Waves in a warm ion-electron plasma: a two-fluid analysis and model applications
Date:
Seminar at the Centre for mathematical Plasma Astrophysics, KU Leuven, Belgium.
Abstract. In the literature the two-fluid formalism is often utilised to give an overview of waves in a cold plasma as a stepping stone towards kinetic theory. This cold plasma analysis is then used to introduce wave labels based on wave properties at propagation parallel and perpendicular to the background magnetic field. However, comparing parallel or perpendicular propagation to oblique angles reveals that they differ significantly due to the introduction of avoided crossings in the frequency-wavenumber diagrams at oblique angles. Therefore, we adopt a new wave labelling scheme that does not rely on properties at parallel and perpendicular propagation and highlights the frequency ordering at all oblique angles of propagation.
The first part of the talk will focus on the two-fluid dispersion relation, which is a polynomial of sixth order in the frequency squared, and thus describes six forward-backward propagating pairs. Analysis of this dispersion relation allows for identification of mode crossings and avoided crossings. These mode crossings depend on the model’s parameters and six different regimes can be identified. For any regime the model allows for the computation of phase and group speed diagrams at arbitrary wavelengths, and animation of these diagrams for varying wavenumber illustrates the avoided crossing behaviour clearly.
In the second part, we focus on two applications of the general dispersion relation. As a first application, the warm ion-electron dispersion relation can be used to extend the Appleton-Hartree relation, which describes the high-frequency waves in a cold plasma, to include the contribution from the electron thermal velocity. As a second application, the analytic group speed expressions are evaluated at different propagation angles and in different regimes to reveal whistling behaviour across all modes.