Solar Wind Magnetic Field Correlation Length: Correlation Functions versus Cross-field Displacement Diffusivity Test

Abstract

Abstract The estimate of the solar wind magnetic fields’ parallel correlation length, λ, be it from the measured fields’ correlation functions or their spectral power at “zero” frequency, have long pointed toward short values on the order of 0.01 au. Evaluation of the mean cross-field displacements (CFDs), however, fails to show the decorrelation and resulting diffusion at the expected scales, pointing instead toward λ values on the order of 0.1 au or more. In an effort to understand this “order-of-magnitude” discrepancy and reconcile the approaches using correlation functions and the CFD diffusivity test, both approaches are applied here, with renewed attention to the “details” as well as the broader sense of the calculations, to a large, 20 yr long set of magnetic field and flow data from the ACE spacecraft. It is found that solar wind intervals too short relative to λ are a likely reason for some underestimate through the correlation-function approach, causing a premature drop of the correlation functions. Once converged to their long-time limit, however, the correlation functions produce magnetic field correlation lengths very much consistent with the magnetic-field-line (MFL) correlation lengths of the diffusivity test, with nearly matching distributions of the correlation lengths corrected by the proper ratio of their theoretical estimates. The fields’ correlation lengths mostly range from 0.03 to 0.08 au, and the MFL correlation lengths from 0.04 to 0.3 au, with peaks at 0.075 and 0.15 au, likely due to nonlinear and quasilinear regimes of MFL wandering. As for the power-at-zero-frequency approach, it is doomed by the solar rotation.