Alfvénic Turbulence and Kinetic Instabilities in the Expanding Solar Wind: Two-Dimensional Hybrid Simulations

Abstract:

Couplings between large and small scales in solar wind turbulence are further complicated by the expansion, which acts at all scales and directly influences the particle thermodynamics. We present 2-D hybrid simulations of kinetic turbulence, including the effects of radial expansion by means of the hybrid expanding model (HEB). We investigate properties of the cascade first in standard hybrid simulations and then we analyze the effects of a slow expansion on the turbulent spectrum (spectral break, residual energy, cross-helicity). Incoherent spectra of balanced and inbalanced counter-propagating Alfvén waves are taken as initial conditions and the dependence on the plasma beta and the amplitude of fluctuations is investigated. We focus on the properties of the ion parallel and perpendicular heating driven by the turbulence, and on how this is modulated by the expansion. Turbulence shapes the properties of the plasma, generating local temperature anisotropy in the distribution functions, however the associated perpendicular heating and parallel cooling are not strong enough to counteract the expansion-driven anisotropic cooling. As a consequence, the plasma is driven towards the fire hose instability threshold with increasing heliocentric distance, in agreement with solar wind observations. Once the plasma enters into the fire hose unstable region, electromagnetic fluctuations driven by the ion temperature anisotropy are generated on top of the background turbulence. Despite the configuration of our simulations - out-of-plane mean magnetic field - which allows for the growth of only a subset of fire hose fluctuations, these waves are able to locally scatter the protons and partially reduce their unstable temperature anisotropy. Our findings show that kinetic instabilities driven by anisotropic distributions, like fire hose, can play a role also in turbulent and inhomogeneous plasmas, and suggest that these mechanisms are at work in the solar wind expansion.