Recent work suggests that the cosmic ray spectrum may be dominated by Galactic sources up to \sim 10 ^ { 17.5 } eV , and by an extra-Galactic component beyond , provided this latter cuts off below the transition energy .
Here it is shown that this cut-off could be interpreted in this framework as a signature of extra-galactic magnetic fields with equivalent average strength B and coherence length l _ { c } such that B \sqrt { l _ { c } } \sim 2 - 3 \cdot 10 ^ { -10 } G \cdot Mpc ^ { 1 / 2 } , assuming l _ { c } < r _ { L } ( Larmor radius at \lesssim 10 ^ { 17 } eV ) and continuously emitting sources with density 10 ^ { -5 } Mpc ^ { -3 } .
The extra-Galactic flux is suppressed below \sim 10 ^ { 17 } eV as the diffusive propagation time from the source to the detector becomes larger than the age of the Universe .