The Transport of Low-Frequency Turbulence in Astrophysical Flows. II. Solutions for the Super-Alfvenic Solar Wind

Abstract:

Zank et al. 2012 developed a low-frequency turbulence transport model for any magnetized inhomogeneous flow. The model describes the energy corresponding to forward and backward propagating modes, the residual energy, and the correlation lengths corresponding to forward and backward propagating modes and the residual energy. We apply the Zank et al. model to the super-Alfvénic solar wind, considering i) the heliosphere from 0.29 to 5 AU with and without the Alfvén velocity, and ii) the entire heliosphere from 0.29 to 100 AU in the absence of the Alfvén velocity. The model shows that (1) shear driving is responsible for the in situ generation of backward propagating modes, (2) the inclusion of the background magnetic field modifies the transport of turbulence in the inner heliosphere, (3) the correlation lengths of forward and backward propagating modes are almost equal beyond ∼30 AU, and (4) the fluctuating magnetic and kinetic energies in MHD turbulence are in approximate equipartition beyond ∼30 AU. Model results for each case are compared to observations, using Helios 2 and Ulysses observations for the first case, and Voyager 2 data for the second case. For the Voyager 2 observations, we calculate the turbulent quantities corresponding to a positive and negative sign of B_r and B_t, and the azimuthal angle φ=tan−1(B_t /B_r ). The model reproduces the observations quite well from 0.29 to 5 AU. The outer heliosphere (>1 AU) observations are well described by the model. The temporal and latitudinal dependence of the observations makes a detailed comparison difficult but the overall trends are well captured by the models. We conclude that the results reasonably validate the Zank et al. model for the super-Alfvénic solar wind.