OK. So I asked ...
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Originally Posted by Thompson
So, is it your position that the observed features are mostly made of iron, because the number of iron lines observed is greater than that for other elements?
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And Michael replies ...
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Originally Posted by Mozina
No. The number of photon emissions from iron is greater in the coronal loops. Even by your logic, it's mostly iron plasma and other kinds of metals in these coronal loops. ...
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And I replied ...
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Originally Posted by Thompson
I do not see why my logic would imply that it's mostly iron. Please explain. Do you think that the elements on the list are the only elements in the loop?
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And Michael replies ...
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Originally Posted by Mozina
No, not at all. They simply represent the more prominent wavelengths. Even still, by comparison, the iron concentrations in these areas is far greater than in the full spectrum of the sun. Why? This is direct evidence that the heat distribution of the model will directly affect the abundance calculations. The fact these two numbers are different precludes us from simply "assuming" that photons accurely represent concentrations of elements. We can get some idea from this method, but you can take it only so far.
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I still don't see a recognizable answer to the original question. So let me rephrase: What fact(s) of observation lead directly to the conclusion that the loops are dominated by iron? Since we have established that it is not the relative number of iron spectral lines, then what is it?
Now a couple of other points raised here. First, we don't need to have all this discussion just to conclude that the model affects the abundance determination, that is certainly true. So I presume that you dispute the model: Is it your position that we cannot determine photospheric abundaces via spectroscopy, because we have a false model of the photosphere?
In anticipation of what ever answer crops up, I draw the various readers attention to the relevant literature. For instance, on my shelf is the book
Stellar Atmospheres by Dimitri Mihalas, W.H. Freeman and co., 1978 (2nd ed). A tad old perhaps, I used it in my student days, but the basics have not changed. It presents a thorough introduction into how stellar atmospheres (including the solar atmosphere) are modeled. More recently, and perhaps more to the current point, a couple of relevant papers:
Chemical abundances from inversions of stellar spectra: Analysis of solar-type stars with homogeneous and static model atmospheres, Carlos Prieto,
et al., Astrophysical Journal 558: 830-851, 2001;
Line formation in solar granulation II. The photospheric Fe abundance, M. Asplund,
et al., Astronomy and Astrophysics 359: 743-754, 2000.
The former paper describes how abundances are determined by inverting the absorption spectra. The point of using this technique is that the spectra involve multiple elements simultaneously, so the method amounts to solving many simulataneous equations. There are many elements, and different abundances, but they are all in the same atmosphere, so the simultaneous solution will recover that atmosphere. This makes it sound simpler than it really is. The inversion is not unique, meaning there is more than one mathematical atmosphere that will satisfy the equations. We can select out the physical atmosphere by constraining the solutions through physics, the radiative transfer equation being one element.
The latter paper is quite specific to iron, studies how iron lines get their shape, and derives a photospheric abundance, which happens to be very close to the meteoric abundance. Was that just an accident? (The log of the derived abundace is 7.44 +/- 0.05 for FeI and 7.45 +/- 0.10 for FeII, compared to 7.46 +/- 0.01 for meteorites; these are relative to a hydrogen abundance with a log of 12.0).
The point of this, the book & papers (and there is a lot more where they came from) is that there is a strong body of physics involved in modeling stellar atmospheres, constrained by an equally strong body of detailed observations. Since I have studied stellar atmospheres in some detail myself, it is not enough for me to simply be told it's all wrong, or that we don't know something, without some more specific hints. What exactly is wrong with the standard way of deriving abundances?