Evolution of turbulent magnetic fluctuation power with heliospheric distance

On the basis of transport theories appropriate to a radially expanding solar wind, new results for the evolution of the energy density in solar wind fluctuations at MHD scales are derived. The models, which represent a departure from the well‐known WKB description, include the effects of “mixing”, driving by stream‐stream interactions (compression and shear) and interstellar pick‐up ions as well as non‐isotropic MHD turbulence. Magnetometer data from Voyager 1 and 2 and Pioneer 11 are compared to the turbulence‐based models and close agreement is found between theory and data for a reasonable choice of parameters.