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As happens frequently, this science nugget picks up where another one left off. Even though it's solar maximum, and we have other things to divert ourselves with, still it is astonishing that the Yohkoh SXT database has not been used more systematically for the study of coronal holes. A phenomenon that is present on the Sun all the time, independent of the level of solar activity, is the large-scale flow that we call "supergranulation" We return to Hoeksema's "gorgeous coronal hole" with the following question in mind: Do the boundaries of coronal holes reflect the motions in supergranulation? At a first guess, the answer to this question should be "of course", since the convective flows these supergranulation cells represent certainly can move the coronal magnetic field around. It's feeble by comparison and has nothing to hang onto to resist the stress.
The first thing we do is compare images half a day apart. Those of you with broad-band Internet connections could look also at the movie in full Javascript detail, but the essential point is made by the comparison below:
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At first glance these two pairs of images look identical (click to enlarge and be sure) but they are not. On the left one has the contours from the complementary images sketched; on the right the direct contours just for a sanity check. Little differences near the poles where no care was taken will show the difference. The images have rotation-correction to a point near disk center.
"Hmmm," we conclude. Although the large-scale convective motions (supergranulation) are reputed to have time scales on the order of one day, we don't see much change in a major fraction of one. So, let's look at a longer time scale in just the same way.
First, please check out the movie - it's only two frames this time, so you could even get it on a modem link. Now, let's make the same set of comparisons as above:
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Wow. These are very interesting comparisons. Again, the pair on the left is (image1 plus contour2; image2 plus countour1) if that makes sense; the pair on the right has matching images and contours so you can check for operator error. The image pair on the left is the operational one, and I don't see much evidence for a random pattern of convective cells at the correct scale. That should be about 1/30 of the solar diameter, which one could clearly perceive here. So, again as frequently happens with science nuggets, we've come face-to-face with an apparent quandary.
Our suspicions have been amply confirmed. Things are not so simple. We expected that the boundaries of the coronal hole would flutter like a Kansas curtain in a thunderstorm, but with time scales of one day and spatial scales of one arc minute. It doesn't look that way. We must delve deeper!
Obviously, one simple way to delve deeper would be to make detailed comparisons with the supergranulation pattern as observed simultaneously with an instrument (Kitt Peak? Mauna Loa?) viewing the chromosphere, where the chromospheric network guides us to the supergranulation; or to the SOHO/MDI doppler data, which show it directly. We can do this in a future nugget - please stay tuned.
January 5, 2001