Lighting up a filament channel

Science Nugget: October 13, 2000


Filament channels are the large-scale coronal features that provide the support of a filament (if seen on the disk) or prominence (if seen on the limb) against gravity. A filament is cold and dense and has no right to be floating high up in the corona - thus magnetic levitation with a hotly-debated geometry is required. We have not done many weekly science nuggets on filament channels, and in fact this research subject moves much more slowly than it should. However with the "chewy nougat" observation we found that one could sometimes see the cores of filament channels as hot, rather than cool, coronal inclusions -- there are also wonderful pictures from TRACE showing a hot shroud of material around one of these cold inclusions in the corona. Upon writing up the "chewy nougat," we found a convenient theory due to Antiochos and Klimchuk: siphon flow that elevates chromospheric material against the strong solar gravity (fifty times that of Earth at their respective surfaces). Coronal siphons had seemed necessary theoretically for some time, but the new wrinkle of this theory was that an energy source needed to be found near the footpoint of a long coronal field line. A filament channel contains such field lines - we know this because a concave-up segment would be needed to support the cold prominence material, or otherwise it would slide right down the field lines and drain out of the corona.

The FlareGenesis opportunity

The flight of the polar-circling balloon carrying the "FlareGenesis" telescope provided Yohkoh with some opportunities for joint observations not only with the high-resolution optical telescope/magnetograph on the balloon, but also with TRACE and other resources for solar observation. The observation described in this nugget resulted from this opportunity. A nice M-class solar flare occurred in the balloon's (and therefore everybody's) target active region. A review of the Yohkoh SXT movie for a few days around this flare showed something interesting: the flare apparently caused a brightening to spread along the length of the filament channel. Moreover this transient brightening, or quite similar ones, occurred half a dozen times in the same location while it was visible on the solar disk, including a pair of brightening events that occurred before the M flare.

The spine brightens!

The image above shows the channel we're talking about. In the left-hand frame, the SXT image from 22-JAN-00, 05:48:19 UT, is shown with dashed lines outlining the approximate location of the filament channel boundaries. The right-hand panel shows the appearance of the same region just 4 hours later: the difference should be obvious. But because we like to show lots of pretty pictures in these Nuggets, an SXT movie is linked below. The images span the interval 05:48 to 23:20 UT on 22-JAN-00. The second brightening is during the aftermath of the M flare.

MPEG movie, 100 kbyte

JavaScript movie, 2.3 Mbyte

As one can see, the coronal loops along the length of the channel brighten in X-rays, implying that either heat or hot plasma is injected into those loops. The fact that this recurs several times over the course of a few days (only two episodes are depicted here) indicates that the event must not be too uncommon, and the flux tubes lying along that dimension of the filament channel must be fairly stable. Please note that after the loops brighten, they appear to stay hot for a few hours; this will be discussed below.

Do the flux tubes in question lie along the center of the filament channel, threading its axis like the "chewy nougat" phenomenon? Or do they lie on the outside, as part of the shell of field lines closing around the channel (like the X-ray image in the earlier nugget)? From this viewing angle it is difficult to know for sure, and risky to speculate, but either geometry requires the heating event to affect the entire length of the channel without catastrophically disrupting the magnetic stability. (Sound familiar?)

And in this corner...

Just for fun, let's look at the same channel-brightening event in SOHO/EIT images. The movie below provides the 195-Angstrom images from EIT, between 08:24 and 15:36 UT; the temperature of the plasma shown here is mostly close to 1.5 million kelvins. We can see a small "front" of brightness enhancement, propagating down the length of the channel, but the coronal loops sure don't stay bright, like they do in SXT.

MPEG movie, 148 kbyte

JavaScript movie, 2+ Mbyte

The transient nature of the brightening is the difference image below: this shows the frame of 09:00:33 minus 08:48:11. The bright, wedge-shaped feature (lower arrow) has brightened in the intervening 12 minutes, while the rest of the loops north of that bright wedge have faded (particularly the dark smudge indicated by the upper arrow).

A JavaScript movie of these difference images is linked here

Why the difference?

Clearly there's something different about the images. We must recall that SXT is a broadband instrument, and as such it shows plasma over a wide range of temperatures, from about 2 to 20 million kelvins. This is in contrast to EIT, which is a narrowband instrument: the 195-Angstrom channel is sensitive to 1.5-MK plasma, and not much else. So one (un-refereed) interpretation is that the loops are heated from the northwest end, and each EIT image is merely showing us that plasma which happens to be at 1.5 MK at that time. Thus the propagating brightness enhancement seen in the EIT images tracks the progress of heating from one end of the channel to the other.

What does this have to do with the filament-creation theory mentioned above? Possibly nothing. They're both related to heat sources near the footpoints of long field lines, but the Antiochos & Klimchuk heating source was not described as so episodic as the brightenings we're monitoring here. Nonetheless, the observations would support the general plausibility of the mechanism they envisioned. And perhaps there is a connection to the "chewy nougats" -- hot, X-ray emitting cores which are sometimes observed inside otherwise dark filament cavities. Again, those brightenings are often less episodic than the current observations; but variations in the X-ray brightness of nougats have certainly been measured.

October 13, 2000

David McKenzie <>
Hugh Hudson <>