# Cosmic Ray Acceleration in Hydromagnetic Flux Tubes

## TL;DR

This paper proposes that hydromagnetic flux tubes in radio galaxy lobes can accelerate cosmic rays to ultra-high energies through Fermi acceleration mechanisms, even without strong turbulence.

## Contribution

It introduces a novel environment for cosmic ray acceleration and demonstrates that weak shocks can effectively accelerate particles to ultra-high energies.

## Key findings

- CR can reach Hillas energies in flux tubes
- First-order Fermi acceleration is equivalent to second-order in this context
- Acceleration occurs even with weak turbulence

## Abstract

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetised turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.