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 test-particle approach. The CME is modelled as a spheromak propagating through a Parker-like solar wind and the trajectories of 5 GeV protons are integrated in the guiding-centre approximation limit. Our results show that the CME can significantly accelerate the protons up to tens of GeV.
Particles gain energy through an adiabatic heating mechanism as they access to regions of compressed plasma downstream of the CME-driven shock. In our configuration, the maximum energy gain, which is about 50 % per crossing, occurs when the perturbation reaches about 0.3 AU, which corresponds to ~9 hours after the spheromak is injected.
The parallel diffusion plays an important role by confining the particles within the simulation domain long enough for them to encounter the disturbance multiple times and gain energy at each interaction. Particles' energy spectra at 1 AU shows the energy gain depends on the longitude of the magnetic field line on which the particle is located. They also show that the spectra are harder for smaller values of the parallel mean free path λ.