NEW DATA ON INFRARED EMISSION OF THE NIGHT SKY AND AURORAS

The question of the infrared emission of the night sky, which in recent years has attracted much attention and has already been discussed more than once in the pages of Uspekhi Fizicheskikh Nauk \(^{1,2,3}\), is most closely connected with the problem of the structure of the upper layers of the atmosphere and, above all, with the problem of nitrogen dissociation at great altitudes. Let us recall briefly the essence of the matter. After the discovery in 1939 by L. A. Kubetskii \(^{1}\), and also by Slipher, of the infrared emission of the night sky, Stebbins, Whitford, and Swings \(^{5}\) reported in 1944–1945 that, with the aid of a photoelement and a set of light filters, they had detected very strong infrared radiation with a wavelength of about 10,440 Å. The wavelength indicated by them could be attributed either to atomic nitrogen \((\lambda = 1039.8\ \mathrm{m}\mu,\ \lambda_{2} = 1040.7\ \mathrm{m}\mu)\), or to molecular nitrogen \((\lambda = 1043\ \mathrm{m}\mu)\). Thus an exact determination of the wavelength of the infrared emission of the night sky promised to solve one of the central questions in the physics of the upper atmospheric layers—the question of nitrogen dissociation. However, an exact measurement of the wavelength of such weak \((\sim 10^{-2}\ \mathrm{erg}/\mathrm{cm}^{2}\,\mathrm{sec}\,\mathrm{sterad})\) radiation in the indicated region of the spectrum presents exceptional difficulties. In 1948 S. F. Rodionov and E. N. Pavlova \(^{6}\), as a result of measurements made with the aid of a photoelectron multiplier with a set of light filters, confirmed the presence in the spectrum of the night-sky glow of radiation with wavelengths close to \(\lambda = 8600\) Å and 10,440 Å, and also obtained indications of the possible existence of radiation with \(\lambda = 9100\) Å. A decisive step forward was made by V. I. Krasovskii \(^{7}\). The method he developed for coupling an electron-optical converter with a photographic plate (see, for example, \(^{2}\)) made it possible to carry out direct spectrographic investigations of the night-sky glow in a broad region of the spectrum, up to \(\lambda = 12\,000\) Å. Preliminary measurements carried out in 1948 \(^{7}\) showed that there is indeed intense radiation in the wavelength region of about 8600 Å and 10,400 Å (having, rather, the character of diffuse bands than of lines), as well as a fairly intense continuous spectrum. However, these circumstances did not permit sufficiently reliable measurements of the wavelengths to be made. At the present time V. I. Krasovskii has published \(^{8}\) the results of more careful spectrographic measurements of the infrared glow of the night sky, carried out by him in the summer of 1949, which radically change the situation.

The spectrograph he used in combination with the electron-optical converter and the photographic plate had a linear dispersion from 1200 to 2400 Å·mm and made it possible to carry out measurements in the wavelength interval from 8800 to 11,000 Å. The resolving power was comparatively low—it was possible to distinguish lines separated from one another by 0.05 mm, i.e., differing by 50–100 Å.

The study of the microphotograms showed that, contrary to the data of the American authors, no intense radiation near 10,400–10,440 Å exists at all and, consequently, the discussion raised by them concerning the nature of this imaginary line has no basis. In reality, the infrared spectrum of the night-sky glow proved to be much more complex. Along with a rather intense continuous spectrum there is observed a whole series of lines corresponding to wavelengths \(\lambda = 8870 \pm 2,\ 9391 \pm 2,\ 9976 \pm 2,\ 10217 \pm 3,\ 10374 \pm 4,\ 10827 \pm 3\) Å. Apparently there is also radiation with \(\lambda = 9700\) Å. The weak line \(\lambda = 10\,374\) Å almost merges with the line \(\lambda = 10\,276\) Å, practically ob-

rolling down its gentle slope. Each of the indicated wavelength values was obtained as the mean from 10–12 spectrograms. The author points out that all the named lines are greatly broadened and, apparently, are not individual lines, but the result of the superposition of a number of lines not resolved by the spectrograph. Thus, further investigations are necessary for deciphering the infrared spectrum of the night-sky glow.

The author processed the microphotograms by the usual methods of photographic photometry. The greatest intensity, as it turned out, is possessed by radiation falling in the region around 8900 Å.

In the same issue of the journal there is a communication by S. F. Rodionov and L. M. Fishkova\(^{9}\) on their spectrophotometry of the radiation of polar auroras in the infrared region of the spectrum. The method applied by the authors remained the same as in the photometry of the night sky in 1948.\(^{6}\) The measurements were carried out with the aid of a photoelectric multiplier and a set of light filters, which isolated (taking into account the selective sensitivity of the multiplier) the spectral regions: 9000–10 800, 8000–10 800, and 7000–10 800 Å. Thus it was possible to isolate three spectral intervals: 7000–8000, 8000–9000, and 9000–10 800 Å. (The earlier observations of Vegard—1939—and Meinel—1948—extended into the infrared region to 8300 and 8900 Å, respectively.)

The intensity of the infrared radiation was compared with the intensity of the green line (5577 Å), isolated with the aid of an interference light filter. The results obtained are characterized by the following figures (for different types of auroras):

intensity in the region 9000–10 000 Å

\[ (3.8 \div 9)\cdot 10^{-2}\ \frac{\mathrm{erg}}{\mathrm{cm}^{2}\sec\ \mathrm{sterad}} \]

intensity of the green line

\[ (9 \div 37)\cdot 10^{-4}\ \frac{\mathrm{erg}}{\mathrm{cm}^{2}\sec\ \mathrm{sterad}} . \]

Measurements of the night-sky glow carried out at the same latitude gave:

intensity in the region 9000–10 800 Å

\[ (1.43 \div 1.94)\cdot 10^{-2}\ \frac{\mathrm{erg}}{\mathrm{cm}^{2}\sec\ \mathrm{sterad}}, \]

intensity of the green line

\[ (1.55 \div 3.52)\cdot 10^{-4}\ \frac{\mathrm{erg}}{\mathrm{cm}^{2}\sec\ \mathrm{sterad}} . \]

The somewhat greater intensity of the night-sky glow, in comparison with that measured in 1948 on Elbrus \(\left(1.25\cdot 10^{-2}\right.\) and \(1.17 \times 10^{-4}\ \frac{\mathrm{erg}}{\mathrm{cm}^{2}\sec\ \mathrm{sterad}}\), respectively\(^{6}\)), the authors explain by the fact that during the period of the measurements (March) twilight conditions occur in the north.

For the relative intensities of different portions of the spectrum (\(I_{9000-10800\,\text{Å}}\) taken as 10), the following values were obtained (for different types of auroras): \(I_{8000-9000}=1 \div 3.8;\quad I_{7000-8000}=0.2 \div 0.85;\quad I_{5577}=0.23 \div 0.61.\)

An exception is provided by arcs that visually have a red tint. For them \(I_{7000-8000}=5.5\text{–}6.3\).

For the night sky, \(I_{8000-9000}=1.3 \div 1.95\); \(I_{7000-8000}=0.12 \div 0.66\), \(I_{5577}=0.12 \div 0.18\).

Thus, the green emission of auroras is relatively more intense than in the glow of the night sky, and for different types of auroras it varies more strongly than their infrared emission. On average, the intensity of the infrared emission of auroras is approximately 20 times greater than the intensity of the green line.

There is no doubt that further detailed study of the infrared emission of the night sky and auroras, made possible by the method developed by V. I. Krasovskii, will shed new light on questions concerning the structure of the upper layers of the atmosphere and the nature of the night-sky glow.

G. Rozenberg

References Cited

1. I. A. Khvostikov, UFN 33, 570 (1947).
2. G. V. Rozenberg, UFN 38, 446 (1949).
3. M. V. Shishkina, UFN 38, 450 (1949).
4. V. M. Slipher, Monthly Nat. Roy. Astr. Soc. 93, 666 (1938).
5. I. Stebbins, A. E. Whitford and P. Swings, Phys. Rev. 66, 225 (1944), Astrophys. Journ. 101, 39 (1945).
6. S. F. Rodionov and E. N. Pavlova, DAN 65, 831 (1949).
7. V. I. Krasovskii, DAN 66, 53 (1949).
8. V. I. Krasovskii, DAN 70, 999 (1950).
9. S. F. Rodionov and L. M. Fishkova, DAN 70, 1001 (1950).