Numerical Simulations Of High Cross-Helicity Turbulence From 0.2 To 1 Au

Turbulence in the fast stream of the solar wind is maintained despite the small compressibility and a dominance of outward-propagating fluctuations (z(+) > z(-)), in contrast to its rapid decay in imbalanced homogenous MHD turbulence. We numerically study if the inhomogeneity introduced by solar wind expansion can be an effective source of z(-) that maintains turbulence. Starting at 0.2 AU with z(-) = 0, we obtain a damping with distance of z(+) and a quasi-steady level of z(-). The z(+) spectrum steepens with distance toward a -1.4 power-law at 1 AU, while the z(-) spectrum has a - 5/3 power-law index at all distances. These properties are robust against variations of the input spectrum and expansion rate and are in agreement with in- situ data, suggesting that imbalanced turbulence can be maintained by expansion alone.