Reynolds Number and Viscosity in the Solar Wind

Abstract The Reynolds number is a key dimensionless parameter used to distinguish turbulent flow from laminar flow. We present generalized expressions for both the Reynolds number and viscosity, incorporating the spectral indices of the inertial range, applicable across Kolmogorov, Iroshnikov–Kraichnan (IK), and k −2 scaling regimes. We show that turbulence governed by Kolmogorov scaling yields a larger Reynolds number than that governed by IK scaling, while viscosity behaves oppositely. We apply these generalized formulations to the data from Parker Solar Probe encounters 1 through 17 and Solar Orbiter encounters 1 through 4 to study the radial evolution of Reynolds number and viscosity from ∼15 R ⊙ to 1 au. We find that the Reynolds number is highest for the steepest spectral slopes and lowest for the shallowest ones, while viscosity shows the opposite trend, being lowest for the steepest slopes and highest for the shallowest ones throughout the inner heliosphere.