Velocity statistics from spectral line data: effects of density-velocity correlations, magnetic field, and shear

In a previous work Lazarian and Pogosyan suggested a technique to extract velocity and density statistics, of interstellar turbulence, by means of analysing statistics of spectral line data cubes. In this paper we test that technique, by studying the effect of correlation between velocity and density fields, providing a systematic analysis of the uncertainties arising from the numerics, and exploring the effect of a linear shear. We make use of both compressible MHD simulations and synthetic data to emulate spectroscopic observations and test the technique. With the same synthetic spectroscopic data, we also studied anisotropies of the two point statistics and related those anisotropies with the magnetic field direction. This presents a new technique for magnetic field studies. The results show that the velocity and density spectral indices measured are consistent with the analytical predictions. We identified the dominant source of error with the limited number of data points along a given line of sight. We decrease this type of noise by increasing the number of points and by introducing Gaussian smoothing. We argue that in real observations the number of emitting elements is essentially infinite and that source of noise vanishes.Comment: 12 pages, 10 figures. Accepted for publication in MNRA

Magnetic Reconnection and Turbulent Mixing: From ISM to Clusters of Galaxies

Magnetic reconnection, or the ability of the magnetic field lines that are frozen in plasma to change their topology, is a fundamental problem of magnetohydrodynamics (MHD). We briefly examine the problem starting with the well-known Sweet-Parker scheme, discuss effects of tearing modes, anomalous resistivity and the concept of hyperresistivity. We show that the field stochasticity by itself provides a way to enable fast reconnection even if, at the scale of individual turbulent wiggles, the reconnection happens at the slow Sweet-Parker rate. We show that fast reconnection allows efficient mixing of magnetic field in the direction perpendicular to the local direction of magnetic field. While the idea of stochastic reconnection still requires numerical confirmation, our numerical simulations testify that mixing motions perpendicular to the local magnetic field are up to high degree hydrodynamical. This suggests that the turbulent heat transport should be similar to that in non-magnetized turbulent fluid, namely, should have a diffusion coefficient \sim LV_L, where V_L is the amplitude of the turbulent velocity and L is the scale of the turbulent motions. We present numerical simulations which support this conclusion. The application of this idea to thermal conductivity in clusters of galaxies shows that this mechanism may dominate the diffusion of heat and may be efficient enough to prevent cooling flow formation.Comment: 12 pages, 2 figures, invited talk at JENAM2002 - The Unsolved Universe:Challenges for the Future (v2: minor changes

Scaling, Intermittency and Decay of MHD Turbulence

We discuss a few recent developments that are important for understanding of MHD turbulence. First, MHD turbulence is not so messy as it is usually believed. In fact, the notion of strong non-linear coupling of compressible and incompressible motions along MHD cascade is not tenable. Alfven, slow and fast modes of MHD turbulence follow their own cascades and exhibit degrees of anisotropy consistent with theoretical expectations. Second, the fast decay of turbulence is not related to the compressibility of fluid. Rates of decay of compressible and incompressible motions are very similar. Third, viscosity by neutrals does not suppress MHD turbulence in a partially ionized gas. Instead, MHD turbulence develops magnetic cascade at scales below the scale at which neutrals damp ordinary hydrodynamic motions. Forth, density statistics does not exhibit the universality that the velocity and magnetic field do. For instance, at small Mach numbers the density is anisotropic, but it gets isotropic at high Mach numbers. Fifth, the intermittency of magnetic field and velocity are different. Both depend on whether the measurements are done in local system of reference oriented along the local magnetic field or in the global system of reference related to the mean magnetic field.Comment: 12 pages, Invited Review, Workshop on Theoretical Plasma Physics, Trieste, Italy, 5-16 Jul

Observation of confined propagation in Bragg waveguides

A new type of waveguiding in a slab dielectric bounded on one side by air and on the other by a periodic layered medium (grown by molecular beam epitaxy) has been demonstrated

Density scaling and anisotropy in supersonic MHD turbulence

We study the statistics of density for supersonic turbulence in a medium with magnetic pressure larger than the gaseous pressure. This study is motivated by molecular cloud research. Our simulations exhibit clumpy density structures, which contrast increases with the Mach number. At 10 Machs densities of some clumps are three orders of magnitude higher than the mean density. These clumps give rise to flat and approximately isotropic density spectrum corresponding to the random distribution of clumps in space. We claim that the clumps originate from our random, isotropic turbulence driving. When the contribution from those clumps is suppressed by studying logarithm of density, the density statistics exhibit scale-dependent anisotropy consistent with the models where density structures arise from shearing by Alfv\'en waves. It is noteworthy that originally such models were advocated for the case of low-Mach, nearly incompressible turbulence.Comment: 4 pages, 4 figures, using emulateap

A Technique for Foreground Subtraction in Redshifted 21 cm Observations

One of the main challenges for future 21 cm observations is to remove foregrounds which are several orders of magnitude more intense than the HI signal. We propose a new technique for removing foregrounds of the redshifted 21 cm observations. We consider multi-frequency interferometer observations. We assume that the 21 cm signals in different frequency channels are uncorrelated and the foreground signals change slowly as a function of frequency. When we add the visibilities of all channels, the foreground signals increase roughly by a factor of ~N because they are highly correlated. However, the 21 cm signals increase by a factor of ~\sqrt{N} because the signals in different channels contribute randomly. This enables us to obtain an accurate shape of the foreground angular power spectrum. Then, we obtain the 21-cm power spectrum by subtracting the foreground power spectrum obtained this way. We describe how to obtain the average power spectrum of the 21 cm signal.Comment: 5 pages, 1 figure; To appear on the Astrophysical Journa