Compressive Solar-Wind Turbulence and Anisotropy-Driven Micro-Instabilities

Compressive fluctuations are a minor yet significant component of solar-wind plasma turbulence. In-situ observations indicate that about ten percent of the energy of turbulent fluctuations is associated with compressive slow magnetosonic modes. Slow-mode fluctuations lead to changes in β and in the temperature anisotropy of all plasma species. If the amplitude of the compressive fluctuations is large enough, the temperature anisotropy crosses one or more instability thresholds for anisotropy-driven micro-instabilities. We explore the ansatz that these instabilities reduce the amplitude of the compressive fluctuations. We discuss this nonlinear damping mechanism and analytically determine its thresholds as well as the relevant wave-polarization properties, using magnetohydrodynamics (MHD) to describe the large-scale fluctuations. We compare our results with numerical solutions to the hot-plasma dispersion relation and find good agreement. The threshold of this nonlinear damping mechanism is about 0.1 for β of order unity in agreement with solar-wind observations. The threshold decreases with increasing β. If our ansatz above is correct, then this damping mechanism has important implications for the composition of plasma turbulence in astrophysical plasmas such as the solar wind as well as for the Fermi-acceleration of energetic particles.