Scott,
I think that you're ultimately correct that the DEM is not the "final answer" to any questions in solar physics. If you think of what we're doing as an iterative process, however, it does provide a useful starting point.
For example, if I gave you the intensities from some hot loops in an active region:
Si VII 275.368 47.0 (erg/cm^2/s/sr) Ca XV 200.972 238.9
How else would start to think about them? With the DEM you quickly see that the amount of plasma at 4 MK appears to be much, much higher than it is at 0.6 MK, and that gets you thinking about physical models.
Of course at the end of the day, as you're saying, one needs to forward model the observations.
Best wishes,
Harry
On 9/23/10 8:53 PM, "Scott W. McIntosh" mscott@ucar.edu wrote:
Can I point out a paper to the list:
http://adsabs.harvard.edu/abs/2010ApJ...708.1238J
This paper encapsulates the problem in a concise form. I could point out a couple more, but those are nearly 10 years old now, wow how time flies. I have drawn my line in the sand with emission measures as a practical diagnostic - once we have a DEM/EM, etc what can we do with it? As far as I can tell there are only a few papers in the literature that deal with that particular issue in a real sense and those are from further back in the 80s [look for papers by Carole Jordan].
I am all for learning something about the solar plasma from a zeroth order approach. However, no EM/DEM/Line Ratio estimation deals with the real time-varying corona (or outer solar atmosphere as we need to deal with a intrinsically coupled system). By definition, these diagnostic deal with equilibrium processes and thermal populations - I am not entirely sure that we have that in place and if we do is, is that the process doing the "heavy lifting" or just the after effect procing the apparent stationary state.....there is strong evidence of non-thermal processes affecting the chromospheric, TR and coronal plasmas in the very same spectra from which such diagnostics are derived [probably even worse for broadband EUV imagers].
Sorry, to be a bummer!
-S.