. As the observing times tended to be quite short (i.e. 2-3 hours max) it wasn’t possible to see the entire evolutionary sequence of any given prominence, but even in these short times it was possible to see them occasionally change type. For example, on one occasion a fork evolved into a regular pyramid. On another, an anomalous prominence (similar to a mound or inclined pillar) evolved into an unconnected arch. On yet another, a straight pillar evolved into a tall fork, and a mound prominence evolved into an inclined pillar. These examples alone prove that prominence evolution between types does occur.

What might be happening to the magnetic field lines in the above examples? With the fork, if we imagine a fork as being made of two magnetic field lines each carrying their own plasma, then this presents the possibility that one of the field lines collapses much more quickly than the other, leaving a regular pyramid for a longer period. Alternatively, perhaps the two field lines moved closer to each other, causing the two bulks of material to merge into a single prominence. The connection between an inclined pillar and unconnected arch is not likely to be to do with changing magnetic fields, but simply the material travelling along the field line, i.e. it appears as an inclined pillar until the material begins to arch back down to the second anchor point at the surface. And again, a mound to inclined pillar might just be material moving off the erupting surface and being channelled by the field lines. A straight pillar evolving into a fork could be caused by the emergence of a second field line close to the first, or perhaps two field lines are moving away from each other.

It is also possible to combine the statistical analysis results with the ideas on evolution to get a feel for the possible lifetimes of certain stages of prominence evolution. For example, a single arch was seen to appear 7.1% of the time and a broken arch just 1.7% of the time. This could therefore suggest either that single arches are stable and rarely break up, and/or that the lifetime of the broken stage is very short. Perhaps some of the broken-off material becomes a detached prominence, which is more often seen, and the rest of the material quickly collapses to the surface. What happens to the field lines? Perhaps some of the field strength is lost in parts of the arch, causing a significant reduction in density in the material in those places before the whole thing becomes unstable and collapses back to the surface. Or perhaps a broken arch is simply a gap in the flow of material and isn’t evolution related to magnetic activity.

Another example is the unconnected (3.4%) to single (7.1%) arch evolution. The complete arch is twice as likely to be seen, potentially implying that the speed of the material travelling along the magnetic fields that constitutes an unconnected arch is very high, and thus the lifetime of that stage is short. We have seen a prominence evolve into an unconnected arch within an hour, backing up this idea. It may also imply that once an arch has been anchored to the surface at both ends, it remains fairly stable for a while until disturbed by other magnetic activity.

The regular pyramid occurs almost twice as often as the fork, and almost seven times as often as the broken pyramid. We’ve seen a fork evolve into a pyramid, so this evolutionary sequence is certainly possible. A fork is a more complicated arrangement of field lines than a regular pyramid, so this is likely to be the main cause of the rarity of the fork, and just because a fork can evolve into a pyramid doesn’t mean a pyramid can only be formed in this way, they can likely be formed on their own as well. Since broken pyramids are so rare, it could be a state with a very short lifetime, i.e. once it starts breaking up, the field lines quickly become unstable and collapse very quickly. Also, similar to broken arches evolving into detached prominences, it is possible to see how a pyramid could break up in such a way that it leaves a detached prominence.

Other likely evolutions include a single arch to double arch (and vice versa), which could be caused by a second magnetic field line intersecting the single arch creating an X-shape. The second field line would then carry its own material, or perhaps deviate some of the material in the original arch. The opposite evolution might then be seen to occur, should this second field line collapse before the first. Alternatively, the original field line collapsing first might leave a pillar or mound type. A fork might evolve into a hedgerow if magnetic activity in the vicinity of the fork increases, causing more field lines to emerge and thus more prominences to appear. This is more likely to occur around active regions like sunspots. Another is that a regular pyramid might evolve into a fork. If we imagine the pyramid as being made up of multiple magnetic field lines with the material travelling along all of them (so it appears as one single bulk) then the loss of a central magnetic field line would create a gap in the bulk, creating a fork shape. A curved pillar might evolve into a single arch with a short distance between anchor points, or other distorted shape. A straight pillar might possibly evolve into a curved pillar if the field line was disturbed so much by other nearby magnetism that it bent significantly.

This discussion gives some idea of the potential of a classification scheme of this sort to make predictions about how prominences and the magnetic activity associated with them can evolve.