von Karman Correlation Similarity in Solar Wind Magnetic Turbulence: Comparison of near-Sun (below 0.17 AU) observations from Parker Solar Probe with ACE measurements at 1 AU

Abstract:

In hydrodynamic turbulence a major development underlying much of modern turbulence theory is the similarity decay hypothesis due to von Karman and Howarth. One implication is that during evolution from a specified initial value, the second order correlation undergoes a continuous transformation based on a universal functional form, evolving due to a rescaling of energy and length. Solar wind turbulence is known to follow many principles adapted from classical fluid turbulence, but until this time the analogous similarity property has not been examined explicitly. Here we analyze a large ensemble of magnetic correlation functions computed from ACE data at 1 AU and Parker Solar Probe data from its first three solar encounters (with perihelia below 0.17 AU), to examine whether the two-point correlation functions undergo a collapse to a similarity form of the type anticipated from von Karman's hypothesis, and also to investigate change in their behavior as a function of heliocentric distance. This study examines for the first time a firm empirical basis for employing the similarity decay hypothesis in space turbulence, a procedure of substantial utility in data analysis as well as dynamical modeling.