EXTRAGALACTIC MAGNETIC-FIELD AND THE HIGHEST ENERGY COSMIC-RAYS

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