Effect of acceleration and escape of energetic particles on spectral steepening at shocks

TL;DR

This paper develops a transport model for energetic particles at shocks, showing how acceleration and escape influence spectral steepening, resulting in different spectral shapes like log-parabolas and broken power laws.

Contribution

It introduces a comprehensive transport equation that incorporates particle acceleration and escape, accounting for turbulence effects near and far from the shock.

Findings

Upstream particle intensity steepens within one diffusion length from the shock.

Spectral shapes can be described by log-parabolas or broken power laws depending on parameters.

The model recovers the standard power law with exponential cutoff under uniform diffusion conditions.

Abstract

Energetic particles spectra at interplanetary shocks often exhibit a power law within a narrow momentum range softening at higher energy. We introduce a transport equation accounting for particle acceleration and escape with diffusion contributed by self-generated turbulence close to the shock and by pre-existing turbulence far upstream. The upstream particle intensity steepens within one diffusion length from the shock as compared with diffusive shock acceleration rollover. The momentum spectrum, controlled by macroscopic parameters such as shock compression, speed, far upstream diffusion coefficient and escape time at the shock, can be reduced to a log-parabola and also to a broken power law. In the case of upstream uniform diffusion coefficient, the largely used power law/exponential cut off solution is retrieved.