Mirror and Proton-cyclotron Instabilities Coexisting with Ambient Turbulence in a Proton-Alpha Plasma

We investigate the effect of ambient turbulence on the mirror and proton-cyclotron instabilities in a proton-alpha particle plasma. We perform three-dimensional hybrid simulations with particle-in-cell ions and a quasi-neutralizing electron fluid. The instabilities are driven by the protons with temperature perpendicular to the mean magnetic field larger than the parallel temperature. The description of these instabilities is usually based on the assumption of a uniform and stationary background. However, this assumption is violated by the ambient turbulence. In particular, the turbulent fluctuations modify the particle distribution function by making it spatially inhomogeneous and time-dependent. We compare the properties of the instabilities to the case of a uniform and stationary background and the same average temperature anisotropy and plasma beta. We find that the initial growth rates of the mirror mode are close, but the saturation level is significantly reduced when the turbulence is present. The saturation level of the proton-cyclotron mode is not affected as strongly.