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

Abstract

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 noncompressible fluctuations.

Energy budget in decaying compressible MHD turbulence

“…For example, characteristics for three separate propagating linear eigenmodes – the Alfvén mode, the slow magnetosonic mode and the fast magnetosonic mode have been studied using numerical simulations (Cho & Lazarian 2002; Kowal & Lazarian 2010; Yang et al. 2018; Makwana & Yan 2020) and observations (Yao et al. 2011; Howes et al.…”

“…A decomposition of the turbulent field is frequently implemented to study different compressible MHD turbulence processes. For example, characteristics for three separate propagating linear eigenmodes -the Alfvén mode, the slow magnetosonic mode and the fast magnetosonic mode have been studied using numerical simulations (Cho & Lazarian 2002;Kowal & Lazarian 2010;Yang et al 2018;Makwana & Yan 2020) and observations (Yao et al 2011;Howes et al 2012;Klein et al 2012). In this work, we decompose the velocity field into solenoidal and compressive parts following Helmholtz decomposition as used in Kida & Orszag (1990, 1992, Miura & Kida (1995), Pan & Johnsen (2017), Wang et al (2019).…”

Properties of Magnetohydrodynamic Modes in Compressively Driven Plasma Turbulence

“…While fast modes can play an important role in scattering of cosmic rays, simulations have shown that the fast modes might only be a marginal component of compressible turbulence [22]. However, these simulations have been driven incompressively by solenoidal forcing [15,19,[22][23][24]. Thus, a natural question to ask is whether and how the nature of the forcing affects the mode composition of turbulence.…”

Magnetohydrodynamic turbulence and propagation of cosmic rays: theory confronted with observations

“…While fast modes can play an important role in scattering of cosmic rays, simulations have shown that the fast modes might only be a marginal component of compressible turbulence. However, these simulations have been driven incompressively by solenoidal forcing [24,[30][31][32]. It is important to identify whether and how changing the nature of the forcing affects the mode composition of turbulence.…”