Cosmic ray acceleration at modified shocks

TL;DR

This paper uses Monte Carlo simulations to study how non-linear effects at shock fronts influence cosmic ray acceleration, revealing that a smooth precursor enhances acceleration efficiency and results in flatter energy spectra.

Contribution

It provides the first detailed numerical analysis of particle acceleration at oblique modified shocks considering non-linear precursor effects.

Findings

Flatter energy spectra compared to sharp shock models.

Precursor confinement increases high-energy particle acceleration.

Results align with theoretical predictions of enhanced efficiency.

Abstract

The non-linear back reaction of accelerated cosmic rays at the shock fronts, leads to the formation of a smooth precursor with a length scale corresponding to the diffusive scale of the energetic particles. Past works claimed that shocklets could be created in the precursor region of a specific shock width, which might energize few thermal particles to sufficient acceleration and furthermore this precursor region may act as confining large angle scatterer for very high energy cosmic rays. On the other hand, it has been shown that the smoothing of the shock front could lower the acceleration efficiency. These controversies motivated us to investigate numerically by Monte Carlo simulations the particle acceleration efficiency in oblique modified shocks. The results show flatter spectra compared to the spectra of the pressumed sharp discontinuity shock fronts. The findings are in accordance with theoretical predictions, since the scattering inside the precursor confines high energy particles to further scattering, resulting in higher energies making the whole acceleration process more efficient.