Abstract
A stochastic model to describe the phase-space diffusion of charged particles induced by random fluctuations of relative gyrophases in the presence of parallel propagating, circularly polarized electromagnetic waves is discussed. The perturbation analysis around the equilibrium points of the noiseless system shows the coexistence of classical diffusion and trapping oscillation. Even if the equation of motions for pitch angle does not include noise terms, the pitch angle diffusion occurs due to the noise term in the equation of the relative gyrophase and the existence of the finite amplitude wave. The resultant theory is validated by using numerical results of test particle simulations; when distributions of the relative gyrophases and pitch angle cosines are close to Gaussian, the classical diffusion and trapping oscillation are observed. With increasing wave amplitude and/or the strength of noise, the pitch angle diffusion becomes subdiffusive.
Highlights
Wave-particle interaction is a fundamental process in collisionless plasmas
The resultant theory is validated by using numerical results of test particle simulations; when distributions of the relative gyrophases and pitch angle cosines are close to Gaussian, the classical diffusion and trapping oscillation are observed
We focus on the characteristics of the phase space diffusion induced by random fluctuations of relative gyrophases, whose probability distributions are often assumed to be the uniform distribution at the initial stage[28,32] or at each time step.[15]
Summary
Wave-particle interaction is a fundamental process in collisionless plasmas. Various electromagnetic and electrostatic waves in plasmas can resonate with charged particles, resulting in particle heating and diffusion. Note that Artemyev et al.[31] have applied the Hamiltonian approach and pointed out effects of random magnetic field fluctuations on trapping of relativistic electrons by whistler waves and electromagnetic ion cyclotron waves They have discussed the relationship between the random fluctuations and the pitch angle diffusion.[31]. We focus on the characteristics of the phase space diffusion induced by random fluctuations of relative gyrophases, whose probability distributions are often assumed to be the uniform distribution at the initial stage[28,32] or at each time step.[15] In order to discuss the phase space diffusion in the presence of nonlinear monochromatic electromagnetic waves, we discuss a simple Langevin model, whose noiseless limit corresponds to the Hamiltonian system for particle dynamics in the monochromatic wave.[18].
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