Fluctuating-Moment Effects in the Solar Wind: A Kinetic Response to Compressive Plasma Turbulence
Abstract: The solar wind is a magnetized plasma that exhibits turbulent fluctuations on many scales. In addition to the non-compressive Alfvénic component of the turbulence, a compressive component with slow-mode-like polarization is commonly observed. These fluctuations are associated with characteristic spatio-temporal changes in the particle distribution functions. If the amplitude of the turbulent fluctuations is large enough, the moments of the fluctuating distribution functions cross the thresholds for various kinetic micro-instabilities. Effects of this type are called "fluctuating-moment effects." Collisional relaxation rates are often large compared to the expansion time of the solar wind. Therefore, non-equilibrium structures in the particle distribution functions can survive. Although a number of kinetic micro-instabilities are known to limit these deviations from equilibrium, in-situ observations indicate that the plasma is closer to the equilibrium state than predicted, even when collisions are insufficient to account for this reduction in the non-equilibrium features. Fluctuating-moment effects offer an explanation for this discrepancy between theoretical predictions and observations. They can reduce common non-equilibrium features such as field-aligned beams, relative drifts among particle species, temperature anisotropies, and non-zero heat flux below the linear instability thresholds. These properties can affect the global energetics as well as the behavior of waves, turbulence, and energetic particles in the plasma.