Ultra-high-energy cosmic-ray sources and large-scale magnetic fields

Protons of energies up to about 10(20) eV can be subject to significant deflection and energy-dependent time delay in large-scale extragalactic or halo magnetic fields of strengths comparable to current upper limits. By performing three-dimensional Monte Carlo simulations of nucleon propagation, we show how observations of arrival direction and time distributions can be used to measure the structure and strength of large-scale magnetic fields and constrain the nature of the source of ultra-high-energy cosmic rays.