Alfv\'{e}nic turbulence in an expanding, collisionless, magnetized plasma*

Using hybrid-kinetic particle-in-cell simulations, we study the evolution of an expanding, collisionless, magnetized plasma in which Alfv\'{e}nic 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 Alfv\'{e}nic fluctuations is modified by this pressure anisotropy. For a sufficiently large plasma beta, the plasma eventually becomes unstable to the oblique firehose instability, 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's expansion continues. The resulting evolution of parallel and perpendicular temperatures is non-adiabatic. Predictions may be tested by measurements of high-beta plasma in the near-Earth solar wind and have implications for understanding the interplay between macro- and micro-scale physics in hot, dilute, astrophysical plasmas.