Cabrera1 has reported the possible detection of a magnetic monopole in flight with magnetic charge g given by the Dirac condition 2 2eg = hc. Here, we accept the Cabrera candidate as a t'Hooft-Polyakov3,4 monopole of mass M ∼ 1016 GeV as expected5 in SU(5) or other grand unified theories. The monopole flux on Earth, on the basis of the single candidate, is f∼(0.1) cm-2 yr-1 (2πsr)-1, presumably consisting of roughly equal numbers of north and south monopoles. Galactic or intergalactic monopoles will have typical velocities of ∼300 km s-1. The Cabrera flux would correspond to a mass density f M/v ∼1 GeV cm-3. Because the mean mass density of the Universe cannot exceed 10-5 GeV cm-3, the mean flux of monopoles in the Universe must be at least five orders of magnitude smaller than f (refs 6,7). This would not be a problem if the monopoles were concentrated in the Galaxy. However, Parker has shown8 that the existence of galactic magnetic fields is inconsistent with a mean galactic monopole flux of 10-7 cm-2 yr-1. It follows that f must be a local flux resulting from the special nature of the observation site. Three possibilities come to mind: the local monopole flux may be associated with the Earth, the Sun, or with the Solar System as a whole.
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