Structured Turbulence Collisionless Dynamics and Magnetic Field Generation in Astrophysical Plasmas

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ABSTRACT - J. C. McWilliams and S. Cowley Structured Turbulence, Collisionless Dynamics, and Magnetic Field Generation in Anisotropic Astrophysical Plasmas This research project is an investigation of some basic problems of plasma turbulence which are particularly relevant to astrophysical dynamos. The study will use the galactic dynamo as the primary astrophysical example, but the results will be relevant also to protogalactic and accretion-disk dynamos. In particular, the early stages of magnetic amplification at very small scales in large-Prandtl-number plasmas and the late stages of magnetic energy buildup at large scale by inverse cascade will be studied with the following objectives: (1) To determine mechanisms for magnetic energy scale growth at scales within the viscous dissipation range of fluid turbulence, by ambipolar diffusion, anisotropic viscosity, or self-consistent nonlinear dynamics. (2) To explore the physics of plasmas on scales short compared to the collisional mean free path, but when the evolution is slow compared to the collision frequency, and to derive fluid-like equations that describe this physics. (3) To study the inverse cascade in a rotating disk with small-scale forcing, such that the anisotropic environment provides a physical mechanism for helicity generation. These problems have not been thoroughly studied in the past, yet they address key processes which must occur in actual physical dynamos. Analytical theory, turbulent closure model calculations, and direct numerical solutions of high Reynolds number fluid equations will be used to attack these problems. Asymptotic analysis of kinetic equations will be used to describe low collisionality behavior. Dissipation-range magnetohydrodynamics (MHD) will be explored by closure calculations and direct numerical solutions at high Prandtl number, including non-MHD effects such as partial ionization and low collisionality. Direct numerical solutions of turbulent MHD in an anisotropic environment w ill be also be calculated, making use where possible of asymptotic reductions to decrease computational complexity and increase the range of resolvable scales while retaining the ingredients necessary for self-consistent dynamo action. This theoretical study will address often overlooked questions of small- and large-scale dynamos and provide basic understanding of high Prandtl-number and anisotropic plasma turbulence. ***