Abstract
We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes - namely the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving, while varying the Alfven Mach number (MA), plasma beta, and the sonic Mach number from sub-sonic to trans-sonic. 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 Alfven mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high MA. The slow mode properties are similar to the Alfven 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 MA. 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.
Highlights
Plasma turbulence plays an important role in various astrophysical processes
We have identified a regime of low-Mach number (MA) compressively driven turbulence where the isotropic fast mode can dominate over the Alfven mode in the kz spectrum, which is the relevant one for gyroresonance of particles (ω − kzvk 1⁄4 nΩ, with Ω the gyrofrequency)
We have analyzed the properties of the different MHD modes in solenoidally and compressively driven turbulence, which is highly relevant for astrophysical plasmas
Summary
Plasma turbulence plays an important role in various astrophysical processes. It is important in solar wind heating and acceleration [1], it regulates star formation processes [2,3,4], and it scatters cosmic rays [5], to name a few. Unlike Alfven modes, which preferentially cascade in the field-perpendicular direction, fast modes seem to show an isotropic cascade This result has led to several important implications for astrophysical 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] These simulations have been driven incompressively by solenoidal forcing [15,19,22,23,24]. We perform higher-resolution studies and find the isotropic nature of the fast modes throughout the inertial range, suggesting no scale-dependent anisotropy. Another related question is whether fast modes show an MA-dependent behavior like Alfven modes in terms of weak or strong turbulence. This study shows that the nature of MHD turbulence can be different depending on a variety of parameters, the nature of driving, and this has important implications for understanding the effect of this turbulence on related problems
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