The effect of the thermal properties of electrons on the dispersion of kinetic Alfvén waves

The kinetic Alfvén wave (KAW) is an extension of the Alfvén mode to the kinetic scales that propagates at quasi-perpendicular angles with respect to the background magnetic field in a magnetized plasma. The KAW is characterized by a right-handed polarization in the plane orthogonal to the background magnetic field. This allows the KAW to resonate with electrons, as contrary to the electromagnetic ion-cyclotron (EMIC) wave, which resonates with positive ions due to it’s its left-handed polarization. The EMIC mode is also a kinetic extension of the classical Alfvén wave, and propagates at quasi-parallel wavenormal angles. Due to the relevance and overall presence of both of these modes in space plasmas, understanding the nature of the transition from the EMIC mode to the KAW is a matter of great interest for the study of the micro-scale physics of these systems.

The transition from left-hand to right-hand polarized Alfvén waves depends on the wavenumber, plasma beta, temperature anisotropy, and 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 characterization of the thermal properties of a plasma. This ratio varies significantly between different space plasma environments. Thus, studying how variations on this ratio affect the polarization properties of electromagnetic waves becomes of high relevance highly relevant
for our understanding of the dynamics of space plasmas.

In this work, 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 EMIC to KAW. We show that the temperature ratio T_e/T_p has strong and non-trivial effects on the polarization of the Alfvénic modes, especially at kinetic scales (k_┴ρ_L>1, where k_┴=k sinθ and ρ_L= cs/Ω_p, with c_s the plasma sound speed and Ωp the proton’s gyrofrequency) and β_e+β_p>0.5, where β_s=8πnkT_s/B² is the ratio between thermal and magnetic pressure. We conclude that electron inertia plays an important role in the kinetic scale physics of the KAW in the warm plasma regime, and thus cannot be excluded in hybrid models for computer simulations.