Turbulence, waves, and thermodynamics in expanding, collisionless plasma

The plasma composing many different astrophysical systems of interest – from the solar wind to the intracluster medium of galaxy clusters – is often weakly collisional or collisionless, with the Larmor radii of the constituent particles being many orders of magnitude below their Coulomb mean free paths. This feature results in a complex interplay between a plasma's macrophysical evolution (e.g., due to expansion, compression, or large-scale shear) and its microphysical response (e.g., triggering of kinetic instabilities). In this talk, the results of several hybrid-kinetic simulations that elucidate this phenomenon will be presented. We show how the nonlinear dynamics of strong Alfvénic turbulence in a collisionless plasma efficiently adapts to changes in fundamental wave physics that are induced by the effect of macroscopic expansion on microscopic particle motions. This adaptation holds irrespective of a qualitative transformation to the plasma’s thermodynamics caused by pressure-anisotropy-driven kinetic instabilities. We also demonstrate that different rates of expansion can lead to qualitatively distinct thermodynamic states, with dramatic ramifications for both macroscopic wave and turbulent dynamics. Our results may help to disentangle the signatures of kinetic instabilities and strong Alfvénic turbulence in key observables in the near-Earth solar wind, such as magnetic power spectra and ion velocity distribution functions.