Whistler-mode waves are circularly polarized electromagnetic waves generated by hot, anisotropic electrons. These waves are excited by the Earth’s radiation belts and accelerate electrons in various space plasma environments such as stellar wind and planetary magnetospheres. Inspired by a method used for simpler wave systems, Artemyev et al. developed a mapping technique to better understand the long-term electron dynamics in these phenomena as characterized by the nonlinear interaction of electromagnetic waves and electrons.

Whereas existing mapping techniques are limited to diffusive systems, the authors’ version is unique in that it can include nonlinear effects. By applying a Hamiltonian of electron motion with a finite wave-field perturbation, the technique considers two main processes – the long-term evolution produced by changes in electron energy caused by trapping, and by scattering.

Over time, particle trajectories in the map fill the entire available phase space, regardless of their initial energy distribution.

“Such stochastic dynamics is quite expected for diffusive resonance, but was unexpected for systems with nonlinear resonances,” said author Anton Artemyev. “This stochasticity means that long-term system dynamics can be modeled with the very powerful mapping technique.”

Looking forward, the authors plan to combine the developed mapping scheme with a realistic wave model – one that includes whistler-mode waves with significant variations in amplitude and phase coherence.

“Such combination would allow us to perform simulations of electron acceleration in radiation belts and compare these simulations with in situ spacecraft observations,” Artemyev said.

Source: “Mapping for nonlinear electron interaction with whistler-mode waves,” by A. V. Artemyev, A. I. Neishtadt, and A. A. Vasiliev, Physics of Plasmas (2020). The article can be accessed at https://doi.org/10.1063/1.5144477.