From Current Literature

The Spectrum of the Aurora Borealis and the Structure of the Upper Layers of the Atmosphere

L. Vegard. Das Nordlichtspektrum und die Konstitution der oberen Atmosphärenschicht.
ZS. für Phys. 16, p. 367, 1923

Many hypotheses and suppositions of various kinds have at one time been advanced concerning the causes of the aurora borealis. Stark’s experiment1 supports the hypothesis put forward by Vegard2 in 1917, according to which positively charged particles are taken to be the agents exciting the aurora borealis. But in view of the fact that positively charged particles alone cannot explain all the features of aurorae, nor many other cosmic phenomena, Vegard in the present work assumes that in the formation of aurorae, along with electric rays (electrons and positive particles), no lesser role is played also by ultraviolet rays, X-rays, and $\gamma$-rays, whose presence in solar radiation is not excluded. The behavior of these latter rays becomes comprehensible if one takes into account their great ionizing power. This expanded hypothesis of Vegard in no way contradicts the mechanical picture of aurorae developed mathematically by Störmer3 in his numerous memoirs.

The only means that gives us the possibility of drawing any conclusions about the physical causes and modes of origin of the aurora borealis is its spectrum. Unlike all other spectra, in the spectrum of the aurora borealis we are struck by the comparative poverty of lines having any more or less considerable intensity. Up to now, in the spectrum of the aurora borealis a total of about 35 lines and bands have been discovered, and of these only four lines—$5577.1$–$5578.4\ \text{Å}$, $4182.5\ \text{Å}$, $3432.7\ \text{Å}$, and $3208.3\ \text{Å}$—cannot be identified quite precisely with the lines of elements known to us. Since all the remaining lines undoubtedly belong to nitrogen, Vegard considers that the four indicated lines also belong to nitrogen, and not to the hypothetical geocoronium.

Vegard relates the cause of their appearance both to the physical state of the nitrogen excited to luminescence and to the methods of excitation themselves. A laboratory study of the luminescence of $N$ and of mixtures $H$–$N$ and $He$–$N$ under the action of cathode rays led Vegard to the conclusion that in the upper layers of the atmosphere (above 100 km) there is apparently neither hydrogen nor helium, since even a small admixture of these gases to nitrogen gives the spectral lines characteristic of them. The existence, however, of nitrogen alone

in the upper layers of the atmosphere is confirmed by the fact that, if, when nitrogen is excited by cathode rays, its temperature is simultaneously lowered, then a gradual disappearance of certain lines in the spectrum is observed, and at the temperature of liquid air, finally, a spectrum is obtained which, in the number of its lines, differs considerably from the spectrum of nitrogen at room temperature, but which is very close—if one excludes from consideration the yellow-green line \(5577.2 \mathring{\mathrm A}\), most characteristic of the aurora—to the spectrum of the aurora. True, Vegard did not succeed in obtaining the yellow-green line in this experiment, but he explains this by the fact that in the laboratory it is very difficult to realize those conditions of excitation which are present in the earth’s atmosphere.

The chief reason lies, in his opinion, in the fact that under laboratory conditions we usually excite gaseous nitrogen to luminescence, whereas in reality in the upper layers of the atmosphere nitrogen is present in the form of snow dust. Vegard expresses the hope that at the temperature of liquid hydrogen he will be able to realize suitable conditions of excitation, and then, in his opinion, the lines characteristic of the aurora must necessarily arise. As is known, in the very recent past this hope was justified.

From Vegard’s point of view the structure of the earth’s atmosphere is as follows: up to a height of about \(100\) km it consists of gaseous nitrogen, oxygen, argon, etc.; above \(100\) km it consists exclusively of snow dust, solid nitrogen and, perhaps, oxygen, the centers of condensation of the latter being able to be positively charged particles or molecules \(NH_3\).

As regards the law of distribution of all these gases with height, for small heights the ordinary barometric formula is applicable,

\[ dp=-\rho g\,dh=-\frac{gM}{RT}p\,dh \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots (1) \]

where \(p\) is pressure, \(\rho\) is density, \(g\) is the acceleration of gravity, \(M\) is the molecular weight, \(R\) is the gas constant, and \(T\) is the absolute temperature; for greater heights, however, it has to be supplemented somewhat in order to explain the independence of the intensity of the auroral lines from height. Rays of this kind have repeatedly been observed, when the intensity of the light over a stretch of several hundreds of km in height—from \(100\)–\(110\) km to \(400\)–\(750\) km—remained almost constant. It is therefore necessary to suppose that, beginning approximately at a height of \(100\) km, the density of the atmosphere, if it decreases at all, does so only very slightly. Vegard assumes that the upper atmospheric layers are electrically charged; the electric field arising in this case causes the charged particles of nitrogen to move upward and thereby to compensate to some extent the action of gravity.

Formula (2) passes into

\[ dp=-(\varepsilon g-\varepsilon F)\,dh, \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots (2) \]

where \(F\) is the electric force and \(\varepsilon\) is the electric density. Since, under the influence of the photoelectric action of solar radiation and of the field \(F\), the atmosphere would have to lose nitrogen continuously, it must be admitted that this loss is compensated by the same sun.

A simple consideration leads Vegard to the conclusion that the differential equation derived from formula (2),

\[ \frac{d^{2}\varepsilon}{dr^{2}} -\frac{1}{\varepsilon}\left(\frac{d\varepsilon}{dr}\right)^{2} +\frac{2}{r}\frac{d\varepsilon}{dr} +\frac{2q\varepsilon}{u\xi} -\frac{4\pi\gamma^{2}}{a}\varepsilon^{2} =0 \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots (3) \]

is not confirmed if one supposes that all nitrogen molecules are in the ionized state. It remains, therefore, to suppose that the charged particles of nitrogen in the upper layers are formed from a large number of separate molecules, condensed around electric charges or molecules \(NH_3\), as nuclei.

Depending on their size and charge, these particles of nitrogen (in the form of small crystals or frozen vapor) will either move upward or fall. In the latter case, at a certain altitude they begin to melt and, having turned into gas, again rise upward in order once more to condense, etc. The melting of the particles may also be caused by the bombardment of frozen nitrogen vapor by electric rays (electrons or positive particles). These processes of successive melting and freezing open up new possibilities for a very simple explanation of the change in the coloration of the aurora—from green at the top to red at the lower boundary of the aurora. Apparently, the green coloration is associated with the crystalline form of nitrogen, while the red is associated with the gaseous form.

The thickness of the layer of condensed nitrogen increases with distance from the poles toward the equator. An approximate judgment of it may be made on the basis of observations of the height of auroras at various latitudes. In regions close to the pole this thickness apparently does not exceed 300–350 km, whereas in Christiania, for example, Størmer observed an aurora extending to an altitude of 700–800 km.

The assumption of the existence in the upper layers of the atmosphere of charged nitrogen in the form of small crystals also makes it possible simply to explain: a) the blue color of the sky, b) the zodiacal light, c) the change in the coloration of meteorites as they pass through the earth’s atmosphere (above—white or greenish, but beginning at a certain altitude—red), d) the twinkling of fixed stars and the non-twinkling of planets, e) the reflection of acoustic and electromagnetic waves from the upper atmospheric layers, f) secondary phenomena of the aurora borealis and the presence of the green line in the glow of the night sky.

P. Vasil’ev.

1. J. Stark. Über die Natur der Nordlichtstrahlen. “Die Naturwissenschaften,” 6, p. 145, 1918. Cf. “Uspekhi fizich. nauk,” vol. I, p. 233, 1918. ↩

2. L. Vegard. Jahrbuch d. Rad. u. Elektronik. 14, 1917. ↩

3. C. Störmer. Arsch. des Sc. phys. et nat. de Genève, 4-periode XXIV, 1907; C. R. 1908–1910. A. N. Krylov. “Uspekhi fizich. nauk,” vol. I, p. 1, 1918. ↩