Looking for evidence of mixing in the solar wind from 0.31 to 0.98 AU

Abstract

To determine the efficacy of turbulence in the solar wind, the amount of mixing of the solar wind plasma is quantified. If magnetohydrodynamic turbulence in the solar wind acts as does Navier‐Stokes turbulence, the chunk mixture of scalar quantities in the solar wind plasma should evolve with time. In particular, (1) the sizes of plasma chunks should decrease with distance from the Sun owing to the stretching and folding action of turbulence and (2) the occurrence distributions of passive‐scalar values in the plasma should narrow with distance from the Sun owing to homogenization. The evolution of the chunk mix is studied through the use of interface‐crossing statistics. Using Helios 1 and Helios 2 slow‐solar wind measurements from 0.31 to 0.98 AU, temporal jumps in the proton number density, proton specific entropy, and proton beta are used to locate plasma interfaces in the solar wind and the plasma chunk sizes are determined from the distances between the plasma interfaces. No evidence for the evolution of the chunk‐size distribution is seen, indicating an absence of mixing in the solar wind. The distribution of plasma density broadens by 11% with distance, the distribution of proton flux stays constant, while the distribution of proton specific entropy narrows by 14%; collectively the three distributions do not show clear evidence for evolution toward homogeneity, hence they do not show clear evidence for mixing. Implications of the lack of evidence for mixing in the solar wind are discussed.