Orateur
Description
Ever since the classic papers of Lagage & Cesarsky (1983) and Hillas (1984), the Bohm limit uBR has been taken as the measure of the maximum energy to which cosmic rays can be accelerated by shocks. The obvious problem in the 1980s was that the Bohm limit for supernova remnants fell well short of the knee in the spectrum at a few PeV. This problem appeared to be solved when fields of a few hundred microGauss fields were observed in young supernova remnants and the theory of non-resonant magnetic field amplification said this should be so. But the problem was not solved. Gamma-ray observations imply a turnover in the cosmic ray spectrum at a few hundred TeV. Furthermore, the time-dependent theory of non-resonant amplification said again that this should be so since there is insufficient time to amplify the field on the Larmor scale of PeV cosmic rays.
The Bohm limit is based on the assumption that cosmic ray transport is diffusive and that the minimum mean free path is the Larmor radius. As early as 1966 Jokipii pointed out the possibility that cosmic rays could be confined by mirrors ahead of the shock, but he did not integrate this into a broader model in which cosmic rays cross the shock many times. Mirroring has recently been receiving attention in cosmic ray transport in general. Here I examine the possibility that non-diffusive transport with mirroring may explain cosmic ray acceleration beyond the Bohm limit (Bell et al 2025).