Current status of turbulent dynamo theory: From large-scale to small-scale dynamos

Several recent advances in turbulent dynamo theory are reviewed. High resolution simulations of small-scale and large-scale dynamo action in periodic domains are compared with each other and contrasted with similar results at low magnetic Prandtl numbers. It is argued that all the different cases show similarities at intermediate length scales. On the other hand, in the presence of helicity of the turbulence, power develops on large scales, which is not present in non-helical small-scale turbulent dynamos. At small length scales, differences occur in connection with the dissipation cutoff scales associated with the respective value of the magnetic Prandtl number. These differences are found to be independent of whether or not there is large-scale dynamo action. However, large-scale dynamos in homogeneous systems are shown to suffer from resistive slow-down even at intermediate length scales. The results from simulations are connected to mean field theory and its applications. Recent work on helicity fluxes to alleviate large-scale dynamo quenching, shear dynamos, nonlocal effects and magnetic structures from strong density stratification are highlighted. Several insights which arise from analytic considerations of small-scale dynamos are discussed.Comment: 36 pages, 11 figures, Spa. Sci. Rev., submitted to the special issue "Magnetism in the Universe" (ed. A. Balogh

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Perceived physical and social causality in animated motions: Spontaneous reports and ratings

Michotte argued that we perceive cause-and-effect, without contributions from reasoning or learning, even in displays of two-dimensional moving shapes. Two studies extend this line of work from perception of mechanical to social causality. We compared verbal reports with structured ratings of causality to gain a better understanding of the extent to which perceptual causality occurs spontaneously or depends on instruction or context. A total of 120 adult observers (72 in the main experiment, 48 in an initial experiment) saw 12 (or 8) different computer animations of shape A moving up to B, which in turn moved away. Animations factorially varied the temporal and spatial relations of the shapes, and whether they moved rigidly or in a non-rigid, animal-like manner. Impressions of social as well as physical causality appeared in both free reports and ratings. Perception of physical causality was stronger than perception of social causality, particularly in free reports. No differences of this nature appear in infants and children, so the asymmetry may reflect learnt knowledge. Physical causality was relatively unspecific initially, but discrimination of causal and delayed control events improved with exposure to multiple events. Experience seems to affect the causal illusion even over a short timeframe; the idea of 'one-trial causality' may be somewhat misleading. Regardless of such effects on the absolute level of responses, the different measures showed similar patterns of variation with the spatio-temporal configuration and type of motion. The good fit of ratings and reports validates much recent work in this area. (c) 2006 Elsevier B.V. All rights reserved