Properties of Magnetohydrodynamic Modes in Compressively Driven Plasma Turbulence

Abstract

We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfvén, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfvén Mach number (M A), plasma β, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfvén-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvénic turbulence as we go from low to high M A. The slow-mode properties are similar to the Alfvén mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed.

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).…”

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

“…Turbulence & Cosmic ray propagation Huirong Yan magnetic pressure, the magnetosonic modes should be considered when studying such turbulence. Studies have shown that the energy spectrum and the scale-dependent anisotropy of slow modes are quite similar to Alfvén modes [24,25,29]. On the other hand, fast modes seem to show an isotropic cascade.…”

“…Our view on the transport of CRs has been rapidly changing, largely thanks to the advances in MHD turbulence [7,8]. MHD turbulence can be decomposed and the interaction of turbulence with CRs can be studied separately in each of the three MHD modes, Alfven, fast and slow, the latter of two are compressible modes [24,25]. It has been demonstrated based on the tested model of turbulence that the scattering of CRs ( 100 GeV) is dominated by fast modes instead of the often-adopted Alfven modes, which indicates the inhomogeneous scattering of CRs [19,20,23] and inefficiency of scattering on low energy CRs.…”

Acceleration of ultrahigh-energy cosmic rays in the early afterglows of gamma-ray bursts

“…Particle scattering and diffusion largely rely on the properties of plasma turbulence. Fast modes waves show an isotropic cascade and it could be the most effective scatterers of cosmic-rays [3]. The spectrum of the isotropic cascade was claimed to be k −3/2 .…”