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Observations in the solar wind suggest that the compressive component ofinertial-range solar-wind turbulence is dominated by slow modes. The lowcollisionality of the solar wind allows for non-thermal features to survive,which suggests the requirement of a kinetic plasma description. Theleast-damped kinetic slow mode is associated with the ion-acoustic (IA) waveand a non-propagating (NP) mode. We derive analytical expressions for theIA-wave dispersion relation in an anisotropic plasma in the framework ofgyrokinetics and then compare them to fully-kinetic numerical calculations,results from two-fluid theory, and MHD. This comparison shows majordiscrepancies in the predicted wave phase speeds from MHD and kinetic theory atmoderate to high $\beta$. MHD and kinetic theory also dictate that all plasmanormal modes exhibit a unique signature in terms of their polarization. Wequantify the relative amplitude of fluctuations in the three lowest particlevelocity moments associated with IA and NP modes in the gyrokinetic limit andcompare these predictions with MHD results and in-situ observations of thesolar-wind turbulence. The agreement between the observations of the wavepolarization and our MHD predictions is better than the kinetic predictions,suggesting that the plasma behaves more like a fluid in the solar wind thanexpected.

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