in collaboration with K. Yuen, S. Du (LANL); Z. Gan, X. Fu (NMC), H. Yan (DESY)
Recent Parker Solar Probe (PSP) measurements of the near-Sun solar wind have provided in-situ measurements of compressible magnetized turbulence. Understanding their properties has led us to examine several physical processes including the temporal properties of compressible MHD turbulence, scalings of density fluctuations and the magnetic energy dissipation via compressible effects in reconnecting regions. Specifically, through 4D FFT analysis (temporal plus 3D spatial) of compressible MHD turbulence, most fluctuations are found to have very low frequencies with finite wavenumbers and they do not follow the dispersion surfaces of linearized MHD waves. We propose a broadened Lorentzian model in frequency dependence and find quite good agreement between the theory and simulations (Yuen et al. 2023). One implication of this understanding is the re-examination of the mechanisms responsible for density fluctuations in compressible turbulence, along with the properties observed by PSP (Fu et al. 2023). In addition, we present simulations of 3D reconnection to demonstrate that the compressible processes give rise to an additional phase that produces even more overall magnetic energy conversion than that by the initial relaxation of magnetic curvature (Du et al. 2022). These processes could play a role in the continued heating of the background plasma such as solar wind and magnetized outflows.