Stochastic Diffusion of Electrons interacting with Whistler-mode Waves
in the Solar Wind
Abstract
Whistler-mode waves have often been proposed as a plausible mechanism
for pitch angle scattering and energization of electron populations in
the solar wind. Recent studies reported observations of obliquely
propagating and narrowband waves consistent with the whistler mode at 1
AU. Close to 0.3 AU, similar waves have also been observed in PSP data,
where evidence of strong scattering of strahl electrons indicates that
these waves regulate the electron heat flux. At both radial distances,
the wave amplitude can be as high as 10% of the ambient magnetic field.
The oblique propagation angle enables resonant interactions without
requiring that the electrons counter-stream with the waves.
Self-consistent PIC simulations by Roberg-Clark et al (2019) and Micera
et al (2020) studied the strahl scattering and subsequent halo formation
due to anomalous resonant interactions enabled by oblique whistlers
generated from the heat flux fan instability. Observational studies of
whistlers near the Sun have also concluded that they are connected to
this instability. Cattell and Vo (2021) also demonstrated the same
features of the scattering from a particle tracing simulation, one
advantage of which is the ability to calculate kinetic quantities such
as the diffusion coefficients. Also, the tracing code includes
variational calculations to ensure energy conservation in the presence
of highly chaotic dynamics. In this study, we investigate in more detail
the resonant interactions of electrons with these high amplitude and
oblique whistlers. We will show that these waves at 0.3 AU may exceed
the stochasticity condition where resonance overlap occurs. Furthermore,
the stochastic width around the primary islands might be large enough
that diffusion is enabled even before they overlap. In simulations with
1 AU parameters, the particle motion is strongly stochastic where all
harmonics significantly overlap, leading to an isotropic pitch angle
diffusion which forms the halo population. Our calculations also
indicate the presence of higher-order effects, allowing for sub- and
super-harmonic resonant interactions.