Plasma Emission Induced By Electron Beam in Weakly Magnetized Plasmas

TL;DR

This study uses kinetic electromagnetic particle-in-cell simulations to explore plasma emission mechanisms in weakly-magnetized plasmas akin to the solar corona, revealing the roles of various wave modes and nonlinear processes.

Contribution

It provides the first detailed simulation-based analysis of plasma emission processes in weakly-magnetized solar corona-like conditions, highlighting the decay of the beam-Langmuir mode as a key mechanism.

Findings

The primary excited mode is the beam-Langmuir (BL) mode via bump-on-tail instability.

Generalized Langmuir waves include superluminal Z-mode and thermal Langmuir components.

The decay of the BL mode likely drives the plasma emission processes.

Abstract

Previous studies on the beam-driven plasma emission process were done mainly for unmagnetized plasmas. Here we present fully-kinetic electromagnetic particle-in-cell simulations to investigate such process in weakly-magnetized plasmas of the solar corona conditions. The primary mode excited is the beam-Langmuir (BL) mode via the classical bump-on-tail instability. Other modes include the whistler (W) mode excited by the electron cyclotron resonance instability, the generalized Langmuir (GL) waves that include a superluminal Z-mode component with smaller wave number $k$ and a thermal Langmuir component with larger $k$, and the fundamental (F) and harmonic (H) branches of plasma emission. Further simulations of different mass and temperature ratios of electrons and protons indicate that the GL mode and the two escaping modes (F and H) correlate positively with the BL mode in intensity, supporting that they are excited through nonlinear wave-wave coupling processes involving the BL mode. We suggest that the dominant process is the decay of the primary BL mode. This is consistent with the standard theory of plasma emission. Yet, the other possibility of the Z+W$\rightarrow$O--F coalescing process for the F emission cannot be ruled out completely.