Propagation of waves in weakly ionized two-fluid plasmas. I. Small-amplitude Alfv\'enic waves

TL;DR

This paper investigates how weak ionization and collisions affect the propagation and damping of Alfvénic and other MHD waves in astrophysical plasmas, revealing polarization-dependent damping and energy distribution effects.

Contribution

It derives a dispersion relation for small-amplitude waves in weakly ionized plasmas including collisions and Hall effects, highlighting polarization-dependent damping and energy imbalance.

Findings

Low-frequency waves are weakly damped and strongly coupled.

Higher frequencies cause phase shifts and increased damping.

Magnetic energy often exceeds kinetic energy, breaking equipartition.

Abstract

The large abundance of electrically neutral particles has a remarkable impact on the dynamics of many astrophysical plasmas. Here, we use a two-fluid model that includes charge-neutral elastic collisions and Hall's current to study the propagation of magnetohydrodynamic (MHD) waves in weakly ionized plasmas. we derive the dispersion relation for small-amplitude incompressible transverse waves propagating along the background magnetic field. Then, we focus on the polarization relations fulfilled by the eigenmodes and their corresponding ratios of magnetic to kinetic energies, and we study their dependence on the relations between the oscillation, collision and cyclotron frequencies. For low wave frequencies, the two components of the plasma are strongly coupled, the damping due to the charge-neutral interaction is weak and the effect of Hall's term is negligible. However, as the wave frequency increases, phase shifts between the velocity of charges, the velocity of neutrals, and the magnetic field appear, leading to enhanced damping. The effect of collisions on the propagation of waves strongly depends on their polarization state, with the left-handed circularly polarized ion-cyclotron modes being more efficiently damped than the linearly polarized Alfv\'en waves and the right-handed circularly polarized whistler modes. Moreover, the equipartition relation between the magnetic energy and the kinetic energy of Alfv\'en waves does not hold in general when the collisional interaction and Hall's current are taken into account, with the magnetic energy usually dominating over the kinetic energy. This theoretical result extends previous findings from observational and numerical works about turbulence in astrophysical scenarios.