Non-Maxwellianity of Ion Velocity Distributions in the Earth's Magnetosheath

We analyze the ion velocity distribution function (iVDF) deviations from local thermodynamic equilibrium (LTE) in collisionless plasma turbulence. Using data from the Magnetospheric Multiscale (MMS) mission, we examine the non-Maxwellianity of 441,577 iVDFs in the Earth's magnetosheath. We find that the iVDFs' anisotropy is overall limited, while high-order non-LTE features can be significant. Our results show that the complexity of the iVDFs is strongly influenced by the ion plasma beta and the turbulence intensity, with high-order non-LTE features emerging with large-amplitude magnetic field fluctuations. Furthermore, our analysis indicates that turbulence-driven magnetic curvature contributes to the isotropization of the iVDFs by scattering the ions, emphasizing the complex interaction between turbulence and the velocity distribution of charged particles in collisionless plasmas.