Proton-Helium Spectral Anomaly as a Signature of Cosmic Ray Accelerator

TL;DR

This paper proposes that the spectral difference between helium and protons observed by PAMELA is a signature of diffusive shock acceleration in supernova remnants, providing evidence for cosmic ray origins.

Contribution

It introduces a model considering the initial injection phase of DSA, explaining the spectral anomaly as a result of elemental injection bias due to Alfven wave interactions.

Findings

He2+ ions are injected more efficiently during stronger shocks.

The spectral difference is explained by larger gyroradii of He2+ ions affecting wave trapping.

Fitting PAMELA data supports DSA as the source of cosmic rays.

Abstract

The much-anticipated proof of cosmic ray (CR) acceleration in supernova remnants (SNR) must hinge on full consistency of acceleration theory with the observations; direct proof is impossible because of the orbit scrambling of CR particles. The PAMELA orbital telescope revealed deviation between helium and proton CR spectra deemed inconsistent with the theory, since the latter does not differentiate between elements of ultrarelativistic rigidity. By considering an initial (injection-) phase of the diffusive shock acceleration (DSA), where elemental similarity does not apply, we demonstrate that the spectral difference is, in fact, a unique signature of the DSA. Collisionless plasma SNR shocks inject more He2+ relative to protons when they are stronger and so produce harder helium spectra. The injection bias is due to Alfven waves driven by the more abundant protons, so the He2+ ions are harder to trap by these waves because of the larger gyroradii. By fitting the p/He ratio to the PAMELA data, we bolster the DSA-case for resolving the century-old mystery of CR origin.