Proton Temperature-anisotropy Instability Coexisting with Ambient Turbulence in the Solar-wind Plasma

Abstract

The description of the thermal-ion instabilities in the solar wind is usually based on the assumption of a uniform and stationary background. However, the instabilities have to coexist with omnipresent turbulence that makes the ambient medium inhomogeneous and time dependent. We perform three-dimensional hybrid simulations with particle-in-cell protons and a quasi-neutralizing electron fluid to investigate the effect of the turbulence on a particular instability. The instability is driven by the proton temperature anisotropy and it would generate Alfvén/proton-cyclotron waves propagating nearly parallel to the mean magnetic field if the turbulence were not present. The turbulence in our simulations is approximately two-dimensional with wavevectors highly oblique to the magnetic field. We find that, first, the turbulence results in a considerable modification of the proton distribution function. This contributes to the properties of the instability, such as its growth rate and saturation level. Second, the spatial inhomogeneity associated with the turbulent fluctuations extends the spectrum of the unstable waves to higher perpendicular wavenumbers making them more oblique. This is similar to the effect exerted on an instability by a static nonuniform background. Despite the quantitative changes introduced by the turbulence, we conclude that the instability remains qualitatively the same.