Self-consistent particle-in-cell simulations of fundamental and harmonic plasma radio emission mechanisms

TL;DR

This study uses 2D fully kinetic particle-in-cell simulations to demonstrate the conditions under which plasma radio emissions occur, resolving previous contradictions and emphasizing the importance of realistic beam densities.

Contribution

First self-consistent kinetic simulation showing fundamental and harmonic plasma emission from a single electron beam system.

Findings

Plasma emission depends on initial wave frequency generated by bump-in-tail instability.

Unrealistically dense beams suppress plasma emission due to non-Langmuir wave modes.

Single-beam plasma emission can occur without counter-propagating beams.

Abstract

Aims. The simulation of three-wave interaction based plasma emission, thought to be the underlying mechanism for Type III solar radio bursts, is a challenging task requiring fully-kinetic, multi-dimensional models. This paper aims to resolve a contradiction in past attempts, whereby some studies indicate that no such processes occur. Methods. We self-consistently simulate three-waved based plasma emission through all stages by using 2D, fully kinetic, electromagnetic particle-in-cell simulations of relaxing electron beams using the EPOCH2D code. Results. Here we present the results of two simulations; Run 1 (nb/n0 = 0.0057, vb/{\Delta}vb = vb/Ve = 16) and Run 2 (nb/n0 = 0.05, vb/{\Delta}vb = vb/Ve = 8), which we find to permit and prohibit plasma emission respectively. We show that the possibility of plasma emission is contingent upon the frequency of the initial electrostatic waves generated by the bump-in-tail instability, and that these waves may be prohibited from participating in the necessary three-wave interactions due to frequency conservation requirements. In resolving this apparent contradiction through a comprehensive analysis, in this paper we present the first self-consistent demonstration of fundamental and harmonic plasma emission from a single-beam system via fully kinetic numerical simulation. We caution against simulating astrophysical radio bursts using unrealistically dense beams (a common approach which reduces run time), as the resulting non-Langmiur characteristics of the initial wave modes significantly suppresses emission. Comparison of our results also indicates that, contrary to the suggestions of previous authors, an alternative plasma emission mechanism based on two counter-propagating beams is unnecessary in an astrophysical context.