Linear unstable whistler eigenmodes excited by a finite electron beam

TL;DR

This paper investigates the linear growth and structure of whistler eigenmodes excited by a finite electron beam, combining theoretical analysis and experimental relevance in Earth's space plasma environment.

Contribution

It provides a detailed linear instability analysis for finite gyrating electron beams and matches eigenmodes to experimental observations of beam-generated whistler waves.

Findings

Whistler waves are excited via cyclotron and Landau resonances.

Eigenmodes peak near the beam boundary and decay outward.

A linear growth rate for each mode is derived.

Abstract

Electron beam-generated whistler waves are widely found in the Earth's space plasma environment and are intricately involved in a number of phenomena. Here we study the linear growth of whistler eigenmodes excited by a finite gyrating electron beam, to facilitate the interpretation of relevant experiments on beam-generated whistler waves in the Large Plasma Device at UCLA. A linear instability analysis for an infinite gyrating beam is first performed. It is shown that whistler waves are excited through a combination of cyclotron resonance, Landau resonance and anomalous cyclotron resonance, consistent with our experimental results. By matching the whistler eigenmodes inside and outside the beam at the boundary, a linear growth rate is obtained for each wave mode and the corresponding mode structure is constructed. These eigenmodes peak near the beam boundary, leak out of the beam region and decay to zero far away from the beam.