Correlation anisotropy of the solar wind density fluctuations at 1 au

Plasma turbulence in the solar wind has been widely studied to exhibit local anisotropy in its velocity and magnetic field fluctuations, in which fluctuations are greater in directions perpendicular to the large scale mean magnetic field. Density spectra have been previously measured using in situ solar wind data [1], although density anisotropy in the inertial range is less frequently examined. Here, we explore the autocorrelations of the density fluctuation field in the energy containing and inertial ranges, using 10 years of ACE observations taken from 1998 to 2008. We observe an increase in plasma density correlation length in the mean magnetic field direction. Additionally, the density correlation lengths follow a lognormal distribution, similar to that of the magnetic correlation lengths. What we find at 1 au using in situ data agree with the previously reported inertial range density anisotropy in the solar corona using remote sensing techniques [2].

[1] Bellamy et al, JGR 110, A10104 (2005). doi:10.1029/2004JA010952.

[2] Armstrong et al, ApJ, 358, 685 (1990). doi:10.1086/169022.