ANTISYMMETRIC ROTATION MEASURES IN OUR GALAXY - EVIDENCE FOR AN A0 DYNAMO

Rotation measures for extragalactic sources, after selection to emphas ize the large-scale structure of the Galactic magnetic field, reveal a striking antisymmetric pattern about the Galactic plane and the merid ian through the Galactic Centre. This pattern is also observed in the rotation measures of pulsars with \b\ > 8 degrees. The symmetry of the pattern, the magnitude of the rotation measures and the distance depe ndence of the pulsar rotation measures show that this pattern is not t he result of local perturbations, but is on a large and possibly Galac tic scale. This antisymmetric pattern and the existence of magnetic fi elds perpendicular to the plane in the Galactic Centre region, indicat e that a dynamo mode of odd symmetry, possibly of A0 type, makes a sub stantial contribution to the magnetic fields in the thick disk and hal o of our Galaxy, at least inside the Solar circle.