Acceleration of relativistic protons in a CME-perturbed solar wind

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 $λ_{\|}$. 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 $λ_{\|}^{-3/2}$. Energy spectra harden for decreasing parallel mean free path $λ_{\|}$.

Figures (10)

• Figure 1: Top and side views of the magnetic field lines in a spheromak. Note that despite appearances the magnetic field strength of the spheromak increases monotonically towards its center where it peaks at $2B_0/3$ (see equations (\ref{['eq:spheromak_1']}) and (\ref{['eq:spheromak_2']})). At the boundary of the spheromak, at $r^\prime=R_s$, there is only a non-zero poloidal component $B_p^\prime = 0.2172\:B_0\sin(\theta^\prime)$.
• Figure 2: Injection of the spheromak into the numerical domain. Top panel : the spheromak is injected in the equatorial plane at a constant radial speed $V_s$ in a pre-existing stationary wind.
• Figure 3: Equatorial cuts of the simulation domain as a function of time, where the time t=0 corresponds to the injection time of the spheromak. The color maps are for the radial fluid velocity (top panel) and temperature (bottom panel).
• Figure 4: CME induced variations of the plasma parameters at 1 AU. The vertical lines approximately delimit the time interval during which the local plasma can be considered to be perturbed by the CME.
• Figure 5: Time evolution of three selected magnetic field lines, denoted 0,1 and 2 from est to west.