Coexistence of Slow-mode and Alfvén-mode Waves and Structures in 3D Compressive MHD Turbulence

The compressible component of solar wind turbulence displays a slow-mode feature. However, the nature of the slow-mode fluctuations remain open. In this work, based on numerical simulations of the driven compressible magnetohydrodynamic (MHD) turbulence with a uniform mean magnetic field, we use polarization of the MHD modes to decompose turbulent velocity and magnetic fields into Alfvén modes, slow modes, and fast modes. The numerical results with different cross-helicity, plasma beta β, and Alfvén Mach number note that fast modes are a marginal component among the three decomposed modes, and the compressible component of the MHD turbulence behaves mainly as the slow modes. Both of the decomposed slow modes and Alfvén modes exhibit a Kolmogorov-like power-law spectrum and evident anisotropy, with wavevectors mainly distributing around the directions perpendicular to the uniform mean field. For the first time, it is found that the propagating slow magnetosonic waves as well as the non-propagating slow-mode structures are combined to contribute to the compressible fluctuations, and the propagating Alfvén waves as well as the non-propagating Alfvén-mode structures coexist for the non-compressible fluctuations. However, there is unlikely a one-to-one match between the identified slow waves and Alfvén waves, or between the identified slow-mode structures and Alfvén-mode structures. These findings provide a new perspective on our understanding of the compressible and non-compressible fluctuations.