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−. Thez+ 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.