Wade: You didn't discuss much about what goes on between the Lyman edge and 911 Angstroms; is that an instrumental limitation?
Rottman: Yes, it's difficult to make the measurements as you move further and further into the UV. But also, we are allowed to do Ly alpha with UARS only because of its importance to water vapor in the atmosphere. To go next step, you've got to make convincing arguments about why you need to make these measurements. Now, the solar physics community has ample reason; we would love to do the whole Lyman series, Ly beta especially. The TIMED SEE instrument (Tom Woods, PI) will do that; the TIMED community recognizes the importance of this spectral range because of their atmospheric interest above 100 km. But this is a constant struggle, trying to justify why we need to make these kinds of observations.
Wade: The irony of it is, of course, that Voyager has been looking at objects other than the Sun for years, in just that region, although not specifically for variability in solar-like stars.
Rottman: Well, TIMED will do it also, and there are a few rocket flights recently that cover that spectral region, but these provide only snapshots of the Sun, with no reliable synoptic data to establish solar variability.
Ayres: You kind of glossed over the extreme UV, particularly the 10 to 30 nm region. While it is true that this is a very difficult region to observe for most stars, there is a horizon that varies in depth, starting at a few parsecs, for which it is possible to observe stars, as has been done with EUVE. It's an extremely useful spectral region, because it contains a variety of ionization states, particularly for iron. But I think it's fair to say that our knowledge of the stars from 10 to 30 nm is better than our knowledge of the Sun, because we don't have solar measurements of even moderate spectral resolution in this region. This lack of information for the Sun makes it very difficult to interpret the stellar measurements.
Rottman: There have been infrequent observations in this area, especially back toward the beginning of Cycle 22, but I think they were troubled with their short duration and troubled with calibration problems. It's certainly an area that needs attention, though I don't see any hope for it before the TIMED mission. It is our point of view that to really understand then Sun we will need uninterrupted data sets in all of these wavelengths.
White: The irony of it is, Tom, that TIMED is really driven by the needs of the upper atmosphere folks, and the band you're talking about is important to them. But, we can't get the solar folks in space physics to recognize we need these solar data.
Ayres: And the Harvard spectrometer on SMM, which would have looked at this region in some detail, with fairly good resolution and in time for the solar maximum, was dropped. Gary, let me make one more comment -- actually two more comments. One on the X-ray region, and the tremendous dichotomy between solar physics and stellar physics. There are people like Loren Acton who have worked for a long time with the YOHKOH experiment, who see these spectacular changes from solar minimum to maximum, largely because YOHKOH's bandpass is pretty hard compared to that of its stellar counterparts. So as you were saying, as you go to shorter and shorter wavelengths, you see more and more variability across the cycle. It's a factor of 100 in Lx across the cycle in the YOHKOH data. But if you go to the somewhat softer bands that are typically observed with the stellar instruments, like EINSTEIN, or BEPPOSAX, or even AXAF when it goes, you find a variation over the cycle that's more like 5 or 10. So a lot of stellar folks look at the solar community, and they say, what's going on in the Sun that there's this tremendous variation being quoted, yet we don't see anything like this in the solar proxies we have in our stellar data? I won't even show a viewgraph, at risk of running over my time [laughter] -- or of running over your time, actually [laughter], I'd like to make one more comment concerning the absolute accuracy of these missions. I know that with SOLSTICE, you've taken a lot of pains to observe ensembles of hot stars regularly, to track the degradation of the instrument with time. And I know it's very difficult to take an absolute measurement in the laboratory and ensure that it's carried to orbit. But there are stellar targets, like hot white dwarf stars, that are known to be very stable in their luminosity, and very predictable in their energy distributions, that might be useful as radiometric calibrators. They'd essentially give you absolute calibrations, to the extent that you know the distribution. So they might be useful as standards for the solar observations, if one could design an instrument that could observe these very faint objects.
Rottman: Well, it took lot of work to bridge the brightness gap between the Sun and these bright blue stars. To go even further to the stars you're suggesting would require careful consideration. Perhaps you could use HST, and go from those standards to our blue stars. But each time you do a step like this, and transfer the calibration from one data set to another, you introduce at least a 1 % error. It doesn't take very long before you're back at our present capability of 3 to 5 %.}
Wade: Well, clearly the problem is that the Sun is too bright, so you ought to send your laboratory to Pluto and observe from there [laughter].
Rottman: You know, the more I work in this area, the more it seems phenomenal to me that the Sun is as stable as it is. When you talk about a tenth of a percent over time periods of five years -- we can't do that in the laboratory; we cannot make standard sources that stable. So from many standpoints, the Sun is a more reliable source than any other we have, so maybe [laughter] we ought to be using the Sun to calibrate our lab standards!