Generation of Large-Scale Density Fluctuations near the Sun

As a signature of compressible solar wind, enhanced density fluctuations are reported by in-situ and remote sensing observations near the Sun. However, their generation mechanism remains elusive. In this study, we use MHD simulations to study the nature of fluid-scale density fluctuations and explore two possible generation mechanisms. The first one is parametric decay instability (PDI), where coherent density fluctuations are associated with the slow mode excited by a large amplitude Alfven wave in the low-beta environment. The PDI process is essentially 1D and it could explain some observed density fluctuations within ~15 solar radii, even those in the lower solar atmosphere. The second mechanism is nonlinear coupling to incompressible Alfvenic structures, which are found in 3D driven turbulence simulations. The structures are perpendicular to the background magnetic field. They have very low frequency and do not follow dispersion relation of any linear MHD wave. We examine the relation and interplay between these two mechanisms.