Evolution of energy‐containing turbulent eddies in the solar wind

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Abstract
Previous theoretical treatments of fluid‐scale turbulence in the solar wind have concentrated on describing the state and dynamical evolution of fluctuations in the inertial range, which are characterized by power law energy spectra. In the present paper a model for the evolution of somewhat larger, more energetic magnetohydrodynamic (MHD) fluctuations is developed by analogy with classical hydrodynamic turbulence in the quasi‐equilibrium range. The model is constructed by assembling and extending existing phenomenologies of homogeneous MHD turbulence, as well as simple two‐length‐scale models for transport of MHD turbulence in a weakly inhomogeneous medium. A set of equations is presented for the evolution of the turbulence, including the transport and nonlinear evolution of magnetic and kinetic energy, cross helicity, and their correlation scales. Two versions of the model are derived, depending on whether the fluctuations are distributed isotropically in three dimensions or restricted to the two‐dimensional plane perpendicular to the mean magnetic field. This model includes a number of potentially important physical effects that have been neglected in previous discussions of transport of solar wind turbulence. Numerical solutions are shown for several cases of interest that demonstrate the advantages of this approach. We suggest that this model may prove useful in studies of solar wind heating and acceleration, as well as in describing the response of interplanetary turbulence to wave energy injected by pickup ions and planetary upstream waves.