Comparative study of gyrokinetic, hybrid-kinetic and fully kinetic wave physics for space plasmas

TL;DR

This study compares gyrokinetic, hybrid-kinetic, and fully kinetic models to understand wave physics in space plasmas, revealing their respective strengths and limitations in accurately capturing wave behaviors relevant to solar wind conditions.

Contribution

It provides a comprehensive numerical comparison of three kinetic models for space plasma waves, highlighting their validity ranges and differences in wave physics representation.

Findings

Gyrokinetic model accurately reproduces Alfvén wave physics near its validity range.

Hybrid-kinetic model captures high-frequency waves but underpredicts wave damping.

Gyrokinetic lacks high-frequency solutions and cyclotron effects.

Abstract

A set of numerical solvers for the linear dispersion relations of the gyrokinetic, the hybrid-kinetic, and the fully kinetic model is employed to study the physics of the kinetic Alfv\'en wave and the fast magnetosonic mode in these models. In particular, we focus on parameters that are relevant for solar wind oriented applications (using a homogeneous, isotropic background), which are characterized by wave propagation angles averaging close to 90{\deg}. It is found that the gyrokinetic model, while lacking high-frequency solutions and cyclotron effects, faithfully reproduces the fully kinetic Alfv\'en wave physics close to, and sometimes significantly beyond, the boundaries of its range of validity. The hybrid-kinetic model, on the other hand, is much more complete in terms of high-frequency waves, but owing to its simple electron model it is found to severely underpredict wave damping rates even on ion spatial scales across a large range of parameters, despite containing full kinetic ion physics.