On the Nature of the Variable Component of Radio Emission from Discrete Sources

Almost all works devoted to the radio emission of discrete sources indicate the presence in it of a variable component. Summarizing the data bearing on the nature of the sources, Ryle[^1], for example, considers that if the intensity fluctuations are inherent in the radio emission incident on the Earth’s ionosphere and troposphere, and are not introduced by them, then the minimum observed period of the fluctuations makes it possible to estimate the upper limit of the size of the source. This circumstance alone shows how important is the question of the origin of the variable component of the radio emission, a question covered quite fully in the paper under review[^2].

The conclusions drawn by the authors of the paper are based on the results of observations of the emission of the sources Cygnus and Cassiopeia at wavelengths of 3.7–6.7 m at two points separated by a distance of 210–170 km. Interpreting these results, Smith—the author of the first part of the paper—distinguishes two types of fluctuations of radio emission, the first of which are the rapid random intensity fluctuations at a wavelength of 3.7 m (Fig. 1), which show no noticeable correlation if the distance between the receiving points exceeded 20 km, in connection with which they are attributed an ionospheric origin1.

Fig. 1. Radio emission of the source Cygnus at a wavelength of 3.7 m on August 3, 1949. A—in Cambridge. B—in Jodrell Bank (England). Distance between receiving points—210 km.

However, such an explanation of the nature of the fluctuations does not cover the whole range of experimentally established facts. It has been found,

for example, that simultaneous records of radio emission at different wavelengths (3.7 and 6.7 m) at one receiving station sometimes show a significant correlation (Fig. 2), whereas even the simplest

Fig. 2. Fluctuations of the radio emission of the source Cassiopeia at wavelengths 3.7 and 6.7 m (July 4, 1949).

ionospheric mechanisms should have given, at such different wavelengths, entirely different patterns of intensity variation. Moreover,

Fig. 3. “Bursts” of emission of a discrete source at a wavelength of 6.7 m. The distance between the receivers is 160 km. The upper record is for Cambridge, the lower for Defford (England).

the periods of various kinds of ionospheric disturbances correlate only very weakly with the magnitude of the fluctuations.

Fig. 4. Radio emission of the source Cassiopeia. Records A and B belong to different receiving stations separated by a distance of 3.9 km (October 9, 1949).

Another type of fluctuation, which was sometimes observed at a wavelength of 6.7 m, consists of separate rare “bursts” of intensity of considerable amplitude, lasting 10–20 sec. The excellent coincidence

moments at which such “bursts” appeared when received at points separated by a distance of 210 km (Fig. 3) compel one to regard them as genuine variations in the radio emission of discrete sources.

Thus, according to Smith, the intensity fluctuations are produced by two different mechanisms, one of which is associated with the source of the radio emission, and the other with the terrestrial ionosphere.

The authors of the second part of the work speak decisively in favor of a local source for the origin of the fluctuations. Additional grounds for such a conclusion are seen by them in the fact that the periods of the presence and absence of fluctuations coincide to a considerable extent for the sources Cygnus and Cassiopeia. Moreover, if reception on two identical receivers separated by a distance of 100 m gave completely identical patterns of emission fluctuations, then when the distance between the receivers was increased to 3.9 km the correlation coefficient fell to 0.95 (Fig. 4).

The existence in the ionospheric layers of clouds of ionized gas with densities different from the mean densities of the layer indicates a probable mechanism for the occurrence of radio-emission fluctuations. A cloud of ionized gas of thickness ~1 km, located in the F-layer, with an electron density twice the normal density, is capable of changing the phase of meter waves by 180°. Such phase changes over considerable portions of the wave front may lead to intensity fluctuations similar to those observed experimentally. In this interpretation, the fluctuations of the radio emission of discrete sources are analogous to the twinkling of stars at optical frequencies.

G. Getmantsev

CITED LITERATURE

1. M. Ryle, Proc. Phys. Soc., A 62, 491 (1949).
2. F. G. Smith, C. G. Little, A. C. B. Lovell, Nature 165, 422 (1950).

1. The experiments of Bolton, apparently, also lead to the same conclusion; in them a discrete source in Cygnus was observed simultaneously in Australia and New Zealand. Mentioning these observations, Smith notes that they were made at insignificant elevations of the source, so that different refractive effects could mask the true changes in the intensity of the radiation incident on the Earth’s atmosphere. The observations mentioned above, however, are to a considerable extent free from interference from atmospheric refraction, since they were made at sufficiently large angles of elevation of the source above the horizon. ↩