M. Ryle

An investigation of radio-frequency radiation from the sun

It has been known for some time that the sun emits radio-frequency radiation whose intensity greatly exceeds the value expected from a black-body at 6000°K. In the present paper, experiments are described in which measurements have been made of the solar radiation at frequencies of 175 and 80 Mcyc. /sec. Measurement of the small powers which can be abstracted from practical aerial systems requires special types of receiving equipment if absolute measurements are to be recorded automatically over long periods of time. An apparatus has been developed in which the output power of a local source of random ‘noise’ is automatically and continuously adjusted so as to be equal to the aerial power; in this way the receiver is used only as an indicator of balance, and errors due to variation of its gain or internal noise are eliminated. A special type of aerial has been devised which enables the solar radiation to be recorded separately from the galactic radiation, and so enables continuous observation of the sun to be made with aerials of comparatively low directivity. The results obtained on these two frequencies show that the sun normally emits radiation whose intensity corresponds to a surface temperature of the order of 106°K. Large fluctuations in the intensity occur, however, and during the passage of large sunspots, equivalent temperatures as high as 108 to 109°K have been observed. In addition to these day-to-day variations the radiation is subject to sudden brief increases of intensity lasting only for a few seconds. Measurements of the diameter of the source, by a method analogous to Michelson’s stellar interferometer, have shown that during periods of very great intensity the radiation originates in an area of the sun of the same order of size as a sunspot. This result means that equivalent temperatures of 109 to 1010°K must exist. Measurements of the polarization of the radiation have shown that during periods of increased activity the radiation is mainly circularly polarized. The present account covers the experimental methods and the results obtained up to the present time. It is hoped to consider these results theoretically in a future paper.

The spatial distribution and the nature of radio stars

An investigation of the number-intensity distribution of the observed radio stars shows that they cannot be interpreted in terms of a homogeneous distribution of sources; there is an apparent increase in the spatial density or absolute luminosity of distant sources. It is also found that this increase is, within rather small statistical errors, independent of direction. It therefore appears that the solar system is situated at the centre of a spherical region in which the spatial density or luminosity of the sources increases with distance, uniformly in all directions. It may also be concluded that this increase does not persist for distances much greater than those to which sources have been observed individually. It appears impossible to explain the results in terms of sources situated within the galaxy, or by irregular clustering of extra-galactic sources. Furthermore, it may be shown from arguments based on the integrated radiation that the minimum radius of the spherical region is comparable with that of the optically observable universe. It is therefore suggested that the observed features are due to effects on a cosmical scale; such an explanation provides an immediate reason for the remarkably isotropic distribution of radio stars. Attempts to explain the observations according to steady-state theories offer little hope of success, but there seems every reason to suppose that they might be explained in terms of an evolutionary theory. No detailed comparison has yet been made between the observations and the predictions of any particular evolutionary model, but a general explanation of the observations seems possible if it is supposed that the majority of radio stars belong to an excessively rare class of object, having a local spatial density of about 2 x 10-26 parsec-3, and an absolute luminosity comparable with that of the intense radio source in Cygnus. On this interpretation, the difficulty of identifying radio stars with optical objects finds a natural explanation; only some tens of the main class of radio star would be within reach of the 200 in. telescope.

Evidence for the Stellar Origin of Cosmic Rays

Several authors have suggested the possibility that cosmic rays are due to the acceleration of charged particles in the atmospheres of certain stars. Recent observations at radio frequencies have provided some evidence in support of this hypothesis which is reviewed in this paper. The observations have shown that at least part of the radio-frequency radiation from the galaxy is emitted by discrete sources of small angular diameter. Measurements of the intensity of the radiation from these sources, and of the time variations of the intensity, indicate that the sources emit radio waves as if they were at a temperature of more than 1014 deg. K. It is concluded that this intense radiation cannot be caused by the coherent oscillation of a large number of electrons, but must be due to a genuine electron temperature of about 1014 deg. K. (corresponding to a mean electron energy of 1010 electron volts). The existence of this electron temperature indicates that in certain stellar bodies there are mechanisms capable of accelerating particles to cosmic ray energies. A previous theory of the emission of radio waves from the sun is extended to other stellar bodies. It is concluded that a star having a surface magnetic field strength and a peripheral velocity somewhat greater than those which have so far been observed could emit radio waves having the observed intensity. A star of this type could also accelerate charged particles to cosmic ray energies. The small visual brightness of the sources of radio waves suggests that these bodies may be characterized by small visual opacity or small photospheric temperature. It is suggested that the difference between the angular distribution of cosmic rays and of the radio waves from the galaxy is due to the deflection of cosmic ray particles by magnetic fields. In addition to previous theories of a general magnetic field in the galaxy, it is shown that the scattering produced by the magnetic fields of the distributed sources themselves should be sufficient to produce an isotropic distribution of cosmic rays at the earth even though the sources may show a marked concentration near the galactic equator.

Observations at the Mullard Radio Astronomy Observatory, Cambridge

When the news of the launching of the first satellite was received at Cambridge, we were not aware of any plans for radio observations in this country. Such observations seemed likely to be important: (a) In order to provide approximate information on the orbit so that accurate visual and radar observations might be made. (b) Because the presence of a transmitter at varying heights and zenith angles offered possibilities for ionospheric investigation. We therefore planned a series of observations which eventually engaged most of the radio astronomy group, and a large part of the ionosphere group of the Cavendish Laboratory. The first observations were aimed at the determination of an approximate orbit, and an interferometer normally used for observing radio stars at a frequency of 38 Mc/s was modified to receive the 40 Mc/s satellite transmission. This instrument was in operation on the night of 5 to 6 October, and a 40 Mc/s receiver for measuring the Doppler shift was installed the following day. A 20 Mc/s interferometer was added for ionospheric investigation, and receivers for measuring the apparent changes of intensity were later installed for 20, 40 and 80 Mc/s, the last being used on the second harmonic of the 40 Mc/s transmitter.

The application of interferometric methods in radio astronomy

Abstract Interferometric methods have been applied to a number of different problems in radio astronomy and the increased resolution has allowed important advances in both solar and galactic problems. Observations using spaced-aerial interferometers of variable aperture have enabled the distribution of radio “brightness” across the solar disk to be determined at wavelengths between 60 cm and 7·9 m. Similar methods have been used to determine the detailed structure of the galactic background radiation in directions near the galactic plane; such observations are important in connection with the effect of the ionized regions of interstellar matter. The use of interferometers, particularly of the “phase-switching” type, has allowed considerable advances in the study of “radio stars”. In addition to providing accurate positions, such observations have made it possible to determine the angular diameter of some of the more intense sources.

RADIO OUTBURSTS AND A POSSIBLE CHANGE OF PERIOD OF ALGOL

Mallama (1978) has suggested an association between a possible change of period of Algol during 1975 with a radio outburst on 1975 January 15-16. Previous extensive observations (Pooley and Ryle 1973) have, however, shown that such outbursts occur quite infrequently.

Is there a case for nuclear power

In this article, one of a continuing series examining nuclear power in the UK, one of the countrys most distinguished scientists examines the case for and against the use of nuclear power