Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements
We study the properties of sub-Alfvénic magnetohydrodynamic (MHD) turbulence, i.e., turbulence with Alfvén mach number M _A = V _L / V _A < 1, where V _L is the velocity at the injection scale and V _A is the Alfvén velocity. We demonstrate that MHD turbulence can have different properties, depen...
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2024-01-01
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| author | A. Lazarian Ka Wai Ho Ka Ho Yuen Ethan Vishniac |
| author_facet | A. Lazarian Ka Wai Ho Ka Ho Yuen Ethan Vishniac |
| author_sort | A. Lazarian |
| collection | DOAJ |
| description | We study the properties of sub-Alfvénic magnetohydrodynamic (MHD) turbulence, i.e., turbulence with Alfvén mach number M _A = V _L / V _A < 1, where V _L is the velocity at the injection scale and V _A is the Alfvén velocity. We demonstrate that MHD turbulence can have different properties, depending on whether it is driven by velocity or magnetic fluctuations. If the turbulence is driven by isotropic bulk forces acting upon the fluid, i.e., is velocity driven, in an incompressible conducting fluid we predict that the kinetic energy is ${M}_{{\rm{A}}}^{-2}$ times larger than the energy of magnetic fluctuations. This effect arises from the long parallel wavelength tail of the forcing, which excites modes with k _∥ / k _⊥ < M _A . We also predict that as the MHD turbulent cascade reaches the strong regime, the energy of slow modes exceeds the energy of Alfvén modes by a factor ${M}_{{\rm{A}}}^{-1}$ . These effects are absent if the turbulence is driven through magnetic fluctuations at the injection scale. We confirm our predictions with numerical simulations. Since the assumption of magnetic and kinetic energy equipartition is at the core of the Davis–Chandrasekhar–Fermi (DCF) approach to measuring magnetic field strength in sub-Alfvénic turbulence, we conclude that the DCF technique is not universally applicable. In particular, we suggest that the dynamical excitation of long azimuthal wavelength modes in the galactic disk may compromise the use of the DCF technique. We discuss alternative expressions that can be used to obtain magnetic field strength from observations and consider ways of distinguishing the cases of velocity and magnetically driven turbulence using observational data. |
| format | Article |
| id | doaj-art-1377c57733064c6c91c54731d8f7e930 |
| institution | DOAJ |
| issn | 1538-4357 |
| language | English |
| publishDate | 2024-01-01 |
| publisher | IOP Publishing |
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| series | The Astrophysical Journal |
| spelling | doaj-art-1377c57733064c6c91c54731d8f7e9302025-08-20T02:50:47ZengIOP PublishingThe Astrophysical Journal1538-43572024-01-0197818810.3847/1538-4357/ad8d49Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength MeasurementsA. Lazarian0https://orcid.org/0000-0002-7336-6674Ka Wai Ho1https://orcid.org/0000-0003-3328-6300Ka Ho Yuen2https://orcid.org/0000-0003-1683-9153Ethan Vishniac3https://orcid.org/0000-0002-2307-3857Department of Astronomy, University of Wisconsin-Madison , Madison, WI, 53706, USA ; lazarian@astro.wisc.edu, kho33@wisc.eduDepartment of Astronomy, University of Wisconsin-Madison , Madison, WI, 53706, USA ; lazarian@astro.wisc.edu, kho33@wisc.edu; Theoretical Division, Los Alamos National Laboratory , Los Alamos, NM 87545, USA ; kyuen@lanl.govTheoretical Division, Los Alamos National Laboratory , Los Alamos, NM 87545, USA ; kyuen@lanl.govPhysics Department, Johns Hopkins University , Baltimore, MD 21218, USA ; evishni1@jhu.eduWe study the properties of sub-Alfvénic magnetohydrodynamic (MHD) turbulence, i.e., turbulence with Alfvén mach number M _A = V _L / V _A < 1, where V _L is the velocity at the injection scale and V _A is the Alfvén velocity. We demonstrate that MHD turbulence can have different properties, depending on whether it is driven by velocity or magnetic fluctuations. If the turbulence is driven by isotropic bulk forces acting upon the fluid, i.e., is velocity driven, in an incompressible conducting fluid we predict that the kinetic energy is ${M}_{{\rm{A}}}^{-2}$ times larger than the energy of magnetic fluctuations. This effect arises from the long parallel wavelength tail of the forcing, which excites modes with k _∥ / k _⊥ < M _A . We also predict that as the MHD turbulent cascade reaches the strong regime, the energy of slow modes exceeds the energy of Alfvén modes by a factor ${M}_{{\rm{A}}}^{-1}$ . These effects are absent if the turbulence is driven through magnetic fluctuations at the injection scale. We confirm our predictions with numerical simulations. Since the assumption of magnetic and kinetic energy equipartition is at the core of the Davis–Chandrasekhar–Fermi (DCF) approach to measuring magnetic field strength in sub-Alfvénic turbulence, we conclude that the DCF technique is not universally applicable. In particular, we suggest that the dynamical excitation of long azimuthal wavelength modes in the galactic disk may compromise the use of the DCF technique. We discuss alternative expressions that can be used to obtain magnetic field strength from observations and consider ways of distinguishing the cases of velocity and magnetically driven turbulence using observational data.https://doi.org/10.3847/1538-4357/ad8d49Magnetic fieldsInterstellar magnetic fieldsAstrophysical fluid dynamicsMagnetohydrodynamics |
| spellingShingle | A. Lazarian Ka Wai Ho Ka Ho Yuen Ethan Vishniac Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements The Astrophysical Journal Magnetic fields Interstellar magnetic fields Astrophysical fluid dynamics Magnetohydrodynamics |
| title | Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements |
| title_full | Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements |
| title_fullStr | Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements |
| title_full_unstemmed | Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements |
| title_short | Sub-Alfvénic Turbulence: Magnetic-to-kinetic Energy Ratio, Modification of Weak Cascade, and Implications for Magnetic Field Strength Measurements |
| title_sort | sub alfvenic turbulence magnetic to kinetic energy ratio modification of weak cascade and implications for magnetic field strength measurements |
| topic | Magnetic fields Interstellar magnetic fields Astrophysical fluid dynamics Magnetohydrodynamics |
| url | https://doi.org/10.3847/1538-4357/ad8d49 |
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