Signatures for a Cosmic Flux of Magnetic Monopoles
  Any early universe phase transition occurring after inflation has the potential to populate the universe with relic magnetic monopoles. Observations of galactic magnetic fields, as well as observations matched with models for extragalactic magnetic fields, lead to the conclusion that monopoles of mass $\lsim 10^{15}$ GeV are accelerated in these fields to relativistic velocities. We explore the possible signatures of a cosmic flux of relativistic monopoles impinging on the earth. The electromagnetically-induced signatures of monopoles are reliable. The hadronically-induced signatures are highly model-dependent. Among our findings are (i) the electromagnetic energy losses of monopoles continuously initiate a protracted shower of small intensity; (ii) monopoles may traverse the earth's diameter, making them a probe of the earth's interior structure; (iii) in addition to the direct monopole Cherenkov signal presently employed, a very attractive search strategy for monopoles is detection of their radio-Cherenkov signal produced by the coherent charge-excess in the $e^+-e^-$ shower - in fact, Cherenkov-detectors have the potential to discover a monopole flux (or limit it) several orders of magnitude below the theoretical Parker limit of $10^{-15}/\rm{cm}^2$/s/sr; (iv) it is conceivable (but not compelling) that bound states of colored monopoles may be the primary particles initiating the air showers observed above the GZK cutoff. 
Stuart D. Wick, Thomas W. Kephart, Thomas J. Weiler, and Peter L.   Biermann