Diffusion Processes in Turbulent Magnetic Fields

We study of the effect of turbulence on diffusion processes within magnetized medium. While we exemplify our treatment with heat transfer processes, our results are quite general and are applicable to different processes, e.g. diffusion of heavy elements. Our treatment is also applicable to describing the diffusion of cosmic rays arising from magnetic field wandering. In particular, we find that when the energy injection velocity is smaller than the Alfven speed the heat transfer is partially suppressed, while in the opposite regime the effects of turbulence depend on the intensity of driving. In fact, the scale $l_A$ at which the turbulent velocity is equal the Alfven velocity is a new important parameter. When the electron mean free path $\lambda$ is larger than $l_A$, the stronger the the turbulence, the lower thermal conductivity by electrons is. The turbulent motions, however, induces their own advective transport, that can provide effective diffusivity. For clusters of galaxies, we find that the turbulence is the most important agent for heat transfer. We also show that the domain of applicability of the subdiffusion concept is rather limited.Comment: 3 figures, 11 pages, to be published in AIP volume of "Turbulence and Non-linear Processes in Astrophysical Plasmas

Statistical Tools of ISM turbulence

MHD Turbulence is a critical component of the current paradigms of star formation, particle transport, magnetic reconnection and evolution of the ISM, to name just a few. Progress on this difficult subject is made via numerical simulations and observational studies, however in order to connect these two, statistical methods are required. This calls for new statistical tools to be developed in order to study turbulence in the interstellar medium. Here we briefly review some of the recently developed statistics that focus on characterizing gas compressibility and magnetization and their uses to interstellar studies.Comment: 4 pages, 1 figure, Proceedings of IAU 274 Advances in Plasma Astrophysic

Cross-Section Alignment of Oblate Grains

This paper provides a quantitative account of a recently introduced mechanism of mechanical alignment of suprathermally rotating grains. These rapidly rotating grains are essentially not susceptible to random torques arising from gas-grain collisions, as the timescales for such torques to have significant effect are orders of magnitude greater than the mean time between crossovers. Such grains can be aligned by gaseous torques during the short periods of crossovers and/or due to the difference in the rate at which atoms arrive at grain surface. The latter is a result of the difference in orientation of a grain in respect to the supersonic flow. This process, which we call cross-section alignment, is the subject of our present paper. We derive expressions for the measure of cross-section alignment for oblate grains and study how this measure depends upon the angle between the interstellar magnetic field and the gaseous flow and upon the grain shape.Comment: 24 pages, Post Script file. To appear in The Astrophysical Journal, Vol. 466, p. 274 - 281, July 199

Spectral Properties of Interstellar Turbulence via Velocity Channel Analysis

In this presentation we review the link between the statistics of intensity fluctuations in spectral line data cubes with underlying statistical properties of turbulence in the interstellar medium. Both the formalism of Velocity Channel Analysis for optically thin lines and its extension to the lines with self-absorption is described. We demonstrate that by observing optically thin lines from cold gas in sufficiently narrow (thin) velocity channels one may recover the scaling of the stochastic velocities from turbulent cascade, in particular, Kolmogorov velocities give $K^{-2.7}$ contribution to the intensity power spectrum. Synthetically increasing the channel thickness separates out the underlying density inhomogeneities of the gas. Effects of self absorption, on the other hand, retain the velocity signature even for integrated lines. As a result, intensity fluctuations tend to show universal but featureless scaling of the power $\propto K^{-3}$ over the range of scales.Comment: 12 pages, 1 figure. To appear in: "Magnetic fields in the Universe", eds. E. M. de Gouveia Dal Pino, A. Lazarian & G. Lugones. Angra dos Reis, Brazil Nov. 23 - Dec 3, 200