Authors: Xiangrong Fu (New Mexico Consortium), Hui Li (Los Alamos National Laboratory), Zhaoming Gan (New Mexico Consortium), John Steinberg (Los Alamos National Laboratory)

The solar wind is a magnetized and turbulent plasma. Its turbulence is often dominated by Alfv ́enic
fluctuations and deemed as nearly incompressible away from the Sun, as shown by in-situ measure-
ments near 1AU. However, for solar wind closer to the Sun, the plasma β decreases (often lower than
unity) while the turbulent Mach number Mt increases (can approach unity, e.g., transonic fluctuations).
These conditions could produce significantly more compressible turbulence, characterized by enhanced
density fluctuations, as seen by several space missions. In this paper, a series of 3D MHD simulations
of turbulence are carried out to understand the properties of compressible turbulence, particularly the
generation of density fluctuations. We find that, over a broad range of parameter space in plasma β,
cross helicity and polytropic index, the turbulent density fluctuations scale linearly as a function of Mt,
with the scaling coefficients showing some dependence on the parameters. Furthermore, through de-
tailed spatio-temporal analysis, we show that the density fluctuations are dominated by low-frequency
nonlinear structures, rather than compressible MHD eigen-waves. These results will be important for
understanding how compressible turbulence contributes to solar wind heating near the Sun.