PHENOMENOLOGY OF HYDROMAGNETIC TURBULENCE IN A UNIFORMLY EXPANDING MEDIUM

A simple phenomenology is developed for the decay and transport of tur bulence in a constant-speed, uniformly expanding medium. The fluctuati ons are assumed to be locally incompressible, and either of the hydrod ynamic or non-Alfvenic magnetohydrodynamic (MHD) type. In order to rep resent local effects of nonlinearities, a simple model of the Karman-D ryden type for locally homogeneous turbulent decay is adopted. A detai led discussion of the parameters of this familiar one-point hydrodynam ic closure is given, which has been shown recently to be applicable to non-Alfvenic MHD as well. The effects of the large-scale flow and exp ansion are incorporated using a two-scale approach, in which assumptio ns of particular turbulence symmetries provide simplifications. The de rived model is tractable and provides a basis for understanding turbul ence in the outer heliosphere: as well as in other astrophysical appli cations.