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On the Light of the Night Sky
In this note are synthesized many papers about the night sky which came to my knowledge. The principal papers were noted on the marginal space.
1. The most exact measurements of the brightness of the night sky, measured in comparison with a star of known magnitude, were made by Dufay. These are as follows:
visual magnitude: 4.60m (35 clear nights)
photographic magnitude: 4.36m (55 clear nights).
With respect to the polarization of the light of the night sky also, the most precise determinations have been made by Dufay. They have shown that the light of the night sky is partially polarized and is always very small (between two and four percent).
From these and other reasons, we are led to think that there is an emission light in the high atmosphere.
2. The earliest observation of the auroral green line in the night sky was made by Slipher in 1915 and then the wave length of the line was precisely determined by Babcock through a Fably-Perot interferometer in 1923.
After that, Sliphere discovered (1929) a group of emission lines in the red and orange regions of the spectrum, up until that time unexplored. These wave-lengths were as follow: 5892, 6315, 6530, 7270 A .
The Presence of certain nitrogen bands in the night sky (in the case of the aurora, the bands 4278 and 3914 A of the negative system of nitrogen have generally an intensity comparable to that of the green line) was not observed up until 1933. In short, up until that time all the observed bands between 3900 and 5100A are much weaker than the line 5577 A.
3. Recent research on the spectrum of the night sky (1933-1935) has been made by Cabannes and Dufay. To simplify the exposition of the results, we divide the spectral intervals studied (800-2950 A) into three regions.
(a) Region 5000-8000A
The principal emission spectrum of the night sky between 5000 and 8000 A belong to the three forbidden lines of neutral oxygen atom: 5577,6300 & 6360 A respectively, and next to the first positive system of the nitrogen bands. The vibrational bands of water vapour a, C, α and D are also observed in the night sky.
(b) Region 3800-5000 A
All the emission lines or bands in this region are quite weak. We find in the first place several bands of the first negative bands and of the second positive bands of nitrogen, and 32 Vegard-Kaplan bands are included between 3400 and 5400 A. Other than these, certain intense lines of the red spectrum of argon and the most typical lines of the spectrum of cometary nuclei are all very near certain radiations of the sky.
(c) Region 3000-3900 A
In this region 40 lines or bands of the emission spectrum was observed, most of them belong very probably to the negative and second positive systems of the nitrogen bands.
Several other radiations are very near the cometary nucleus lines. But at the present time, it seems difficult to decide definitely about these attributions.
4. In this section the variations of the light of the night sky is described. Since 1923, Lord Rayleigh made a long series of measurements on the brightness of the sky through coloured filters. According to Rayleigh and his collaborators, important variations occur from one night to another in certain spectral regions.
From 1923 to 1927, Rayleigh's measurements with a green filter indicated a seasonal variation with a minimum of brightness in December, and two maximum less pronounced in March and in October. In addition, Lord Rayleigh found an increase of the average light from 1923 to 1927. During the winter 1931-32, Dufay found a simultaneous weakening of the green line and also of the blue and violet regions. This slow variation shows a correlation between the intensity of the night sky and the activity of the sun.
The variations which occur during a single night have been studied by Lord Leyleigh and McLennan. These variations seem systematic the brightness passing in general through a maximum around midnight. In contrast, the results of Karandikar and Ramanathan obtained in India are absolutely different. They indicate, for all spectral regions, the following tendency:
a progressive weakening during the first part of the night, a minimum between midnight and two A.M. followed by an increase of intensity during the end of the night.
5. Theories of the origin of the night sky light.
The night sky light is a very complex phenomenon whose origin still remains to a great extent mysterious.
The author indicates briefly the hypothesis which seems acceptable for explaining. In 1932, Dauvillier suggested an extremely attractive theory which tries to interpret simultaneously all the phenomena of the high atmosphere, by attributing them to the electronic emission of the sun.
The solar electrons come into the neighbourhood of the earth, their paths curve into the earth's magnetic field. The earth is then surrounded at a distance of the order of its radius by a quasi-spherical enclosure of paths. Gas molecules (oxygen, nitrogen) which Dauvillier assumes to exist even at this distance from the earth, will then be ionized by primary electrons which thus produce much less rapid secondary electrons.
The primary polar paths (such as P1 in Fig.6) produce the polar aurora, and the primary equatorial path (such as E1) which liberate secondary electrons produce the permanent aurora.
S.Chapmann (1937) also attempted to illustrate the permanent aurora which is originally derived from sun light. The energy represented by the light of the night sky is stored up during the day mainly in the form of dissociation (including ionization as a special case).
The two atoms into which oxygen molecules are dissociated by day may both be in the lowest state (3P) or one may be in this state and the other in the 1D excited state. But very soon after sunset all the 1D atmos then existing will have reverted lowest state. This is the energy of the main source of excitation of the night sky light, though processes yielding higher amounts of energy will also go on to a minor degree.
The nitrogen molecules will mostly be in their lowest state and the dissociation energy of the 3P oxygen atom can raise the nitrogen molecules to the 1S state.
Thus most of the spectrum of the permanent aurora can be illustrated by this theory.
6. Attempts at reproducing in the laboratory the spectra of the aurora and of the permanent aurora have been made by Vegard, Kaplan and McLennan etc., but the author does not touch them here. At the end he alludes to the presence of the three forbidden lines of neutral oxygen atom in novae and in the planetary nebulae.
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