The characteristics and the generation mechanisms of solar radio outbursts seem to be distinct at the frequencies above and below a certain cross-over frequency somewhere between 300 and 600 Mc/s (see Table I).

Most of the background of cosmic radio waves comes from our own Galaxy. In the equatorial plane this origin is evident, and the obvious features can be directly related to the Galaxy's structure. Away from the plane the emission comes from a roughly spherical halo, which accounts for most of the total emission from the Galaxy. Some features in this part of the sky are not evident from optical studies; the most important is a circular belt of emission passing not far from the center and the anti-center. It is impossible that this should be extragalactic in origin, since it is more than twice as bright as the brightest estimates of the whole galactic component, and optical evidence would be expected of such a major irregularity in the distribution of the galaxies, or indeed of a single spiral galaxy close to or enveloping our own. We turn therefore to a suggestion of an origin in our own Galaxy.

The mechanism that generates sporadic (nonthermal) solar radio emission, which is of great interest to radio astronomy and solar physics, is rather obscure. Recently the authors considered this problem [1, 2, 3] and this report gives their summary.

Attempts to explain both the expansion of the universe and the condensation of galaxies must be very largely contradictory so long as gravitation is the only force field under consideration. For if the expansive kinetic energy of matter is adequate to give universal expansion against the gravitational field it is adequate to prevent local condensation under gravity, and vice versa. This is why, essentially, the formation of galaxies is passed over with little comment in most systems of cosmology. Yet the galaxies, and the clusters in which they are often found, are such an important characteristic property of the universe that it is unsatisfactory to dismiss their origin in the vague term “fluctuation phenomenon.”

A paper [1] sent to the Manchester Symposium on Radio Astronomy in 1955 described the views developed earlier [2] concerning a magnetobremsstrahlung (synchrotron) origin of nonthermal cosmic radio emission. Unfortunately, for unknown reasons, it was not included in the Symposium volume [3]. At the same time the paper by Unsöld was published [3] (see also [4]), in which the nonthermal galactic radio emission is connected with radiation of a great number of cold dwarfs (radio stars). At the same time, Unsöld criticizes the theory of cosmic-ray origin, according to which the main sources of these rays are envelopes of supernovae and, perhaps, novae [2, 5, 6, 7, 8]. Therefore, we begin with some critical remarks.

Relativistic electrons moving in a homogeneous magnetic field H emit “synchrotron radiation” with a frequency distribution which may be written in the following form.

Undoubtedly the outstanding advance of the last few years in theoretical radio astronomy has been the widespread recognition of the importance of synchrotron radiation by relativistic electrons. The possibility that high-speed electrons might be of importance in the solar problem was mentioned by Giovanelli [1] and by me [2] in 1948. But real quantitative development in this direction dated from the well-known paper of Schwinger [3] published in 1949. Schwinger's results were put into a form suited to the problems of radio astronomy by Alfvén and Herlofson in 1950, still from the point of view of emission from stellar surfaces, however. In 1953, Ginzburg [5] and Shklovskiì [6] took the additional step of suggesting that relativistic electrons exist in space and that they give rise to the emission from nonthermal sources.