Type III radio bursts are associated with energetic electrons accelerated by solar flares from the lower corona. The standard theory links these emissions to a conversion of plasma oscillations excited by the bump-on-tail instability into electromagnetic waves. Since electron beams can propagate to large heliospheric distances and continue to emit radio waves, the instability must be finely balanced, not to disrupt their propagation (so-called Sturrock’s dilemma). To explain this, many models invoking contrasting processes have been proposed (e.g. quasilinear vs strong turbulence description of interactions between various plasma modes). In this study, we perform 2D PIC simulations of beam injection, propagation, and emissions in a large system without periodic boundary conditions. Results demonstrate that the beam decouples from the excited electrostatic oscillations near the injection site and propagates through the background plasma with relatively small energy loss. Downstream, the instability continues to operate only at the beam front. The main body of the beam between downstream and upstream reaches a quasi-steady state. It may become unstable again where the background plasma is colder or less dense. Background temperature variations affect the beam instability more than background density fluctuations. Radio emissions at plasma frequency and its second harmonic are primarily generated upstream in the region of intense fluctuations, where both classical signatures of three-wave conversion processes and those associated with modulational instability are detected. Our results are consistent with satellite data showing that electron beams often continue to generate type III radio bursts even beyond 1 AU. They illustrate in a first-principle model how a beam state consistent with subsequent quasilinear relaxation emerges shortly after beam injection.

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.