The role of the thermal properties of electrons on the dispersion properties of Alfvén waves in space plasmas

¹ Departamento de Física, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, 7800003 Ñuñoa, Santiago, Chile
² Mullard Space Science Laboratory, University College London, Dorking RH5 6NT, UK
³ Research Center in the intersection of Plasma Physics, Matter, and Complexity ( P 2 mc), Comisión Chilena de Energía Nuclear, Casilla 188-D, Santiago, Chile

Received: 2 February 2024
Accepted: 2 July 2024

Abstract

Context. The transition from left-hand to right-hand polarised Alfvén waves depends on the wavenumber, the ratio of kinetic to magnetic pressure, β, the temperature anisotropy, and the ion composition of the plasma. Along with the temperature anisotropy, the electron-to-proton temperature ratio, T_e/T_p, is of great relevance for the characterisation of the thermal properties of a plasma. This ratio varies significantly between different space plasma environments. Thus, studying how variations in this ratio affect the polarisation properties of electromagnetic waves becomes highly relevant for our understanding of the dynamics of space plasmas.

Aims. We present an extensive study on the effect of the thermal properties of electrons on the behaviour and characteristics of Alfvénic waves in fully kinetic linear theory, as well as on the transition from the electromagnetic ion-cyclotron wave to the kinetic Alfvén wave.

Methods. We solved the fully kinetic dispersion relation for oblique electromagnetic waves of the Alfvén branch in a homogenous Maxwellian electron-proton plasma. We quantified the effect of the thermal properties of electrons by varying the electron-to-proton temperature ratio for different configurations of the propagation angle, β_p = 8πnkT_p/B², and wavenumber.

Results. We show that the temperature ratio, T_e/T_p, has strong and non-trivial effects on the polarisation of the Alfvénic modes, especially at kinetic scales (k_⊥ρ_L > 1, where k_⊥ = k sin θ, θ is the propagation angle, and ρ_L = c_s/Ω_p, with c_s the plasma sound speed and Ω_p the proton’s gyrofrequency) and β_e + β_p > 0.5. We conclude that electron inertia plays an important role in the kinetic scale physics of the kinetic Alfvén wave in the warm plasma regime, and thus cannot be excluded in hybrid models for computer simulations.