Acceleration of relativistic protons in a CME-perturbed solar wind

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 toward the CME from 3 AU, are integrated in the guiding-center approximation and scattered in velocity space with a mean free path lambda. Our results show that the CME can increase the protons' energy by several GeV. The acceleration occurs during the time particles stream along the portion of a magnetic field line 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 passage through the acceleration region, 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.