Jungyeon Cho

The Anisotropy of Magnetohydrodynamic Alfvénic Turbulence

We perform direct three-dimensional numerical simulations for magnetohydrodynamic (MHD) turbulence in a periodic box of size 2π threaded by strong uniform magnetic fields. We use a pseudospectral code with hyperviscosity and hyperdiffusivity to solve the incompressible MHD equations. We analyze the structure of the eddies as a function of scale. A straightforward calculation of anisotropy in wavevector space shows that the anisotropy is scale independent. We discuss why this is not the true scaling law and how the curvature of large-scale magnetic fields affects the power spectrum and leads to the wrong conclusion. When we correct for this effect, we find that the anisotropy of eddies depends on their size: smaller eddies are more elongated than larger ones along local magnetic field lines. The results are consistent with the scaling law ∥ ~ recently proposed by Goldreich & Sridhar. Here ∥ (and ⊥) are wavenumbers measured relative to the local magnetic field direction. However, we see some systematic deviations that may be a sign of limitations to the model or our inability to fully resolve the inertial range of turbulence in our simulations.We perform direct 3-dimensional numerical simulations for magnetohydrodynamic (MHD) turbulence in a periodic box of size

Compressible magnetohydrodynamic turbulence: mode coupling, scaling relations, anisotropy, viscosity‐damped regime and astrophysical implications

2\pi

Simulations of Magnetohydrodynamic Turbulence in a Strongly Magnetized Medium

threaded by strong uniform magnetic fields. We use a pseudo-spectral code with hyperviscosity and hyperdiffusivity to solve the incompressible MHD equations. We analyze the structure of the eddies as a function of scale. A straightforward calculation of anisotropy in wavevector space shows that the anisotropy is scale-{\it independent}. We discuss why this is {\it not} the true scaling law and how the curvature of large-scale magnetic fields affects the power spectrum and leads to the wrong conclusion. When we correct for this effect, we find that the anisotropy of eddies depends on their size: smaller eddies are more elongated than larger ones along {\it local} magnetic field lines. The results are consistent with the scaling law

Turbulence and Magnetic Fields in the Large-Scale Structure of the Universe

\tilde{k}_{\parallel} \sim \tilde{k}_{\perp}^{2/3}

Simulations of MHD Turbulence in a Strongly Magnetized Medium

proposed by Goldreich and Sridhar (1995, 1997). Here

Compressible sub-alfvenic MHD turbulence in low-Beta plasmas

\tilde{k}_{\|}

Magnetic Field Structure and Stochastic Reconnection in a Partially Ionized Gas

(and

The Generation of Magnetic Fields through Driven Turbulence

\tilde{k}_{\perp}

Thermal Conduction in Magnetized Turbulent Gas

) are wavenumbers measured relative to the local magnetic field direction. However, we see some systematic deviations which may be a sign of limitations to the model, or our inability to fully resolve the inertial range of turbulence in our simulations.

SIMULATIONS OF ELECTRON MAGNETOHYDRODYNAMIC TURBULENCE

We present numerical simulations and explore scalings and anisotropy of compressible magnetohydrodynamic (MHD) turbulence. Our study covers both gas-pressure-dominated (high β) and magnetic-pressure-dominated (low β) plasmas at different Mach numbers. In addition, we present results for super-Alfvenic turbulence and discuss in what way it is similar to sub-Alfvenic turbulence. We describe a technique of separating different magnetohydrodynamic modes (slow, fast and Alfven) and apply it to our simulations. We show that, for both high- and low-β cases, Alfven and slow modes reveal a Kolmogorov k - 5 / 3 spectrum and scale-dependent Goldreich-Sridhar anisotropy, while fast modes exhibit a k - 3 / 2 spectrum and isotropy. We discuss the statistics of density fluctuations arising from MHD turbulence in different regimes. Our findings entail numerous astrophysical implications ranging from cosmic ray propagation to gamma ray bursts and star formation. In particular, we show that the rapid decay of turbulence reported by earlier researchers is not related to compressibility and mode coupling in MHD turbulence. In addition, we show that magnetic field enhancements and density enhancements are marginally correlated. Addressing the density structure of partially ionized interstellar gas on astronomical-unit scales, we show that the viscosity-damped regime of MHD turbulence that we reported earlier for incompressible flows persists for compressible turbulence and therefore may provide an explanation for these mysterious structures.