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120,000 miles high...the flames consist of calcium, hydrogen, and several other elements.
We are concerned not so much with the prominences as with the layer from which they spring. The ordinary atmosphere of the sun terminates rather abruptly, but above it there is a deep though very rarefied layer called the chromosphere consisting of a few selected elements which are able to float--float, not on the top of the sun's atmosphere, but on the sunbeams. The art of riding a sunbeam is evidently rather difficult, because only a few of the elements have the necessary skill. The most expert is calcium. The light and nimble hydrogen atom is fairly good at it, but the ponderous calcium atom does it best.
The layer of calcium suspended on the sunlight is at least 5,000 miles thick. We can observe it best when the main part of the sun is hidden by the moon in an eclipse; but the spectroheliograph enables us to study it to some extent without an eclipse... the conclusions about the calcium chromosphere that I am going to describe rest on a series of remarkable researches by Professor Milne.
P. 71. How does an atom float on a sunbeam? The possibility depends on the pressure of light to which we have already referred (p.26). The sunlight travelling outwards carries a certain outward momentum; if the atom absorbs the light it absorbs the momentum and so receives a tiny impulse outwards. This impulse enables it to recover the round it is losing in falling back towards the sun. The atoms in the chromosphere are kept floating above the sun like tiny shuttlecocks, dropping a little and then ascending again from the impulse of the light. Only those atoms which can absorb large quantities of sunlight in proportion to their weight will be able to float successfully. We must look rather closely into the mechanism of absorption of the calcium atom if we are to see why it excels the other elements.
The ordinary calcium atom has two rather loose electrons in its attendant system... each of these electrons possesses a mechanism for absorbing light. But under the conditions prevailing in the chromosphere one of the electrons is broken away, and the calcium atoms are in the same smashed state that gives rise to the mixed lines" in the interstellar cloud. The chromospheric calcium thus supports itself on what sunlight it can gather in with the one loose electron remaining. To part with this would be fatal; the atom would no longer be able to absorb sunlight, and would drop like a stone. It is true that after two electrons are lost there are eighteen remaining; but these are held so tightly that the sunlight has no effect on them....
P. 72. There are two ways in which light can be absorbed. In one the atom absorbs so greedily that it bursts, and the electron scurries off with the surplus energy (process of ionization)... Clearly this cannot be the process of absorption in the chromosphere because, as we have seen, the atom cannot afford to lose the electron. In the other method of absorption the atom is not quite so greedy. It does not burst, but it swells visibly. To accommodate the extra energy the electron is tossed up into higher orbit. This method is called excitation (cf. p. 59). After remaining in the excited state for a little while the electron comes down again spontaneously. The process has to be repeated 20,000 times a second in order to keep the atom balanced in the chromosphere.
The point we are leading up to is this. why should calcium be able to float better than other elements? It has always seemed odd that a rather heavy element... should be found in these uppermost regions where one would expect only the lightest atoms. We see now that the special skill demanded is to be able to toss up an electron 20,000 times a second without ever making the fatal blunder of dropping it. That is not easy even for an atom. Calcium scores because it possesses a possible orbit of excitation only a little way above the normal orbit so that it can juggle the electron between the two orbits without serious risk....
P. 73. The average time occupied by each performance is 1/20,000 of a second. This is divided into two periods. There is a period during which the atom is patiently waiting or a light wave to run into it and throw up the electron. There is another period during which the electron revolves readily in the higher orbit before deciding to come down again. Professor Milne has shown how to calculate from observations of the chromosphere, the durations of these periods. The first period depends on the strength of the sun's radiation. But we focus attention on the second because it is a definite property of the calcium atom, having nothing to do with local circumstances . . . Milne' s result is that a electron tossed into the higher orbit remains for an average time of a hundred-millionth of a second before it spontaneously drops back again. (note: The Urantia Book, p. 462, says one one-millionth of a second) I may add that during this brief time it makes something like a million revolutions in the upper orbit....
P. 74. There is no prospect of measuring the time of relaxation of the excited calcium atom in a different way. (Is this still true in 1996???)
The excitation of the calcium atom is performed by light of two particular wave-lengths, and the atoms in the chromosphere support themselves by robbing sunlight of these two constituents. It is true that after a hundred millionth of a second a relapse comes and the atom has to disgorge what it has appropriated; but in re-emitting the light it is as likely to send it inwards as outwards, so that the out/flowing sunlight suffers more loss than it recovers. Consequently, when we view the sun through this mantle of calcium the spectrum shows a gap or dark lines at the two wave lengths concerned. These are denoted by the
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