On the Existence of Fast Modes in Compressible Magnetohydrodynamic Turbulence

Gan, Zhaoming; Li, Hui; Fu, Xiangrong; Du, Senbei

doi:10.3847/1538-4357/ac4d9d

Cited by 23 publications

(28 citation statements)

References 42 publications

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“…Furthermore, the majority of wave power resides in the slow mode (8%), and the power of the fast mode is negligible (<1%), which can also be seen in Panel (c). Similar results were reported by Gan et al (2022). Finally, Panel (d) shows the anisotropy of the density fluctuation, consistent with Figure 4(a).…”

Section: Nature Of Density Fluctuationssupporting

confidence: 90%

“…For example, nonlinear structures, which do not belong to any linear mode, can still be decomposed spatially into three MHD modes. Therefore, as pointed out by Gan et al (2022), this decomposition may overestimate the contribution from MHD waves.…”

Section: Nature Of Density Fluctuationsmentioning

confidence: 93%

“…Starting from a quiet uniform plasma (ρ 0 = 1, B 0 = 1, and zero fluctuations), we perturb the MHD system by adding driving terms in both momentum and magnetic field equations (Equations ( 2) and (3)). We maintain the divergence-free condition for the magnetic field by keeping ∇ • f b = 0 to machine precision (Gan et al 2022). For all of the runs, we also keep ∇ • f v = 0 to model incompressible driving at large scales.…”

Section: Numerical Simulationmentioning

confidence: 99%

“…To better analyze fluctuations in the simulations, we employ 4D FFT analysis (3D for space and 1D for time), in which both spatial and temporal information are used to calculate the Fourier power in the ω-k space (Gan et al 2022). Using this approach, the power along the dispersion surfaces of MHD waves (numerically we sum over power within 10% of theoretical frequencies) will be regarded as "waves", and fluctuations elsewhere will be called nonwave structures.…”

Section: Nature Of Density Fluctuationsmentioning

confidence: 99%

“…In principle, the fluctuations in compressible MHD turbulence do not need to satisfy the full dispersion relationships of eigenmodes. Indeed, recent studies of compressible turbulence by Gan et al (2022) showed that velocity and magnetic fluctuations were dominated by nonlinear structures rather than MHD waves in the simulations using an advanced spatiotemporal 4D fast Fourier transform (FFT) analysis. This conclusion is quite different from many previous analyses where the decomposition of fluctuations is based solely on the spatial variations without their temporal properties (see also Dmitruk & Matthaeus 2009;Andrés et al 2017;Yang et al 2019;Brodiano et al 2021).…”

Section: Introductionmentioning

confidence: 99%

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Nature and Scalings of Density Fluctuations of Compressible Magnetohydrodynamic Turbulence with Applications to the Solar Wind

Gan

et al. 2022

ApJ

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The solar wind is a magnetized and turbulent plasma. Its turbulence is often dominated by Alfvénic fluctuations and often deemed as nearly incompressible far away from the Sun, as shown by in situ measurements near 1 au. However, for solar wind closer to the Sun, the plasma β decreases (often lower than unity) while the turbulent Mach number M t increases (can approach unity, e.g., transonic fluctuations). These conditions could produce significantly more compressible effects, characterized by enhanced density fluctuations, as seen by several space missions. In this paper, a series of 3D MHD simulations of turbulence are carried out to understand the properties of compressible turbulence, particularly the generation of density fluctuations. We find that, over a broad range of parameter space in plasma β, cross helicity, and polytropic index, the turbulent density fluctuations scale linearly as a function of M t , with the scaling coefficients showing weak dependence on parameters. Furthermore, through detailed spatiotemporal analysis, we show that the density fluctuations are dominated by low-frequency nonlinear structures, rather than compressible MHD eigenwaves. These results could be important for understanding how compressible turbulence contributes to solar wind heating near the Sun.

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Section: Nature Of Density Fluctuationssupporting

confidence: 90%

Section: Nature Of Density Fluctuationsmentioning

confidence: 93%

Section: Numerical Simulationmentioning

confidence: 99%

Section: Nature Of Density Fluctuationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Nature and Scalings of Density Fluctuations of Compressible Magnetohydrodynamic Turbulence with Applications to the Solar Wind

Gan

et al. 2022

ApJ

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Magnetohydrodynamic instabilities and transition to turbulence

Terradas,

Magyar

2024

Magnetohydrodynamic Processes in Solar Plasmas

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Fast magneto-acoustic wave turbulence and the Iroshnikov–Kraichnan spectrum

Galtier

2023

J. Plasma Phys.

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An analytical theory of wave turbulence is developed for pure compressible magnetohydrodynamics in the small $\beta$ limit. In contrast to previous works where the multiple scale method was not mentioned and slow magneto-acoustic waves were included, we present here a theory for fast magneto-acoustic waves for which only an asymptotic closure is possible in three dimensions. We introduce the compressible Elsässer fields (canonical variables) and show their linear relationship with the mass density and the compressible velocity. The kinetic equations of wave turbulence for three-wave interactions are obtained and the detailed conservation is shown for the two invariants, energy and momentum (cross-helicity). An exact stationary solution (Kolmogorov-Zakharov spectrum) exists only for the energy. We find a $k^{-3/2}$ energy spectrum compatible with the Iroshnikov–Kraichnan (IK) phenomenological prediction; this leads to a mass density spectrum with the same scaling. Despite the presence of a relatively strong uniform magnetic field, this turbulence is characterized by an energy spectrum with a power index that is independent of the angular direction; its amplitude, however, shows an angular dependence. We prove the existence of the IK solution using the locality condition, show that the energy flux is positive and hence the cascade direct and find the Kolmogorov constant. This theory offers a plausible explanation for recent observations in the solar wind at small $\beta$ where isotropic spectra with a $-3/2$ power-law index are found and associated with fast magneto-acoustic waves. This theory may also be used to explain the IK spectrum often observed near the Sun. Besides, it provides a rigorous theoretical basis for the well-known phenomenological IK spectrum, which coincides with the Zakharov–Sagdeev spectrum for acoustic wave turbulence.

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On the Existence of Fast Modes in Compressible Magnetohydrodynamic Turbulence

Cited by 23 publications

References 42 publications

Nature and Scalings of Density Fluctuations of Compressible Magnetohydrodynamic Turbulence with Applications to the Solar Wind

Nature and Scalings of Density Fluctuations of Compressible Magnetohydrodynamic Turbulence with Applications to the Solar Wind

Magnetohydrodynamic instabilities and transition to turbulence

Fast magneto-acoustic wave turbulence and the Iroshnikov–Kraichnan spectrum

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