The Fluid-like Behavior of Kinetic Alfv\'{e}n Turbulence in Space Plasma

TL;DR

This study compares two theoretical models of kinetic Alfvén wave turbulence in space plasmas with MMS observations, revealing that a collisional two-fluid approach better explains small-scale fluctuations, which behave fluid-like despite weak collisionality.

Contribution

The paper demonstrates that collisional two-fluid theory more accurately predicts KAW polarization in space plasma turbulence than kinetic theory, highlighting fluid-like behavior at small scales.

Findings

Two-fluid theory aligns better with MMS data than kinetic theory.

Small-scale KAW fluctuations exhibit fluid-like behavior in the magnetosheath.

Predictions for KAW polarizations in the inner heliosphere are provided for future tests.

Abstract

Kinetic Alfv\'{e}n waves (KAWs) are the short-wavelength extension of the MHD Alfv\'{e}n-wave branch in the case of highly-oblique propagation with respect to the background magnetic field. Observations of space plasma show that small-scale turbulence is mainly KAW-like. We apply two theoretical approaches, collisional two-fluid theory and collisionless kinetic theory, to obtain predictions for the KAW polarizations depending on $\beta_\mathrm{p}$ (the ratio of the proton thermal pressure to the magnetic pressure) at the ion gyroscale in terms of fluctuations in density, bulk velocity, and pressure. We perform a wavelet analysis of MMS magnetosheath measurements and compare the observations with both theories. We find that the two-fluid theory predicts the observations better than kinetic theory, suggesting that the small-scale KAW-like fluctuations exhibit a fluid-like behavior in the magnetosheath although the plasma is weakly collisional. We also present predictions for the KAW polarizations in the inner heliosphere that are testable with Parker Solar Probe and Solar Orbiter.