Coronal Faraday rotation during solar occultation of PSR0525 + 21

The apparent position of the pulsar PSR0525 + 21 is within 1.3° (4.72 R\cirċ) of the south solar pole on or about 13 June of each year. During these times of superior conjunction, the linearly polarised component of the pulsar radiation is subjected to an additional Faraday rotation due to the high electron density and strong magnetic field of the solar corona. We report here measurements of this coronal Faraday rotation in 1978 and 1979 at the 100-m radio telescope of the Max-Planck-Institut für Radioastronomie in Effelsberg which compared pulsar polarisation profiles at λ =17.4 cm (f=1,720 MHz) for times before, during and after occultation by the Sun. A significant residual rotation measure, which exceeded the interstellar value, was observed both years at times close to that of minimum solar approach.