Tuesday

An ever-expanding collection of astronomical observations indicate that strongly magnetized plasma turbulence is ubiquitous in many different astrophysical contexts, from the solar wind to the intracluster medium (ICM) of galaxy clusters. While much is now understood about magnetized turbulence in collisional (magnetohydrodynamical) plasmas, the fundamental properties of such turbulence in collisionless plasmas remain uncertain. In this talk, I will discuss results from hybrid-kinetic particle-in-cell simulations that help to address this uncertainty by modelling the evolution of an expanding, collisionless, magnetized plasma in which strong Alfvénic turbulence is persistently driven. Pressure anisotropy generated adiabatically by the plasma expansion (and consequent decrease in the mean magnetic-field strength) gradually reduces the effective elasticity of the field lines, causing residual-energy build-up in the turbulent fluctuations and modifying their spatial anisotropy. Critical balance is maintained even as the linear frequency of the Alfvénic fluctuations is modified by this pressure anisotropy. For a sufficiently large ion plasma-beta parameter, the plasma ultimately becomes unstable to both the oblique and parallel firehose instabilities, which excites rapidly growing magnetic fluctuations at ion-Larmor scales. Through associated pitch-angle scattering of particles, the ion pressure anisotropy is maintained near marginal firehose stability, even as the plasma continues to expand. The resulting evolution of parallel and perpendicular temperatures does not satisfy double-adiabatic conservation laws. Our results have implications for understanding the complex interplay between macro- and micro-scale physics in various hot, dilute, astrophysical plasmas, and may be tested by measurements of high-beta plasma in the near-Earth solar wind.