A long-lived dark loop

Science Nugget: October 8, 1999

What struck the eye

The solar corona is optically thin to soft X-rays in the range of Yohkoh SXT; i.e., we can see right through it.  We do not expect anything to hide behind something that we see in the corona. And yet sometimes we see things that make us say the scientific equivalent of "Gosh! Sure looks like a dark loop!" This science nugget reports one of these rare occurrences. The movie below shows that it happens after a major flare event just behind the NE limb:

You probably didn't readily see the loop, if that's what it is, so here is a difference image over a four-hour time span:

Here the dark loops show where things have gotten fainter. The interesting one is to the north, but the combination of north and south branches calls to mind "bifurcating cusps," a topic for some future science nugget and not so relevant here. The difference image (on a magnified brightness scale) shows other interesting things, like they always do: the fringe of dark at the limb under the flare results from the reduction of scattered X-rays within the telescope, as the flare dims; the bright tongue running parallel to the limb S of the flare is a large-scale jet that just coincidentally happened.

Now if you'll go back and look at the movie again (sorry it's 300 KB) you will see that the dark structure appears at the top of bright loops resulting from the flare.

How to explain it

Well, the last comment above implies that the highest-lying bright flare loops cooled in place and lost their bright high-pressure contents, thus not containing material capable of radiating. In other words, the darkness results from a strong local vacuum in the form of a loop. Most likely this is not a single loop, but an arcade that we're seeing end-on; this would increase the path length and give better visibility. A single discrete loop might not be able to compete with the bright corona surrounding it.

Are there profound implications?

Possibly. The dark structure persists for almost a day (we're limited in coverage because the final data aren't available at the time of writing):

This pretty strongly implies that a stable static structure was created after the flare, ie the arcade of bright loops, and that the presence or absence of hot plasma did not affect its position very much. In other words, magnetic pressure dominates gas pressure. In plasma-physics parlance, this implies a small "plasma beta", which is the ratio of gas to magnetic pressure - say, on the order of 0.01 at a wild guess.

We've checked, and there is no gross difference in the appearance of the dark structures between two of the SXT filters - this confirms that absorption by a dense foreground object (never mind how to maintain it at such an altitude in the corona!) could not have played a role. Finally, thanks to Keith Strong, who was always a firm believer in dark loops when he was engaged in SXT scientific operations more closely.

Hugh Hudson <hudson@isass1.solar.isas.ac.jp>
October 8, 1999