"Mean-field theories have become indispensable for studying astrophysical hydromagnetic flows which are otherwise hard to solve exactly. The proper modeling of turbulent velocity and magnetic fields, the validity of mean-field theories in astrophysical flows, and its accuracy and precision when compared to astronomical observations are the main focus of this thesis. In particular, the following studies on astrophysical dynamos and accretion disks are presented: (1) We construct a new model of galactic dynamos in which the turbulent kinetic energy and correlation time are coupled to the galactic differential rotation. We find that a strong differential rotation, while contributing to field amplification by line stretching, can ultimately reduce the overall saturated magnetic field strength compared to conventional models because it decreases the turbulent correlation time when its effect on turbulence is included. (2) We present an efficient, concise, physically grounded way of calculating turbulent transport coefficients through averaging the motion of a single fluid blob. The demonstrated configuration-space calculation gives the same result as much lengthier previously used Fourier-space methods. (3) We generalize dynamo quenching theory to anisotropic flows. Understanding how strong magnetic fields can become in astrophysical objects via dynamos is an active area of research, but quenching theories with predictive power so far have almost exclusively been restricted to systems in which turbulence is isotropic and homogenous. We generalize the isotropic dynamical quenching formulae to anisotropic a2 dynamos, where a new 'selective-damping-t' closure is proposed that conserves magnetic helicity as required, reduces to previous results when the turbulence is isotropic, and includes the full Lorentz force back reaction of the magnetic field for anisotropic flows. (4) We study the consequences of finite scale separation on validity and precision of mean-field dynamo theories. The governing equations for mean field dynamos are newly derived to properly include terms in higher order of the scale ratio between mean and fluctuating fields, and the intrinsic and filtering errors as two types of precision are identified. The formalism is then applied to a galactic dynamo model and its Faraday rotation measure, in order to quantify the precision of theoretical predictions. (5) We generalize the technique and approach for mean field precision calculation that we first worked out for dynamos and apply it to a hydrodynamic accretion disk model. The fluctuations in the local black-body emission are carefully averaged to obtain a predicted 1s error bar in the observed spectrum. This produces a precision error of the theory that must be included when assessing agreement or disagreement between theories and observations"--Pages ix-x.

Magnetism is one of the most pervasive features of the Universe, with planets, stars and entire galaxies all having associated magnetic fields. All of these fields are generated by the motion of electrically conducting fluids, the so-called dynamo effect. The precise details of what drives the motion, and indeed what the fluid consists of, differ widely though. In this work the authors draw upon their expertise in geophysical and astrophysical MHD to explore some of these phenomena, and describe the similarities and differences between different magnetized objects. They also explain why magnetic fields are crucial in the formation of the stars, and discuss promising experiments currently being designed to study some of the relevant physics in the laboratory. This interdisciplinary approach makes the book appealing to a wide audience in physics, astrophysics and geophysics.

This book revises the evolution of ideas in various branches of magnetohydrodynamics (astrophysics, earth and solar dynamos, pinch, MHD turbulence and liquid metals) and reviews current trends and challenges. Uniquely, it contains the review articles on the development of the subject by pioneers in the field as well as leading experts, not just in one, but in various branches of magnetohydrodynamics, such as liquid metals, astrophysics, dynamo and pinch.

Mean-Field Magnetohydrodynamics and Dynamo Theory provides a systematic introduction to mean-field magnetohydrodynamics and the dynamo theory, along with the results achieved. Topics covered include turbulence and large-scale structures; general properties of the turbulent electromotive force; homogeneity, isotropy, and mirror symmetry of turbulent fields; and turbulent electromotive force in the case of non-vanishing mean flow. The turbulent electromotive force in the case of rotational mean motion is also considered. This book is comprised of 17 chapters and opens with an overview of the general concept of mean-field magnetohydrodynamics, followed by a discussion on the back-reaction of the magnetic field on motion; the structure of the turbulent electromotive force; homogeneous and two-scale turbulence; turbulent electromotive force in the case of rotational mean motion; and the dynamo problem of magnetohydrodynamics. The dynamo theory, which is based on mean-field magnetohydrodynamics, is explained and its applications to cosmical objects are described. The remaining chapters explore toroidal and poloidal vector fields; a simple model of an a-effect dynamo; and spherical models of turbulent dynamos as suggested by cosmical bodies. This monograph will be of interest to physicists.

Black holes exist in galactic nuclei and in some X-ray binaries found in our own galaxy and the large Magellanic Cloud. This volume focuses on astrophysical high-energy emission processes around black holes, and the development of theoretical frameworks for interesting observational results. Contents: Black Hole Observations; Accretion Disk/Formation of Jets; Energy Extraction from Rotating Black Holes; Supernova and Gamma Ray Bursts; Black Hole Astrophysics. Readership: Graduate students, post-docs and academics in astrophysics, astronomy, cosmology and high energy physics.

This book contains review articles of most of the topics addressed at the conf- ence on Simulations of Magnetohydrodynamic turbulence in astrophysics: recent achievements and perspectives which took place from July 2 to 6, 2001 at the Institut Henri Poincar ́e in Paris. We made the choice to publish these lectures in a tutorial form so that they can be read by a broad audience. As a result, this book does not give an exhaustive view of all the subjects addressed during the conference. The main objective of this workshop which gathered about 90 scientists from di?erent ?elds, was to present and confront recent results on the topic of t- bulence in magnetized astrophysical environments. A second objective was to discuss the latest generation of numerical codes, such as those using adaptive mesh re?nement (AMR) techniques. During a plenary discussion at the end of the workshop discussions were held on several topics, often at the heart of vivid controversies. Topics included the timescale for the dissipation of magneto-hydrodynamical (MHD) turbulence, the role of boundary conditions, the characteristics of imbalanced turbulence, the validity of the polytropic approach to Alfv ́en waves support within interst- lar clouds, the source of turbulence inside clouds devoid of stellar activity, the timescale for star formation, the Alfv ́en Mach number of interstellar gas motions, the formation process for helical ?elds in the interstellar medium. The impact of small upon large scales was also discussed.

Proceedings of the NATO Advanced Research Workshop, Garching, Germany, March 6-10, 1989