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Small-scale behavior of Hall magnetohydrodynamic turbulence
Julia E Stawarz1, Annick Pouquet2
1Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80309, USA and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|January 15, 2016
Summary
Hall magnetohydrodynamic (HMHD) turbulence simulations reveal a transition to a magnetically dominated state at small scales. HMHD exhibits narrower electric field and current structures compared to magnetohydrodynamics (MHD).
Area of Science:
- Plasma Physics
- Astrophysical Turbulence
- Computational Fluid Dynamics
Background:
- Turbulence in magnetized plasmas is crucial for understanding phenomena from solar winds to fusion energy.
- Hall magnetohydrodynamics (HMHD) extends traditional magnetohydrodynamics (MHD) by incorporating electron inertia and Hall effect, essential in low-collisionality plasmas.
- Previous studies often simplified dissipation mechanisms, limiting applicability to specific plasma regimes.
Purpose of the Study:
- To investigate the characteristics of decaying Hall magnetohydrodynamic (HMHD) turbulence.
- To compare HMHD turbulence with analogous magnetohydrodynamic (MHD) turbulence using direct numerical simulations.
- To analyze the impact of different dissipative operators (Laplacian and Laplacian-squared) on HMHD turbulence.
Main Methods:
- Three-dimensional (3D) direct numerical simulations of decaying HMHD turbulence were performed.
- Simulations utilized grids up to 768^3 points and two distinct initial conditions.
- Results were systematically compared against equivalent MHD simulations, examining both Laplacian and hyper-viscosity (Laplacian-squared) dissipation.
Main Results:
- At scales below the ion inertial length, HMHD turbulence transitions to a magnetically dominated state, unlike MHD.
- This transition correlates with the advection term becoming less dominant than dissipation in the momentum equation.
- HMHD simulations show narrower electric field and current structures, and wider vorticity structures compared to MHD. Current structures are more intense and better aligned with the magnetic field.
Conclusions:
- HMHD turbulence exhibits distinct scaling and structural properties compared to MHD, particularly at sub-ion scales.
- The enhanced alignment of intense currents with the magnetic field in HMHD may have significant implications for collisionless plasma instabilities.
- The choice of dissipation operator influences spectral scaling, with hyper-viscosity revealing a longer k^(-7/3) scaling region for right-polarized fluctuations.
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