Monopoles and the Galactic Magnetic Field

Abstract

Polarization of starlight, discovered thirty years ago, was the earliest evidence for an interstellar magnetic field. It remains the only optical manifestation of a general galactic field. Radioastronomy has provided several other lines of evidence. These include observation of synchrotron radiation, of Faraday rotation of radiation from extragalactic sources, a few observations of Zeeman “splitting” in the 21 cm hyperfine line of H, and the measurement of Faraday rotation combined with dispersion in the radiation from pulsars. Here we shall summarize briefly what is known about the interstellar magnetic field, emphasizing aspects relevant to questions about GUT monopoles. Then we shall consider the motion of monopoles in such a field and discuss the limits on monopole flux implied by the existence of the field we observe.