Reynolds Number and Intermittency in the Expanding Solar Wind: Predictions Based on Voyager Observations

Abstract: The large scale features of the solar wind are examined in order to predict small scale features of turbulence in unexplored regions of the heliosphere. The strategy is to examine how system size, or effective Reynolds number, varies, and then how this quantity influences observable statistical properties, including intermittency properties of solar wind turbulence. The expectation based on similar hydrodynamics scalings, is that the kurtosis, of the small scale magnetic field increments, will increase with increasing Reynolds number. Simple theoretical arguments as well as Voyager observations indicate that effective interplanetary turbulence Reynolds number decreases with increasing heliocentric distance. The decrease of scale-dependent magnetic increment kurtosis with increasing heliocentric distance, is verified using a newly refined Voyager magnetic field dataset. We argue that these scalings continue to much smaller heliocentric distances approaching the Alfven critical region, motivating a prediction that the Parker Solar Probe spacecraft will observe increased magnetic field intermittency, stronger current sheets, and more localized dissipation, as its perihelion approaches the critical regions. Similar arguments should be applicable to turbulence in other expanding astrophysical plasmas.