Saturday, August 24, 1996 -- the first clear sunlight in almost a week. The quiet Sun of last Sunday evening has changed. On the leading limb (rotating out of view), I found this example of the rococo architecture of the Sun. It was instantly apparent halfway through my first trip around the Sun with the slow-motion stepper motors. When its tallest spires approached the sweetest spot in the filter's field of view, I froze the telescope's aim and tuned for maximum contrast. The brightest bits faded last and reappeared first and they were visible over a wider range of frequencies than the chromosphere itself is. After taking a few minutes to sketch the general aspect of this prominence, both to get it down (in case clouds returned) and to detect rapid motions (if any), I completed my first circuit of my solar beat. Almost oppositethese complex spires and arches, I found an amorphous haze hanging above the Sun's trailing limb. With tuning, this cloud showed some slight structure, but for the most part all careful examination went unrewarded and I decided I was wasting time better spent elsewhere. I didn't try a drawing of this other active region.

There was a lone sunspot close to one limb and I hoped a very careful look at that limb might be interesting. Only a couple of slightly curved and overgrown spicules grew anywhere nearby, so I attended instead to the clearest detail I've yet seen on the face of the Sun. (Is this face-on detail really more contrasty than before, or am I simply learning to see it and to pay attention to it?) Preceding the spot was a shadowy arc with the spot approximately at its center of curvature -- I have no reason to think the two are physically related, since no dark or light detail connected the spot to this arc. I take this dark region to be a prominence seen in silhouette ("solhouette"?). It's of the same general size (in terms of its angular extent along a great circle) as the larger fields of prominences I have seen, and its most subtle shapes and shadings (which I have not attempted to sketch) were reminiscent of a kind of negative image of limb prominences. Its details were subtle at best. I did not experiment with tuning to determine whether these details or the filament as a whole appeared and disappeared over the same, or a wider, or a narrower band of wavelengths than limb prominences do. Why do the best experiments always come to mind too late to be tried?


Fast Times at Sunspot Central!

I revisitted the amorphous cloud above the trailing limb, made out a little more detail, but elected to go back to the showpiece on the leading limb. I went back by walking the telescope around the limb, back the same way I came. When the region of the sunspot returned to view, the first thing I saw was a brilliant streak angling up from the limb directly "over" the spot. It could not have been there ten minutes earlier! On close inspection, it was not connected by spicules or other visible structure to the surface of the Sun, and no feature of the chromosphere where it "would" touch down seemed out of the ordinary. Instead, it appeared to rise up from beyond the Sun's horizon.

I expected to see rapid changes in this new feature so I watched for a few minutes. None were forthcoming, so I recorded it as exactly as I could (get a pad of graph paper!) and planned to return after another review of the complex show on the leading limb.

When I came back to the spot 10 minutes after first seeing the bright line above it, the streak was gone. No amount of tuning would bring it back into view. There was perhaps a larger than ordinary spicule near where this bright line had been, but certainly nothing more. So -- did something erupt from the Sun? Or have I just seen a "flickering" prominence of the sort the books describe which appear and disappear as their gases are accelerated to and from the observer and their light Dopplar shifted in and out of the tuning interval (+/- 20A corresponds to how many miles/sec? I've only seen this description applied to dark features seen against the solar surface where rapid vertical motions are available to account for their changing visibility, and I understood the range of wavelengths to be much smaller). Or -- my current favorite guess -- am I seeing an extension of that dark filament around and above the limb, a sheet of gas which just happened to change its orientation in space such that for a few minutes only I was coplanar with it? Perhaps it became visible only when I looked along its length rather than across it? Is something like this at work with the faint amorphous prominence -- am I seeing something broadside which would appear brighter and more structured if seen along a sight line which included a greater thickness? (And am I now trying to make too much hay out of an already tenuous guess?)

All this speculation is fueled in part by the appearance of that tallest spire in the leading edge feature -- it looks as delicate as an auroral curtain, and not very different from one for all its difference in scale and color. 1996 August 24. 12:00-12:35 EDT

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