Observations at 6 cm of the solar active region

Abstract

PRECISE knowledge of the fine structure of radio sources in solar active regions, and especially of their angular size and brightness temperature is important for the gyro-resonance absorption theory of the slowly varying component. In particular, such information is important for verifying proposed models1–3 of resonance absorption layers above sun-spots for which measurement of circular polarisation is also extremely useful. The active regions at centimetre wavelengths remain stable in intensity and structure for periods of ∼12 h. Consequently, the existing long baseline interferometers can be used to obtain synthesised maps from observations over those periods. Our first attempt to obtain such maps was in 1973 with the National Radio Astronomy Observatory interferometer at wavelengths of 3.7 and 11.1cm. These maps were not very clean because of the availability of only three baselines4, a problem which does not arise with the Westerbork synthesis radio telescope, which has twenty baselines. Here we describe the preliminary results of synthesis of a solar active region observed on May 8, 1974 from results obtained on May 8–10, 1974 with the Westerbork synthesis radio telescope (WSRT) in Holland5 at a wavelength of 6 cm.