FROM CURRENT LITERATURE

OBSERVATIONS OF THE SOLAR CORONA OUTSIDE ECLIPSE

The solar corona is the name given to the uppermost layers of the Sun’s atmosphere, which during a total solar eclipse form a glow around the solar disk covered by the Moon. Observations of the solar corona are of substantial importance for clarifying the nature of the physical processes taking place on the Sun, and for investigating the various radiations emitted by the Sun and exerting a great influence on the upper layers of the Earth’s atmosphere[^1]. Under ordinary conditions, however, it is extremely difficult to observe the solar corona, because its brightness, in comparison with the brightness of the Sun, is very small. Even in the most intense regions, bordering on the edge of the solar disk, it does not exceed the brightness of the Moon. Therefore for a long time observations of the solar corona were carried out only at the moment of a total solar eclipse and, because of the rarity and short duration of the latter, could not provide a sufficient quantity of systematic material.

Attempts to observe the solar corona outside eclipses were made repeatedly, but only in 1930 did Lyot[^2] solve this problem. The main difficulty, connected with the low relative intensity of the corona, consists in eliminating the influence of the light scattered by the Earth’s atmosphere and by separate parts of the optical instrument. To this end it is necessary, on the one hand, to construct special optical devices (coronagraphs), in which the image of the Sun is removed from the instrument and the image of the corona is transferred to a plane where a photographic plate or the slit of a spectrograph can be placed, and, on the other hand, to raise these instruments to an altitude of not less than 2000 m and to choose sites with the most favorable atmospheric conditions. Even so, for systematic observations of the solar corona in the visible region of the spectrum it is necessary to make observations not in the entire spectrum simultaneously, but to use monochromatic apparatus isolating some one of the most intense lines of the corona. This makes it possible to weaken considerably the influence of scattered light. Such monochromatization was carried out by Lyot in 1939 with the aid of an interference-polarization light filter isolating a narrow region of the spectrum near the line used.[^3] In this work two intense emission lines of the solar corona, with \(\lambda = 5303\) and \(6374\) Å, were used.

In view of the importance of investigations of the solar corona, by decision of the Presidium of the Academy of Sciences of the USSR, a Mountain Astronomical Station of the Main Astronomical Observatory (Pulkovo) was organized in the North Caucasus, 30 km from Kislovodsk, at an altitude of 2130 m above sea level. In addition to non-eclipse observation of the solar ...

systematically conducted there as well, so as to have a complete set of information on solar activity obtained at one point under identical conditions. The coronagraph installed at this station has an objective aperture of 200 mm and a focal length of 2985 mm. A method has been developed for observing the corona outside eclipse and for photographing the green line of coronal radiation with $\lambda = 5303$ Å[^4].

Alongside this, at the Mountain Astronomical Station work has been carried out on observing the corona outside eclipse by photographing in infrared rays[^5]. Non-eclipse observation of the corona in the light of some monochromatic line in the visible part of the spectrum makes it possible to observe only the region of the corona nearest to the edge of the solar disk and the brightest region, since only in this region is the linear spectrum found. Observations in infrared rays are of great interest because the harmful effect of scattered light decreases in proportion to $\lambda^4$, and in the infrared region it becomes possible, instead of spectral lines, to carry out observations in rather broad spectral intervals. This, in turn, makes it possible to observe broader regions of the corona. In addition, in the infrared region of the spectrum of the solar corona, lines with a higher degree of ionization than the lines located in the visible region of the spectrum have been found; as a result, there are additional possibilities there for investigating the excitation of various radiation lines.

Photographing the corona in infrared rays was carried out with the aid of an electron-optical converter; for a normal reproduction of the inner corona on the photographic plate, 5 sec was sufficient. The observations were made with a filter composed of a thin-layer interference light filter, whose transmission maximum is located near 10 750 Å, and marblite glass 2 mm thick. This filter isolated an infrared portion of the spectrum about 0.05 μ wide, including the intense coronal lines 10 746.8 Å Fe XIII and 10 797.95 Å Fe XIII.

A glow of unequal brightness was found, extending to 6–7′ from the edge of the Sun, which cannot be explained by the presence of a halo around the Sun. In addition, in some photographs of a portion of the corona, ray-like structural formations were detected. Thus, photographing the corona in infrared rays opens new possibilities in the study of the solar corona.

V. L.

CITED LITERATURE

1. I. S. Shklovskii, UFN, XXX, 63 (1946).
2. B. Lyot, Zeits. f. Astrophys. 5, 73 (1932); C. R., 200, 219 (1935); see also G. Dimitrov and D. Becker, Telescopes, p. 230, Gostekhizdat (1947).
3. B. Lyot, Astrophys. Journal 101, 255 (1945); see also G. Dimitrov and D. Becker, Telescopes, p. 236, Gostekhizdat (1947).
4. M. N. Gnevyshev and R. S. Gnevysheva, DAN SSSR, LXXII, No. 4 (1950).
5. A. A. Kalinyak, DAN SSSR, LXXII, No. 4 (1950).