Modification of cosmic-ray energy spectra by stochastic acceleration

Abstract

Context. Typical space plasmas contain spatially and temporally variable turbulent electromagnetic fields. Understanding the transport of energetic particles and the acceleration mechanisms for charged particles is an important goal of today's astroparticle physics. Aims. To understand the acceleration mechanisms at the particle source, subsequent effects have to be known. Therefore, the modification of a particle energy distribution, due to stochastic acceleration, needs to be investigated. Methods. The diffusion in momentum space was investigated by using both a Monte-Carlo simulation code and by analytically solving the momentum-diffusion equation. For simplicity, the turbulence was assumed to consist of one-dimensional Alfvén waves. Results. Using both methods, it is shown that, on average, all particles with velocities comparable to the Alfvén speeds are accelerated. This influences the energy distribution by significantly increasing the energy spectral index. Conclusions. Because of electromagnetic turbulence, a particle energy spectrum measured at Earth can drastically deviate from its initial spectrum. However, for particles with velocities significantly above the Alfvén speed, the effect becomes negligible. © ESO, 2013.