The structure of magnetohydrodynamic turbulence.

摘要

Magnetized astrophysical plasmas often exhibit turbulent motions. Such plasmas frequently can be described by magnetohydrodynamics (MHD). I have studied the structure of MHD turbulence numerically. I have performed direct 3-dimensional numerical simulations for MHD turbulence in a periodic box of size 2pi threaded by uniform external magnetic fields. I have considered two extreme limits---strong external magnetic field limits and weak/zero external field limits. In the strong external field limits, I have analyzed the structure of eddies as a function of scale. The results are consistent with the scaling law proposed by Goldreich and Sridhar: smaller eddies are more elongated than larger ones. In the weak/zero external field limits, I have studied the structure and generation of magnetic fields. I have found that the magnetic fields are amplified through field line stretching at a rate proportional to the difference between the velocity and the magnetic field strength times a constant. Equipartition between the kinetic and magnetic energy densities occurs at a scale somewhat smaller than the kinetic energy peak. Above the equipartition scale the velocity structure is, as expected, nearly isotropic. The magnetic field structure at these scales is uncertain, but the field correlation function is very weak. At the equipartition scale the magnetic fields show only a moderate degree of anisotropy, so that the typical radius of curvature of field lines is comparable to the typical perpendicular scale for field reversal. In other words, there are few field reversals within eddies at the equipartition scale, and no fine-grained series of reversals at smaller scales. At scales below the equipartition scale, both velocity and magnetic structures are anisotropic; the eddies are stretched along the local magnetic field lines.