EXTRAGALACTIC MAGNETIC-FIELD AND THE HIGHEST ENERGY COSMIC-RAYS

The strength and spectrum of the extragalactic magnetic field (EGMF) a re still unknown. Its measurement would help answer the question of wh ether galactic fields are purely a primordial relic or whether they we re dynamically enhanced from a much smaller cosmological seed field. I n this Letter, we show that the composition, spectrum, and directional distribution of extragalactic ultra-high-energy cosmic rays, with ene rgies above similar or equal to 10(18) eV, can probe the large-scale c omponent of the EGMF below the present observational upper limit of 10 (-9) G. Cosmic-ray detectors under construction or currently in the pr oposal stage should be able to test the existence of the EGMFs on scal es of a few to tens of megaparsecs and strengths in the range similar or equal to 10(-10) to 10(-9) G.