  The next generation of ultra high energy cosmic ray experiments will probably
detect several dozens of events clustered around the direction towards each of
the most powerful extragalactic sources. We develop a method which could make
possible to reconstruct, from the arrival directions and energies of the
clustered events, the strength and coherence properties of the magnetic field
along the line of sight towards the sources. The method exploits peculiar
signatures arising from magnetic lensing effects, such as the strong flux
magnification of multiple images around caustics. We also discuss how to obtain
information about the cosmic ray composition, and apply this method to samples
of simulated data.
