Authors: Jiaming Wang (Department of Physics and Astronomy, University of Delaware), Rohit Chhiber (Heliophysics Science Division, NASA Goddard Space Flight Center), Sohom Roy (Department of Physics and Astronomy, University of Delaware), Manuel E. Cuesta (Department of Astrophysical Sciences, Princeton University), Francesco Pecora (Department of Physics and Astronomy, University of Delaware), Yan Yang (Department of Physics and Astronomy, University of Delaware), Xiangrong Fu (Los Alamos National Laboratory), Hui Li (Los Alamos National Laboratory), William H. Matthaeus (Department of Physics and Astronomy, University of Delaware)

Plasma turbulence in the solar wind has been widely studied to exhibit anisotropy in both velocity and magnetic field fluctuations, in which the autocorrelations parallel to the large-scale mean magnetic field differ from those in transverse directions. In this study, we explore the related yet understudied aspect of plasma density correlation anisotropy in the energy-containing and inertial ranges of solar wind turbulence. Using decade-long (1998-2008) in situ measurements from the Advanced Composition Explorer (ACE) mission, we find that for all but fast solar wind streams, the density correlation scale in directions quasi-parallel to the mean magnetic field is larger than that in the quasi-perpendicular directions. The correlation scale in fast wind is almost isotropic. Additionally, we investigate the variation of anisotropy with the level of correlation. We find that at lower correlation levels, i.e., at energy-containing scales and larger, the density fluctuations exhibit near-isotropic behavior for fast wind, and slightly favor rapid decorrelation in perpendicular directions for slow and medium winds. At relatively smaller turbulence inertial range scales where correlation values are higher, the sense of anisotropy reverses, suggesting a more “slab-like” structure, in all speed ranges and especially in fast wind samples. This finding stands in contrast to the previously published results on velocity and magnetic field correlations.