The statistics of the discrete sources observed in Cambridge and the interpretation given by Ryle and his colleagues constitute one of the most interesting items of recent astronomy. It is therefore of great importance to check the observational data and this can be done from the independent results being obtained in Sydney by Mills and his colleagues with the 85 Mc./s. Mills Cross. With this in mind Ryle sent me some two months ago a pre-publication account of the Cambridge work. (Now published, Ryle and Scheuer, 1955[1].) The currently available observations with the Mills Cross are not yet sufficient to give a decisive answer, but those available disagree with the Cambridge ones. Because of the importance of the subject it seems desirable to give here an interim account of these observations. The general position of the observations is discussed in a separate paper (paper 18, Pawsey). As stated there observations to date have been aimed at the study of known objects. The beam in each case was adjusted to the appropriate declination and an extended record, including the selected object in a small section, was taken. Most of these records have been examined for discrete sources and such sources listed with their intensities when sufficient records at adjacent declinations were available to delineate them. The list was restricted to sources which, within the 50′ limits of resolution of the equipment, appeared to be discrete point sources. Extended sources were neglected. This method gives an irregular coverage of the sky so that the sampling must be watched.

As part of the programme of observations with the large Cambridge radio telescope, a survey of the integrated radio emission has been made using one of the four elements of the interferometer. At a wave-length of 3·7 metres this aerial has beam-widths to half-power points of 2° in right ascension and 15° in declination. The use of a long wave-length makes it possible to obtain accurate measurements of the brightness temperature of the sky in regions away from the galactic plane. It is with the radiation from these regions that this paper is primarily concerned.

At present we can observe the origin of only the solar component of cosmic rays. The sun emits cosmic rays in connexion with flares and probably also continuously on a smaller scale. Thus the emission of cosmic rays appears to be connected much more closely with non-thermal radio emission than with thermal radiation of light and heat.

The observed cosmical radio-emission has to be divided into the galactic and metagalactic components. The separation of these components is a problem of first importance. It was shown by us (Shklovsky, 1952 [1]) that at least 75% of the observed intensity near the galactic poles is caused by sources located in our stellar system. These sources form an almost spherical sub-system. Another model of the distribution of the cosmical radio-emission sources (Westerhout and Oort, 1951 [2]) is popular in the western countries. The so-called ‘isotropic component’ of metagalactic origin used in that model was considered responsible for the greatest part of the intensity near the galactic poles.

The distribution of radio surface brightness in the Large Cloud has been measured at 3·5 metres by Mills [1], and in the 21-cm. line by Kerr, Hindman and Robinson [2], in Sydney. A comparison with the distribution of the optical surface brightness in red light (λλ 5600–6600)[7] and with the surface density of stars brighter than mpg = 14·0(Mpg = − 4·7)[3], determined by the author at Mount Stromlo, is made in this paper.

It is both convenient and realistic to regard the solar radio-emission as being composed of a quiet permanent part coming from the whole of the sun's surface, together with a superimposed variable part emitted from centres of activity. The variable part can be further subdivided into various components to which separate reference will be made. The components are almost independent and add to one another to give the solar emission recorded by radio telescopes. For our understanding of the phenomena it would be a great advantage if the components could be completely and reliably segregated but this is not always possible. The results from a wide network of observational stations are recorded in the Quarterly Bulletin of Solar Activity, from which the intensities and characteristics of most of the components may be extracted.

The atmosphere of the sun is transparent to visible radiation, is nearly transparent to millimetre and centimetre radio radiation, and becomes opaque to the metre and longer wave radiation. Information about the chromosphere can then be given by observing the radiation from the sun at short radio wave-lengths. In its outer part, the atmosphere of the sun is highly ionized. Absorption in any region is directly proportional to the square of the density and the wave-length squared and inversely to the temperature to the three-halves power This is the familiar equation for the absorption of radio waves in an ionized medium. By consequence of this, the longer wave radiation is absorbed in the outer layers of the sun's atmosphere and can escape only from these outer regions. The shorter wave-length radiation is absorbed very little in the outer part of the solar atmosphere where the density is quite low, and hence radiation from the chromosphere escapes as centimetre and millimetre radio waves. In fact, the principal radiation from the sun in the centimetre and millimetre region comes from the chromosphere.

The observation of the total solar eclipse of 30 June 1954 at 9·4 cm. wavelength was one of the experiments carried out by the Naval Research Laboratory eclipse expedition to Sweden. The apparatus was set up in the city of Oskarshamn at a point about 5 km. north of the centre-line of totality. At this location the eclipse was total, with magnitude 1·035, at 12h 48m u.t.

The Naval Research Laboratory has supported four eclipse expeditions (in the years 1947, 1950, 1952 and 1954) under the direction of Dr J. P. Hagen. The principal purpose was to find the variation of the solar microwave radiation during a total optical eclipse of the sun. During the last two eclipses the sun was sufficiently inactive to enable us to derive the centre-to-limb brightness distribution at a wave-length of 9·4 cm., on the assumption that the distribution was circularly symmetrical. Since these two eclipses were of the same optical magnitude (within 0·12 %) and were measured with the same equipment located at each eclipse near the centre-line of mid-totality, it is of interest to compare their results. The 1954 eclipse observation was described in the last paper by Mayer, Sloanaker and Hagen. The 1952 observation has already been described [1].

It is appropriate at a solar session during a radio astronomy conference to report on an analysis of optical observations to infer the properties of a non-spherically-symmetric chromosphere. For the first detailed model of such a non-symmetric chromosphere was that presented by Giovanelli (1949) [1] in an attempt to reconcile apparent contradictions between radio and optical data. Here we summarize some investigations based only on optical data, obtained by the High Altitude Observatory at the 1952 eclipse. Our observations of this eclipse were obtained as part of a joint programme with the Naval Research Laboratory, which conducted radio observations. Dr Hagen reports on the radio material (papers 46 and 47). The optical data in the present paper come from hydrogen and helium alone, the metallic data being still in reduction.

Observations of radio emission from the quiet sun, which is believed to be due to thermal emission from the ionized gases of the solar atmosphere, provide information about the electron density and temperature throughout the atmosphere. Comprehensive information requires observations, preferably of the brightness distribution over the solar disk, over a large range of wave-lengths. Because such observations are both difficult and tedious it is desirable that a final model atmosphere should be based on a pool of observations taken by independent methods and observers. The following are recent contributions to this pool from the Radiophysics Laboratory.

Several recent papers have dealt with observations of brightness distributions over the solar disk, which were derived either from two-aerial interferometer observations at various spacings and orientations (e.g. O'Brien, 1953) [1], or from multiple-element interferometer fan-beam observations at various orientations (e.g. Christiansen and Warburton, 1954) [2], In each a two-dimensional distribution is derived from a number of essentially one-dimensional observations by a Fourier synthesis method described by O'Brien. The detail given by these methods must be limited by the finite resolution of the individual observations (limited by the maximum aperture of the aerial system), but the form of the limitation is not obvious, though its knowledge is required when relating the observations to a solar model.

The brightness distribution of the quiet sun at radio frequencies is of importance in determining the electron density and temperature in the chromosphere and the corona. Calculations made by Smerd, based on parameters derived from optical observations of the sun, indicate that the brightness distributions for wave-lengths near 1·5 metres are sensitive to the assumed coronal conditions and hence are well suited to checking these parameters.

Previous experiments to determine the distribution of radio brightness across the solar disk at metre wave-lengths using the method of Fourier synthesis have been described by O'Brien (1953) [1]. These results could be explained in terms of a spherically symmetrical corona only if the electron density and temperature differed appreciably from estimates derived by visual methods (O'Brien and Bell, 1954) [2]. In order to check these measurements a similar series of observations was carried out in 1954 during an extended period of exceptionally low solar activity. Additional experiments, which gave information concerning the ellipticity of the brightness distribution, were made at Cambridge during the partial eclipse of June 1954.

The solar eclipse of 30 June 1954 was observed in the neighbourhood of Helsinki on the frequency 81·5 Mc./s. [1]. At the maximum of the optical eclipse 90 % of the solar diameter was occulted by the moon. We may accordingly assume that on the wave-length of observation the eclipse was nearly central. Further, the angle between the orbit of the moon and the solar equator was only 12°.

The laboratory of radio astronomy of the Institut d'Astrophysique at Paris has observed the eclipse of 30 June 1954 at öland (Sweden) and at Meudon (France) [1].

The partial solar eclipse of 20 June 1955 was observed at Tokyo and Toyokawa, where regular solar radio observations are conducted. A party was sent to Kagoshima by the Tokyo Astronomical Observatory. The locations, the frequencies observed and the types of aerial are listed in Table 1. The paths of the northern limb of the moon at three stations are shown by dotted lines in Fig. 3.

Soviet scientists working in the domain of radio astronomy carried out a number of observations of the radio emission from the sun during solar eclipses. The first observation was obtained by Prof. S. E. Khaikin and the author during the eclipse of 20 May 1947 at a wave-length of 1·5 metres. The solar eclipse of 25 February 1952 was observed in Archman, Turkmenian S.S.R., on wave-lengths of 3·2 and 10 cm. by V. S. Troitzky, and on 1, 1·5, 2 and 2·6 metres by the author and by V. V. Vitkevitch. The eclipse of 30 June 1954 was observed in Novomoskovsk, Ukrainian S.S.R., on 3·2 and 10 cm. by V. S. Troitzky, and on 10 and 23 cm. by V. V. Vitkevitch. It was also observed in the Caucasus on a wave-length of 3·2 cm. by A. P. Moltchanov. All the above observations were carried out in places located in the vicinity of the central path of the totality. The eclipse of 30 June 1954 was observed also outside the path of totality on the southern shore of the Crimea by a group of Dr Vitkevitch's colleagues on wave-lengths of 1, 1·5 and 3·5 metres. The maximum phase in this place was 92 %.

A new method for the investigation of the solar corona, suggested by us (Vitkevitch, 1951) [1], consists of observing the radio source identified with the Crab nebula (NGC 1952; α = 05h 31m 40s, δ = 22° 10′) when it is covered by the solar corona. This occurs every year on 14–15 June.

This paper summarizes some recent results obtained with the solar radio-spectroscope at Dapto (Sydney), which has been in operation since 1952. The basic instrument is a receiver which sweeps through the frequency range 40–240 Mc./s. every half a second. Each spectrum is displayed as an intensity-modulated line, and successive spectra are recorded photographically on continuously moving film to give a dynamic record of the spectrum.