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.
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 <email@example.com>
October 8, 1999