# On Kinetic Slow Modes, Fluid Slow Modes, and Pressure-balanced Structures in the Solar Wind

## TL;DR

This paper investigates the nature of slow modes in solar wind turbulence, deriving kinetic dispersion relations, comparing them with fluid models, and analyzing observational data to understand plasma behavior.

## Contribution

It provides analytical gyrokinetic expressions for ion-acoustic wave dispersion and compares kinetic, MHD, and observational results, highlighting discrepancies and fluid-like behavior.

## Key findings

- Kinetic and MHD models show major differences at high plasma beta.
- Observations align better with MHD predictions than kinetic theory.
- Solar wind plasma exhibits more fluid-like behavior than kinetic models suggest.

## Abstract

Observations in the solar wind suggest that the compressive component of inertial-range solar-wind turbulence is dominated by slow modes. The low collisionality of the solar wind allows for non-thermal features to survive, which suggests the requirement of a kinetic plasma description. The least-damped kinetic slow mode is associated with the ion-acoustic (IA) wave and a non-propagating (NP) mode. We derive analytical expressions for the IA-wave dispersion relation in an anisotropic plasma in the framework of gyrokinetics and then compare them to fully-kinetic numerical calculations, results from two-fluid theory, and MHD. This comparison shows major discrepancies in the predicted wave phase speeds from MHD and kinetic theory at moderate to high $\beta$. MHD and kinetic theory also dictate that all plasma normal modes exhibit a unique signature in terms of their polarization. We quantify the relative amplitude of fluctuations in the three lowest particle velocity moments associated with IA and NP modes in the gyrokinetic limit and compare these predictions with MHD results and in-situ observations of the solar-wind turbulence. The agreement between the observations of the wave polarization and our MHD predictions is better than the kinetic predictions, suggesting that the plasma behaves more like a fluid in the solar wind than expected.