Theory and Transport of Nearly Incompressible Magnetohydrodynamic Turbulence: High Plasma Beta Regime

Nearly incompressible magnetohydrodynamic (NI MHD) theory for β ∼ 1 (or β << 1) plasma has been developed and applied to the study of solar wind turbulence. The leading order term in β ∼ 1 or β << 1 plasma describes majority 2D turbulence, while the higher-order term describes a minority slab turbulence. Here, we develop new NI MHD turbulence transport model equations in the high plasma beta regime. The leading order term in a β >> 1 plasma is fully incompressible and admits both structures (flux ropes or magnetic islands) and slab (Alfvén waves) fluctuations. This paper couples the NI MHD turbulence transport equations with a three fluid (proton, electron and pickup ion) equations, and solves the 1D steady-state equations from 1 au to 75 au. The model is tested against 27 years of Voyager 2 data, and Ulysses and NH SWAP data. The results agree remarkably well, with some scatter, about the theoretical predictions.