Radio method of mapping large-scale coronal magnetic fields on active stars

A number of nearby late-type stars, including the Sun, are known to be active at radio wavelengths. Disregarding mass-loss giant stars and novae (where the emission is probably of thermal origin)1, we find that most radio stars are RSCVn binaries and UV Ceti-type flare stars2. Estimated brightness temperatures >109K suggest that the emission mechanisms are non-thermal, and in some cases coherent3. Non-thermal radio emission from active stars is often moderately circularly polarized. In this respect the emission is similar to solar radiation at metre to centimetre wavelengths and may similarly arise from the acceleration and subsequent trapping of mildly relativistic electrons in intense magnetic fields above star spots. I describe here a radio method which measures the longitudinal magnetic sector structure of the solar corona by observing the Sun ‘as a star’ and monitoring the sense of circular polarization of metre-wavelength emission as a function of rotational phase. The method is tested by comparing synoptic plots of solar radio-noise-storm polarities derived from high- and low-resolution observations over a period of four years at Culgoora. This method may be applied to the measurement of large-scale coronal fields on other active stars.