Extragalactic magnetic fields and the second knee in the cosmic-ray spectrum

Recent work suggests that the cosmic-ray spectrum may be dominated by Galactic sources up to ∼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 Bsqrt[l_c]∼2-3·10^-10  G·Mpc^1/2, assuming l_c<r_L (Larmor radius at ≲10¹⁷ eV) and continuously emitting sources with density 10^-5  Mpc^-3. The extra-Galactic flux is suppressed below ∼10¹⁷ eV as the diffusive propagation time from the source to the detector becomes larger than the age of the Universe.