Higher-order Turbulence Statistics in the Sub-Alfvénic Solar Wind Observed by Parker Solar Probe

Abstract

Parker Solar Probe has been the first spacecraft to enter the deep corona below the Alfvén critical point. Here we examine the higher-order statistical properties of magnetic-field fluctuations in the sub-Alfvénic solar wind and compare the results with the neighboring super-Alfvénic region. The intermittency and multifractal properties are analyzed by inspecting the probability density functions, the scale-dependent kurtosis, and fractal spectrum of magnetic-field fluctuations. It is found that the magnetic-field fluctuations present distinct intermittency and multifractal properties in the inertial range and the B R component reveals much higher intermittency and more asymmetrical multifractal spectrum than the other components. The non-Gaussian self-similarity of fluctuations of B R at larger scales has also been observed. Further comparative analysis shows that all the solar wind periods share nearly the same intermittency and multifractal features, and the only difference lies in that the fluctuations of the B T and B N components have slight variations.