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

Yan, Huirong

doi:10.22323/1.395.0038

Cited by 5 publications

(1 citation statement)

References 64 publications

(115 reference statements)

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“…Plasma turbulence, particularly its compressible component, plays a crucial role in numerous astrophysical processes, such as the heating and acceleration of the solar wind, cosmic-ray transport, and star formation (Bruno & Carbone 2013;Yan 2022). The current model of plasma turbulence is typically characterized by three steps: (1) energy injection on large scales (Matthaeus et al 1986;Cho & Lazarian 2003), (2) an inertial energy cascade following some self-similar power-law scaling (Ng & Bhattacharjee 1996;Horbury et al 2008), and (3) dissipation caused by certain kinetic physical processes on small scales (Leamon et al 1998(Leamon et al , 1999Yan & Lazarian 2002;Alexandrova et al 2009;Sahraoui et al 2009).…”

Section: Introductionmentioning

confidence: 99%

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth’s Magnetosheath: Observation Matches Theory

Zhao,

Yan,

Liu

et al. 2024

ApJ

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Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales and affects energetic particle transport. Recent advances in the understanding of compressible magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping in shaping energy distributions on small scales, yet its observational evidence is still lacking. This study provides the first observational evidence of substantial collisionless damping (CD) modulation on the small-amplitude compressible MHD turbulence cascade in Earth’s magnetosheath using four Cluster spacecraft. Based on an improved compressible MHD decomposition algorithm, turbulence is decomposed into three eigenmodes: incompressible Alfvén modes and compressible slow and fast (magnetosonic) modes. Our observations demonstrate that CD enhances the anisotropy of compressible MHD modes because CD has a strong dependence on wave propagation angle. The wavenumber distributions of slow modes are mainly stretched perpendicular to the background magnetic field ( B 0) and weakly modulated by CD. In contrast, fast modes are subjected to a more significant CD modulation. Fast modes exhibit a weak, scale-independent anisotropy above the CD truncation scale. Below the CD truncation scale, the anisotropy of fast modes enhances as wavenumbers increase. As a result, fast-mode fractions in the total energy of compressible modes decrease with the increase of perpendicular wavenumber (to B 0) or wave propagation angle. Our findings reveal how the turbulence cascade is shaped by CD and its consequences for anisotropies in the space environment.

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Section: Introductionmentioning

confidence: 99%

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth’s Magnetosheath: Observation Matches Theory

Zhao,

Yan,

Liu

et al. 2024

ApJ

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Identification of the weak-to-strong transition in Alfvénic turbulence from space plasma

Zhao,

Yan,

Liu

et al. 2024

Nat Astron

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Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales in astrophysical and space plasma systems. Although the theory of anisotropic magnetohydrodynamic (MHD) turbulence has successfully described natural phenomena, its core prediction of an Alfvénic transition from weak to strong MHD turbulence when energy cascades from large to small scales has not been observationally confirmed. Here we report evidence for the Alfvénic weak-to-strong transition in small-amplitude, turbulent MHD fluctuations in Earth’s magnetosheath using data from four Cluster spacecraft. Our observations demonstrate the universal existence of strong turbulence accompanied by weak turbulent fluctuations on large scales. Moreover, we find that the nonlinear interactions of MHD turbulence are crucial to the energy cascade, as they broaden the cascade directions and fluctuating frequencies. The observed connection between weak and strong MHD turbulence systems may be present in many astrophysical environments, such as star formation, energetic particle transport, turbulent dynamos, and solar corona or solar wind heating.

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Diagnosis of 3D magnetic field and mode composition in MHD turbulence with Y-parameter

Malik

Yuen

Yan

2023

Monthly Notices of the Royal Astronomical Society

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Magnetic fields are crucial in numerous astrophysical processes within the interstellar medium. However, the detailed determination of magnetic field geometry is notoriously challenging. Based on the modern magnetohydrodynamic (MHD) turbulence theory, we introduce a novel statistical technique, the ‘Y-parameter’, to decipher the magnetic field inclination in the ISM and identify dominant turbulence modes. The Y-parameter, calculated as the ratio of anisotropies of different Stokes parameter combinations, displays contrasting trends with the mean-field inclination angle in Alfvénic and compressible turbulence modes. A Y-parameter value around 1.5 ± 0.5 provide a statistical boundary to determine the dominant MHD turbulence modes. We have discovered specific correlations between the Y-parameter value and the inclination angle that unveil the dominant turbulence mode. This methodology, when applied to future radio polarisation surveys such as LOFAR and SKA, promises to significantly enhance our knowledge of 3D magnetic field in the ISM and improve our understanding of interstellar turbulence.

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

Cited by 5 publications

References 64 publications

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth’s Magnetosheath: Observation Matches Theory

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth’s Magnetosheath: Observation Matches Theory

Identification of the weak-to-strong transition in Alfvénic turbulence from space plasma

Diagnosis of 3D magnetic field and mode composition in MHD turbulence with Y-parameter

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