How to catch a wave -- on the Sun

Science Nugget:  August 27, 1999

1. What's the idea?

It has been difficult for Yohkoh to see global solar atmospheric waves of the type made famous by Type II bursts, chromospheric Moreton waves, and coronal EIT waves. The reasons for this have been discussed a bit in the links associated with the first likely SXT observation of a global coronal wave. Soft X-ray observations can bring new knowledge about wave properties, because of the good response to higher temperatures, but by golly we need to have more than one wave to work with. Accordingly last week we have been experimenting with a new program aimed explicitly at catching waves: we will have SXT make "quarter resolution" images, ie 10 arc-sec pixels, which it can do every 32 seconds. Thus a wave moving at 700 km/s (conveniently one arc-sec/s) will cover one pixel in ten seconds, or three pixels in 32 seconds (approximately). Such good sampling would in principle give us many images of a given wave, providing we were lucky enough to be using such a mode when the wave happened, and did not go automatically into flare mode. So, this science nugget describes test observations of this sort. Yohkoh has also raised its flare threshold, at least temporarily, so that we can observe global effects on events at the C6 level or below - many EIT waves happen with such flares.

2. Data quality - could we see a wave with the new data mode?

So, let's take a look at some images taken in this mode (AlMg filter, exposure time. It doesn't do much good to look at the movie, because only subtle variations (including some pointing jitter, which is noticeable even at this resolution). Instead the images below show three views of a difference between two consecutive frames (07:10:33 and 07:11:05 UT, August 28, 1999):

From left to right, these show a direct difference; a difference with an expanded scale of +-20 DN (for reference, the active-region areas are saturated at 4,096 DN, so that this expanded scale really is just the faint parts); and a specially-compressed representation that shows the positive and negative excursions as separate square roots. In any case, the point is that there are faint, ghostly features at the level of this +-20 DN of noise. If a wave passed through the corona and were this bright, the pattern would clearly be visible. The May 6, 1998 wave had peak brightnesses many times this level, but it came from an X-class flare. So we don't know exactly what to expect.

A different way to look at these images is via the histogram of the difference: how many pixels showed what brightness level. Left and right below, the histograms from one of the images (07:10:33 UT) in a box near disk center, and from the difference of the two:

The sort-of-unexpected grouping of the histogram into multiple peaks shows the presence of the data compression algorithm. Each pixel value is transmitted essentially as a square root of its value; since the inherent noise (the photon counting statistics) varies as the square root, this can be done in such a way as not to compete. So these big gaps in the histogram in principle don't mean a thing in terms of signal-to-noise ratio.

3. Bottom line

Well (groan) we've done it again! A turgid science nugget with only nuts and bolts in it. But never fear, you will be so pleased with the wave observations that result from this new observing mode (when we actually catch a wave, so to speak in a California way), that you'll be pleased that this explanation of something relatively technical was there close at hand. But really, we are killing time this week because the big flare on August 27, which we observed well, was REALLY DULL:

What the SXT image (left) and HXT light curves (right) show us is a gradual soft-spectrum flare. It had no ejective motions whatsoever, so far as SXT could see, and no CME either according to the LASCO preliminary lists, even though it was an M2.8. In the next nugget we'll probably have excellent observations of the X-class flare later in the day from the same AR (8674), which did have a halo CME and no doubt will be more fun.
 

H. Hudson (hudson@isass1.solar.isas.ac.jp)

August 30, 1999