Type-III Electron Beams: 3D Quasilinear Effects

• Published in: Solar physics 2021-02, Vol.296 (2), Article 42
• Main Authors: Melrose, Donald B., Harding, James, Cairns, Iver H.
• Format: Article
• Language: English
• Subjects: Astrophysics and Astroparticles; Atmospheric Sciences; Diffusion rate; Distribution functions; Electron beams; Langmuir waves; Phase velocity; Physics; Physics and Astronomy; Pitch (inclination); Radio bursts; Scattering; Solar physics; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Three dimensional models; Velocity
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-021-01783-8

A conventional model for the generation of Langmuir waves in Type-III radio bursts is based on a one-dimensional (1D) version of the quasilinear equations. In this model a wave with phase velocity v ϕ resonates with an electron with velocity v = v ϕ , causing the waves to grow at a rate ∝ d F ( v ) / d v > 0 , where F ( v ) is the 1D-distribution function. The backreaction on the electrons drives the electrons towards a plateau distribution: d F ( v ) / d v → 0 . In the 3D-generalization, none of these features apply: waves with phase speed v ϕ can resonate with electrons with speed v < v ϕ , depending on the angle between the wave normal and the electron velocity, wave growth occurs only if the distribution function is both an increasing function of v and also has an anisotropic pitch-angle distribution, and the backreaction involves diffusion in both speed v and in pitch-angle α . In this article we discuss implications of the generalization from 1D to 3D on models for Type-III bursts. An effect that is absent in 1D, but may be important in 3D, is scattering of Langmuir waves by turbulence in the ambient plasma. Pitch-angle scattering by the scattered Langmuir waves may play an important role in the evolution of the Type-III beam.