Magnetic field fluctuations in anisotropic, supersonic turbulence

Abstract

The rich structure that we observe in molecular clouds is due to the interplay between strong magnetic fields and supersonic (turbulent) velocity fluctuations. The velocity fluctuations interact with the magnetic field, causing it too to fluctuate. Using numerical simulations, we explore the nature of such magnetic field fluctuations, $\overrightarrow{\delta B}$, over a wide range of turbulent Mach numbers, $\mathcal{M}=2-20$ (i.e., from weak to strong compressibility), and Alfvén Mach numbers, ${\mathcal{M}}_{\text{A}0}=0.1-100$ (i.e., from strong to weak magnetic mean fields, $B_0$). We derive a compressible quasi-static fluctuation model from the magnetohydrodynamical (MHD) equations and show that velocity gradients parallel to the mean magnetic field give rise to compressible modes in sub-Alfvénic flows, which prevents the flow from becoming two-dimensional, as is the case in incompressible MHD turbulence. We then generalise an analytical model for the magnitude of the magnetic fluctuations to include $\mathcal{M}$, and find $|\overrightarrow{\delta B}|=\delta B=c_s\sqrt{\pi {\rho }_0}\mathcal{M}{\mathcal{M}}_{\text{A}0}$, where $c_s$ is the sound speed and ${\rho }_0$ is the mean density of gas. This new relation fits well in the strong $B$-field regime. We go on to study the anisotropy between the perpendicular ($B_{\perp }$) and parallel ($B_{\parallel }$) fluctuations and the mean-normalised fluctuations, which we find follow universal scaling relations, invariant of $\mathcal{M}$. We provide a detailed analysis of the morphology for the $\delta B_{\perp }$ and $\delta B_{\parallel }$ probability density functions and find that eddies aligned with $B_0$ cause parallel fluctuations that reduce $B_{\parallel }$ in the most anisotropic simulations. We discuss broadly the implications of our fluctuation models for magnetised gases in the interstellar medium.

19 pages, 9 figures. Accepted for publication in MNRAS

References in corpus (35)

• Theory of Star Formation
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• Inefficient star formation through turbulence, magnetic fields and feedback
• The Fractal Density Structure in Supersonic Isothermal Turbulence: Solenoidal versus Compressive Energy Injection
• Magnetic Fields in High-Mass Infrared Dark Clouds
• The link between turbulence, magnetic fields, filaments, and star formation in the Central Molecular Zone cloud G0.253+0.016
• A robust numerical scheme for highly compressible magnetohydrodynamics: Nonlinear stability, implementation and tests
• The effect of magnetic fields on star cluster formation
• Magnetic field amplification in turbulent astrophysical plasmas
• The molecular cloud lifecycle
• Velocity Gradients as a Tracer for Magnetic Fields
• The density structure and star formation rate of non-isothermal polytropic turbulence
• The Turbulent Dynamo in Highly Compressible Supersonic Plasmas
• Mapping the Magnetic Interstellar Medium in Three Dimensions Over the Full Sky with Neutral Hydrogen
• Turbulence and star formation efficiency in molecular clouds: solenoidal versus compressive motions in Orion B
• Tracing interstellar magnetic field using velocity gradient technique: Application to Atomic Hydrogen data
• Moving mesh simulations of star forming cores in magneto-gravo-turbulence
• Magnetic Field Morphology in Interstellar Clouds with the Velocity Gradient Technique
• The impact of stellar feedback on the density and velocity structure of the interstellar medium
• Anisotropy in Quasi-Static Magnetohydrodynamic Turbulence
• MHD Turbulence
• Measuring the Alfvenic Nature of the Interstellar Medium: Velocity Anisotropy Revisited
• Striations in molecular clouds: Streamers or MHD waves?
• Large-scale magnetic fields in Bok globules
• Binary Star Formation and the Outflows from their Discs
• Filaments and striations: anisotropies in observed, supersonic, highly-magnetised turbulent clouds
• On the intermittent character of interplanetary magnetic field fluctuations
• Molecular cloud formation by compression of magnetized turbulent gas subjected to radiative cooling
• The relation between the turbulent Mach number and observed fractal dimensions of turbulent clouds
• The role of initial magnetic field structure in the launching of protostellar jets
• The link between solenoidal turbulence and slow star formation in G0.253+0.016
• The world's largest turbulence simulations

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