Magnetic monopoles and the survival of galactic magnetic fields

The most stringent, mass-independent limit on the flux of magnetic monopoles is based upon the survival of the galactic magnetic fields, the so-called "Parker limit": F10^-16 cm^-2{sr}^{{-}1}sec^-1. We reexamine this limit, taking into account the monopole's mass and velocity distribution, and the observed structure of the galactic magnetic field. We derive flux limits which depend upon the monopole's mass and velocity, and the strength, coherence length, and regeneration time of the galactic magnetic field. The largest monopole flux consistent with both the survival of the galactic magnetic field and the bounds from the mass density contributed by monopoles is F10^-12 cm^-2{sr}^{{-}1}sec^-1, arising for monopoles of mass {} 10¹⁹ GeV with velocity 3×10^-3c which cluster with the Galaxy. An observed flux greater than this would have profound implications for our understanding of the galactic magnetic field, and we briefly explore some exotic possibilities. Of course, this bound is not applicable to a local source (e.g., the Sun, atmospheric cosmic-ray production, etc.).