Numerical simulations of high cross-helicity turbulence from 0.2 to 1 AU
Résumé
Turbulence in the fast stream of the solar wind is maintained despitethe small compressibility and a dominance of outward-propagatingfluctuations ( z^+>z^- , in contrast to its rapid decay in imbalancedhomogenous MHD turbulence. We numerically study if the inhomogeneityintroduced by solar wind expansion can be an effective source of z^-that maintains turbulence. Starting at 0.2 AU with z^-=0 , we obtain adamping 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, whilethe z^- spectrum has a -5/3 power-law index at all distances. Theseproperties are robust against variations of the input spectrum andexpansion rate and are in agreement with in-situ data, suggesting thatimbalanced turbulence can be maintained by expansion alone.