Dynamics of energetic electrons scattered in the solar wind

Magnetohydrodynamics and test-particle simulations

LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 place Jules Janssen, 92195 Meudon, France

^⋆ Corresponding author; ahmed.houeibib@obspm.fr

Received: 9 July 2024
Accepted: 20 December 2024

Abstract

We model the transport of solar energetic particles (SEPs) in the solar wind. We propagated relativistic test particles in the field of a steady three-dimensional magnetohydrodynamic simulation of the solar wind. We used the code MPI-AMRVAC for the wind simulations and integrated the relativistic guiding center equations using a new third-order-accurate predictor-corrector time-integration scheme. Turbulence-induced scattering of the particle trajectories in velocity space was taken into account through the inclusion of a constant field-aligned scattering mean free path λ_∥. We considered mid-range SEP electrons of 81 keV injected into the solar wind at a heliocentric distance of 0.28 AU and a magnetic latitude of 24°. For λ_∥ = 0.5 AU, the simulated velocity pitch angle distributions agree qualitatively well with in situ measurements at 1 AU. More generally, for λ_∥ in the range 0.1–1 AU, an energy-loss rate associated with the velocity drift of about 10% per day is observed. The energy loss is attributable to the magnetic curvature and gradient-induced poleward drifts of the electrons against the dominant component of the electric field. In our case study, which is representative of the average solar wind conditions, the observed drift-induced energy-loss rate is fastest near a heliocentric distance of 1.2 AU. We emphasize that adiabatic cooling is the dominant mechanism during the first 1.5 hours of propagation. Only at later times does the drift-associated loss rate become dominant.