Abstract

Magnetic fields are ubiquitous in the cosmos and play an important dynamical role, as in the solar wind, in stars or in the interstellar medium. In planets and stars, magnetic fields are believed to be generated by a dynamo instability, in which the stretching of magnetic field lines by vigorous ... Show moreMagnetic fields are ubiquitous in the cosmos and play an important dynamical role, as in the solar wind, in stars or in the interstellar medium. In planets and stars, magnetic fields are believed to be generated by a dynamo instability, in which the stretching of magnetic field lines by vigorous motions of electrically conducting fluids exceeds the Joule dissipation. Such magnetohydrodynamic (MHD) flows have large Reynolds numbers and thus nonlinear mode coupling leads to multi-scale interactions and to the formation of complex flows with, in the small scales, the presence of strong intermittent structures. Moreover, tearing mode instabilities develop and reconnection takes place, as in the magnetopause, or in the heating of solar and stellar corona. At what rate does dissipation occur, as the Reynolds number increases? What is the origin of these structures, and how fast are they formed? What is the origin of such magnetic fields (the dynamo problem)? How does the dynamo work when the magnetic Prandtl number PM--the ratio of viscosity to magnetic diffusivity--differs substantially from unity? For example, in the interstellar medium, it can be as large as 1014, whereas in stars such as the Sun and for planets such as Earth, it can be very low (lower than 10⁻⁵, the value for the Earth fluid core); similarly, in liquid breeder reactors and in laboratory experiments with liquid metals, PM ll ≪ 1. Huge scale separation is needed, and several dynamical regimes have been identified, depending on turbulence, rotation, flow helicity, wave interactions, etc. The progress reported in this invited focus issue of New Journal of Physics concerns observations, experiments, modeling, theory and direct numerical simulations. It is the hope of the editors of this collection that a general overview of where this dynamic scientific field is presently, and where it is going in the near future (with several large-scale projects, from detailed satellite observations to experiments and petascale computing), will be valuable to our readers. Show less