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The Reynolds number, Re, is an important quantity for describing a turbulentflow. It tells us about the bandwidth over which energy can cascade from largescales to smaller ones, prior to the onset of dissipation. However, calculatingit for nearly collisionless plasmas like the solar wind is challenging.Previous studies have used "effective" Reynolds number formulations, expressingRe as a function of the correlation scale and either the Taylor scale or aproxy for the dissipation scale. We find that the Taylor scale definition ofthe Reynolds number has a sizeable prefactor of approximately 27, which has notbeen employed in previous works. Drawing from 18 years of data from the Windspacecraft at 1 au, we calculate the magnetic Taylor scale directly and useboth the ion inertial length and the magnetic spectrum break scale asapproximations for the dissipation scale, yielding three distinct Re estimatesfor each 12-hour interval. Average values of Re range between 116,000 and3,406,000, within the general distribution of past work. We also findconsiderable disagreement between the methods, with linear associations ofbetween 0.38 and 0.72. Although the Taylor scale method is arguably morephysically motivated, due to its dependence on the energy cascade rate, moretheoretical work is needed in order to identify the most appropriate way ofcalculating effective Reynolds numbers for kinetic plasmas. As a summary of ourobservational analysis, we make available a data product of 28 years of 1 ausolar wind and magnetospheric plasma measurements from Wind.

Statistics of Turbulence in the Solar Wind. I. What is the Reynolds Number of the Solar Wind?