Occultation of Taurus-A by the Solar Corona at 430 Mc/s in June 1964

Abstract

DURING June of every year, the radio source Taurus-A passes within a small angular distance of the Sun. This particular situation, commonly referred to as the occultation of Taurus-A by the Sun, permits an investigation of the Sun's outer corona as the radio source is viewed through it. The measurements made over the past several years have demonstrated that the radiation from the source is scattered at the electron density irregularities in the corona; as a result one observes an apparent broadening of the source accompanied by a decrease of its intensity. The effect becomes larger with decreasing angular separation of the source from the Sun. This phenomenon has been observed in the range of frequencies 26–178 Mc/s^1–5. At wave-lengths of 6 and 18 cm the effect has been reported to be negative^6,7, except that a positive result has been reported on 10- and 25-cm wave-lengths⁸. The results on all metre and decametre wave-lengths are consistent and are in accordance with the theory of small-angle multiple scattering, in which the angular size of the scattered distribution is proportional to the square of the wave-length. However, the increase of intensity on 169 Mc/s at 7 R₀ (R₀ is the photospheric radius) from the centre of the Sun is not consistent with the theoretical prediction based on the lower frequency data⁵. In order to investigate if the coronal scattering is still effective at frequencies higher than 169 Mc/s, and in particular to see if the increase of intensity is also observed on 430 Mc/s, we undertook to observe at 430 Mc/s the occultation of Taurus-A by the solar corona in June 1964, using the 1,000-ft. dish of the Arecibo Ionospheric Observatory.