Acceleration of relativistic protons in a CME-perturbed solar wind

TL;DR

This study combines MHD simulations and test-particle methods to show that CMEs can significantly accelerate relativistic protons in the solar wind, with energy gains influenced by shock proximity and scattering conditions.

Contribution

It introduces a coupled simulation approach to quantify proton acceleration by CMEs, highlighting the roles of shock dynamics and scattering in energy gain mechanisms.

Findings

CME-driven shocks can increase proton energies by several GeV.

Maximum energy gain occurs near 0.3 AU when the shock approaches.

Acceleration efficiency scales with the inverse of the mean free path to the power 1.5.

Abstract

We investigate the impact of a Coronal Mass Ejection (CME) on the transport and acceleration of relativistic protons in the solar wind using a coupled 3D Magnetohydrodynamics (MHD) simulation and a test-particle approach. The CME is driven by a spheromak injected into a Parker solar wind at a heliocentric distance of 0.139 AU. The trajectories of 5 GeV protons, injected towards the CME from 3 AU, are integrated in the guiding-centre approximation limit and scattered in velocity space with a mean free path $\lambda_{\|}$. Our results show that the CME can increment the protons energy by several GeV. The acceleration occurs during the time particles stream along the portion of a magnetic field line subject to compression downstream of the quasi-perpendicular portion of the CME-driven shock. In our configuration, the maximum energy gain, which is of the order of a few percent per shock crossing, occurs when the shock approaches 0.3 AU. Large energy gains require multiple passes through the acceleration region, which is made possible by the combined action of the mirror force and pitch angle scattering. The efficiency of the acceleration on time scales of the order of hours scales as $\lambda_{\|}^{-3/2}$. Energy spectra harden for decreasing parallel mean free path $\lambda_{\|}$.