Inhomogeneous extragalactic magnetic fields and the second knee in the cosmic ray spectrum

TL;DR

This paper investigates how inhomogeneous extragalactic magnetic fields influence the cosmic ray spectrum's second knee, suggesting that magnetic field variations in voids significantly affect the low-energy cutoff.

Contribution

It introduces a semi-analytical propagation code to study the impact of inhomogeneous magnetic fields on cosmic ray diffusion and the resulting spectral features.

Findings

Void magnetic field strength mainly controls the low-energy cutoff.

Inhomogeneous magnetic fields significantly affect cosmic ray propagation.

The magnetic horizon depends on the large-scale structure distribution.

Abstract

Various experiments indicate the existence of a second knee around energy E=3.10^{17} eV in the cosmic ray spectrum. This feature could be the signature of the end of the galactic component and of the emergence of the extragalactic one, provided that the latter cuts off at low energies. Recent analytical calculations have shown that this cut-off could be a consequence of the existence of extragalactic magnetic fields: low energy protons diffuse on extragalactic magnetic fields and cannot reach the observer within a given time. We study the influence of inhomogeneous magnetic fields on the magnetic horizon, using a new semi-analytical propagation code. Our results indicate that, at a fixed value of the volume averaged magnetic field <B>, the amplitude of the low energy cut-off is mainly controled by the strength of magnetic fields in the voids of the large scale structure distribution.