Measurements of the radiation emitted by the sun at radio-frequencies have shown that the intensity greatly exceeds the value associated with a surface temperature of 6000° K. Under normal conditions the radiation, which appears to be randomly polarized, has an intensity which corresponds to the radiation from a black-body source subtending the same solid angle as the solar disk and at a temperature of about 10
6
°K. During the presence of sunspots very much more intense radiation is emitted by small areas of the solar disk; the intensity at these times corresponds to radiation from a source at a temperature of 10
9
to 10
10
°K, and the radiation is circularly polarized. The experimental results are considered theoretically in this paper, and it is concluded that the radiation in both cases arises from the acceleration of electrons in the solar atmosphere. It is suggested that by the action of the permanent magnetic field of the sun and the non-uniform rotation of the surface matter, a high potential difference is developed between the poles and the equator. Under normal conditions this potential can only produce small discharge currents through the solar atmosphere, although the electric field produced may be sufficient to maintain a mean electron temperature of 10
6
to 10
8
°K in the levels likely to emit radio-frequency radiation. During the presence of sunspots much more intense electric fields can be made available in the solar atmosphere, and in the neighbourhood of the sunspots electron temperatures of the order of 1010 °K should be maintained. A high-temperature electron gas can only radiate appreciably at those frequencies at which it absorbs well. An application of the magneto-ionic theory to the solar atmosphere above a sunspot shows that there are several regions capable of absorbing radiation at each frequency. For one of these regions the absorption (and therefore the radiating power) is very great, but radiation emitted by the region can only be propagated towards the centre of the sun. This region cannot therefore be responsible for the high-intensity radiation associated with sunspots, although the asymmetrical flow of energy from the region must produce an outward radiation pressure; this pressure may be of importance in accounting for the elevation of matter in the solar atmosphere above sunspots. Two other regions have a high absorption (each region absorbing one of the two circularly polarized components) and radiation from both regions can escape from the sun. Owing to the differences of radiating power and electron temperature in the two regions, it is likely that the intensities of the two emitted waves will be different. The radiation which is observed on the earth will therefore appear circularly polarized, the sense of the polarization corresponding to that of the most intense wave.
The generation of radio-frequency radiation in the sun