Diffraction of Radio Star Radiation in the Solar Corona and the Auroral Ionosphere.

Abstract

During the last decade the yearly June occultations of the radio source Taurus A by the solar corona has been observed by several groups with radio interferometers (Hewish; Vitkevich; Blum and Boischot; IAU/ URSI Paris Symposium on Radio Astronomy, Stanford 1959), sometimes combined with fan-beam antennas (Gorgolewski and Hewish, Observatory 80, 99, 1960), and with pencil-beam antennas (Slee, Australian J. Phys. 12, 2, 1959), on wavelengths from 1.9 to 7.9 m. The interferometer observations show visibility reductions indicating an apparent increase in the angular diameter of the source when occulted by the solar corona. Some observers have noted a decrease in the total intensity of the source (Slee; Hewish), others an increase (Blum and Boischot; Vitkevich) during some phase of the occultation. The interpretations of the observations vary. Hogbom (Monthly Notices Roy. Astron. Soc. 120, 530,1960) explains them in terms of refraction (ray optics scattering) in coronal streamers, Hewish invokes multiple scattering in relatively small scale irregularities combined with refraction, while Vitkevich advocates scattering in somewhat larger irregularities and refractive focusing and defocusing by large inhomogeneities associated with coronal rays. All observers seem to agree that the inhomogeneities are anisotropic due to the local magnetic field, although evaluations have been based on the theory for scattering by isotropic irregularities. Radio interferometer observations on 0.67 and 1.35 m of the sources Cassiopeia A and Cygnus A through the auroral ionosphere have been made almost continuously since 1957 at College, Alaska. As might be expected from the similarities of the auroral ionosphere and the solar corona, the same phenomena of visibility reductions and total intensity variations are frequently observed for Cas A and Cyg A during disturbed conditions. An advantage of the ionospheric observations is that the properties of the medium are much better known, and that controlled experiments can be performed. The visibility reductions have been shown to be due to multiple scattering in small scale anisotropic irregularities in a layer of optical thickness of about 4 extending from 400 to 500 km (Owren, Proc. Intern. Conf. Ionosphere, London, 1963), while refractive effects of large ionospheric lenses account for the total intensity variations. In view of the pencilbeam obserVations on 10 and 25 cm of the 1962 Tau A occultation by Basu and Castelli (URSt Meeting, Washington, D. C., 1963) we favor a similar interpretation of the coronal data. Occasional observational results incompatible with present interpretations exist, as also pointed out by Vitkevich.