Compressible turbulence and density fluctuations in the solar wind

Abstract:

The solar wind is a magnetized and turbulent plasma. The turbulence is often dominated by Alfvénic fluctuations and deemed as nearly incompressible away from the Sun, as shown by in-situ measurements near 1AU. However, when it gets closer to the Sun, transonic fluctuations (with turbulent Mach number M_t -- defined as the ratio of fluctuating velocity to sound speed -- approaching unity) drive significantly more compressible turbulence, characterized by enhanced density fluctuations. In this study, we analyze observations from the first two perihelion of Parker Solar Probe (~ 0.16 AU), and show that the turbulence at the injection scale is compressible during perturbed times such as switchbacks, with M_t as high as 0.5 and yields enhanced density fluctuations in the inertial range. These observations are compared with those from ACE at 1AU, where the turbulence is nearly incompressible and typical value of M_t is less than 0.3. A series of 3D MHD simulations of turbulence are carried out to better understand the properties of compressible turbulence and the generation of density fluctuations. We show that with high turbulence Mach number, a large portion of the density fluctuations are associated with low-frequency nonlinear structures, exceeding density fluctuations produced by compressible MHD modes. We also study their dependence on plasma beta, cross helicity and different types of driving. Compressible turbulence can play an important role in the near-the-Sun region, contributing to both parallel and perpendicular ion heating that has distinct signatures from the incompressible turbulence.