ON THE INFRARED RADIATION OF THE NIGHT SKY

One of the central problems of the modern physics of the upper layers of the atmosphere is the problem of atomic nitrogen (see ²). Directly connected with it is the question of the infrared radiation of the night sky.

The point is that the very intense infrared line near 10,440 Å, discovered in 1944, may be assigned either to atomic nitrogen (the forbidden transition \({}^2P_{1/2} \to {}^2D_{5/2}\), \(\lambda_1 = 1039.8\,m\mu\) and \(\lambda_2 = 1040.7\,m\mu\)), or to an electronic transition in the nitrogen molecule \((B^3\pi_{v'=0} \to A^3\Sigma_{v''=0},\ \lambda = 1043\,m\mu)\).

The reviewed work¹ is devoted to clarifying the nature of the infrared glow. The authors used a photometer with a photoelectronic multiplier, selecting the spectral regions they needed in the range 8000–11,000 Å by means of interchangeable light filters. Four light filters were used, with effective wavelengths (taking into account the spectral sensitivity of the photomultiplier) of about 1000, 970, 950, and 830 \(m\mu\).

The measurements were carried out in the summer of 1948 on moonless nights in the mountains, at an altitude of 2200 meters, which excluded the influence of the polluted lower layers of the atmosphere. Control measurements of the intensity of the Moon’s light confirmed a sufficient constancy of atmospheric transparency during the night. The aperture angle of the instrument was 0.9 steradian. The instrument was directed at a region of the sky located at an angle of 40° to the horizon.

The authors consider that in the spectral region of interest to them there can be only 3 components of the spectrum: the problematic line 10 440 Å, the line 8600 Å discovered by Slipher ³ and (possibly) the (1—0) line of molecular nitrogen (8910 Å). The presence of this last one is naturally suspected if the 10 440 Å line in fact belongs to molecular nitrogen (it is (0—0) of the first positive system). Knowing the spectral characteristic of the photometer and the transmission curves of the light filters used, it was possible, from the experimentally measured total intensities corresponding to different light filters, to calculate the relative intensities of these three components. The results of measurements for several typical nights are given in the table (Table 2 of the reviewed work).

Table

Relative intensity of the infrared lines of the night-sky glow

We see that the 10 440 Å line is the brightest. The authors consider the question of the presence of the 8910 Å line still open, since its intensity only slightly exceeds the measurement error. The determination of the intensity in absolute units, carried out by the authors for the 1044 mμ band, led to values ranging from \(1.25\cdot 10^{-2}\) to \(1.8\cdot 10^{-2}\ \mathrm{erg}/\mathrm{cm}^{2}\cdot\mathrm{sec}\cdot\mathrm{steradian}\). For two nights increased values were observed, reaching \(3\cdot 10^{-2}\ \mathrm{erg}/\mathrm{cm}^{2}\cdot\mathrm{sec}\cdot\mathrm{steradian}\). The authors also obtained curves of the change in the total intensity of infrared radiation during the night. The curves obtained on different nights are close to one another: they all have a noticeable maximum at midnight and a second, less clearly expressed, maximum between three and four o’clock in the morning. Since, as was shown, atmospheric transparency did not change during the night, such a course of the curves is connected with the character of the radiation itself. Moreover, according to the authors, the experimental data show that the indicated changes in the intensity of infrared radiation are due precisely to the 10 440 Å band. This can be explained from the point of view of the molecular nitrogen hypothesis. The radiation of the first positive system of atmospheric nitrogen may arise, for example, in the recombination reaction: \(\mathrm{N}+\mathrm{N}+\mathrm{N}_{2}\to\mathrm{N}_{2}+\mathrm{N}_{2}^{*}\) (\(\mathrm{N}_{2}^{*}\) is an excited molecule in the state \(B^{3}\pi\)).

Assuming (see ²) that at night in the upper layers of the atmosphere there is ultraviolet solar radiation causing dissociation

excited nitrogen molecules and thereby hindering their luminescence, we naturally arrive at the intensity curve described above, with a maximum at midnight (whereas the intensity of the ultraviolet radiation passes through a minimum at midnight). Therefore the authors, while not expressing themselves definitely in favor of the atomic or molecular nature of the nitrogen emitting the 10440 Å line, nevertheless believe that their data “testify to the presence of a radiation component directly connected with molecular nitrogen.”

“Possibly,” the authors add, “the radiation of atomic nitrogen (the luminescence of atoms formed during the day) is superposed on the radiation of molecular nitrogen in the form of a component that decreases monotonically during the night; the presence of such a background, diminishing during the night, can be noticed in the figure,” which shows the mean course of the total intensity of the infrared radiation during the night.

M. V. Shishkina.

CITED LITERATURE

1. S. F. Rodionov and E. N. Pavlova, DAN 65, 831 (1949).
2. I. A. Khvostikov, The Luminescence of the Night Sky, Publ. Acad. Sci. USSR (1948).
3. V. M. Slipher, Month. Not. Roy. Astronom. Soc. 93, 664 (1933).