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Search Results: 1 - 10 of 6690 matches for " Alex Lazarian "
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Physics and Chemistry of Purcell's Alignment
Alex Lazarian
Physics , 1995, DOI: 10.1093/mnras/274.3.679
Abstract: Paramagnetic alignment of suprathermally rotating grains is discussed in view of recent progress in understanding subtle processes taking place over grain surface. It is shown that in typical ISM conditions, grains with surfaces of amorphous H$_{2}$O ice, defected silicate or polymeric carbonaceous material are likely to exhibit enhanced alignment, while those of aromatic carbonaceous material or graphite are not. The critical grain sizes and temperature for the Purcell's alignment are obtained and preferential alignment of large grains is established.
Interstellar Filaments and the Statistics of Galactic HI
Alex Lazarian,Dmitri Pogosyan
Physics , 1996, DOI: 10.1086/304927
Abstract: This paper presents a statistical explanation of filament formation in the galactic atomic hydrogen. Recently developed technique allows to determine the 3D spectrum of random HI density. We claim that even in the absence of dynamical factors the Gaussian field corresponding to the measured values of the spectrum of HI density should exhibit filamentary structure, the existence of which has long been claimed.
Cross-Section Alignment of Oblate Grains
Alex Lazarian,Michael Efroimsky
Physics , 1996, DOI: 10.1086/177508
Abstract: 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.
Velocity Field of Compressible MHD Turbulence: Wavelet Decomposition and Mode Scalings
Grzegorz Kowal,Alex Lazarian
Physics , 2010, DOI: 10.1088/0004-637X/720/1/742
Abstract: We study compressible MHD turbulence, which holds key to many astrophysical processes, including star formation and cosmic ray propagation. To account for the variations of the magnetic field in the strongly turbulent fluid we use wavelet decomposition of the turbulent velocity field into Alfven, slow and fast modes, which presents an extension of the Cho & Lazarian (2003) decomposition approach based on Fourier transforms. The wavelets allow to follow the variations of the local direction of magnetic field and therefore improve the quality of the decomposition compared to the Fourier transforms which are done in the mean field reference frame. For each resulting component we calculate spectra and two-point statistics such as longitudinal and transverse structure functions, as well as, higher order intermittency statistics. In addition, we perform the Helmholtz-Hodge decomposition of the velocity field into the incompressible and compressible parts and analyze these components. We find that the turbulence intermittency is different for different components and we show that the intermittency statistics depend on whether the phenomenon was studied in the global reference frame related to the mean magnetic field or it was studied in the frame defined by the local magnetic field. The dependencies of the measures we obtained are different for different components of velocity, for instance, we show that while the Alfven mode intermittency changes marginally with the Mach number the intermittency of the fast mode is substantially affected by the change.
Velocity Anisotropy as a Diagnostic of the Magnetization of the Interstellar Medium and Molecular clouds
Alejandro Esquivel,Alex Lazarian
Physics , 2011, DOI: 10.1088/0004-637X/740/2/117
Abstract: We use a set of magnetohydrodynamics (MHD) simulations of fully-developed (driven) turbulence to study the anisotropy in the velocity field that is induced by the presence of the magnetic field. In our models we study turbulence characterized by sonic Mach numbers M_s from 0.7 to 7.5, and Alfven Mach numbers M_A from 0.4 to 7.7. These are used to produce synthetic observations (centroid maps) that are analyzed. To study the effect of large scale density fluctuations and of white noise we have modified the density fields and obtained new centroid maps, which are analyzed. We show that restricting the range of scales at which the anisotropy is measured makes the method robust against such fluctuations. We show that the anisotropy in the structure function of the maps reveals the direction of the magnetic field for M_A \lesssim 1.5, regardless of the sonic Mach number. We found that the degree of anisotropy can be used to determine the degree of magnetization (i.e. M_A) for M_A \lesssim 1.5. To do this, one needs an additional measure of the sonic Mach number and an estimate of the LOS magnetic field, both feasible by other techniques, offering a new opportunity to study the magnetization state of the interstellar medium.
Statistical Tools of ISM turbulence
Blakesley Burkhart,Alex Lazarian
Physics , 2011, DOI: 10.1017/S1743921311007289
Abstract: 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.
Statistical Description of Synchrotron Intensity Fluctuations: Studies of Astrophysical Magnetic Turbulence
Alex Lazarian,Dmitri Pogosyan
Physics , 2011, DOI: 10.1088/0004-637X/747/1/5
Abstract: We provide a theoretical description of synchrotron fluctuations arising from magnetic turbulence. We derive an expression that relates the correlation of synchrotron fluctuations for an arbitrary index of relativistic electrons with the correlations of squared fluctuations of magnetic field component perpendicular to the line of sight. The latter correlations we study assuming that the turbulence is axisymmetric. We obtain general relations valid for an arbitrary model of magnetic turbulence and analyse the relations for particular example of magnetic turbulence that is supported by numerical simulations. We predict that the synchrotron intensity fluctuations are anisotropic with larger correlation present along the direction of magnetic field. This anisotropy is dominated by the quadrupole component with the ratio between quadrupole and monopole parts being sensitive to the compressibility of underlying turbulence. Our work opens avenues for quantitative studies of magnetic turbulence in our galaxy and beyond using synchrotron emission. It also outlines the directions of how synchrotron foreground emission can be separated from cosmological signal, i.e. from CMB or highly redshifted HI emission. For the sake of completeness we also provide the expressions for the synchrotron polarization (Stocks parameters and their combinations) for the model of axisymmetric magnetic turbulence.
Statistical Tools of Interstellar Turbulence: Bridging the Gap Between Numerics and Observations
Blakesley Burkhart,Alex Lazarian
Physics , 2011,
Abstract: MHD Turbulence is a critical component of the current paradigms of star formation, particle transport, magnetic reconnection and evolution of the ISM. Progress on this difficult subject is made via numerical simulations and observational studies. However, due to limitations of resolution, scale discrepancies, and complexity of the observations, the best approach for connecting numerics to observations is not always obvious. Here we advocate for a approach that invokes statistical techniques to understand the underlying physics of turbulent astrophysical systems. The wealth of numerical and observational data calls for new statistical tools to be developed in order to study turbulence in the interstellar medium. We briefly review some of the recently developed statistics that focus on characterizing gas compressibility and magnetization and their uses to interstellar studies.
MHD Turbulence, Turbulent Dynamo and Applications
Andrey Beresnyak,Alex Lazarian
Physics , 2014, DOI: 10.1007/978-3-662-44625-6_8
Abstract: MHD Turbulence is common in many space physics and astrophysics environments. We first discuss the properties of incompressible MHD turbulence. A well-conductive fluid amplifies initial magnetic fields in a process called small-scale dynamo. Below equipartition scale for kinetic and magnetic energies the spectrum is steep (Kolmogorov -5/3) and is represented by critically balanced strong MHD turbulence. In this paper we report the basic reasoning behind universal nonlinear small-scale dynamo and the inertial range of MHD turbulence. We measured the efficiency of the small-scale dynamo $C_E=0.05$, Kolmogorov constant $C_K=4.2$ and anisotropy constant $C_A=0.63$ for MHD turbulence in high-resolution direct numerical simulations. We also discuss so-called imbalanced or cross-helical MHD turbulence which is relevant for in many objects, most prominently in the solar wind. We show that properties of incompressible MHD turbulence are similar to the properties of Alfv\'enic part of MHD cascade in compressible turbulence. The other parts of the cascade evolve according to their own dynamics. The slow modes are being cascaded by Alfv\'enic modes, while fast modes create an independent cascade. We show that different ways of decomposing compressible MHD turbulence into Alfv\'en, slow and fast modes provide consistent results and are useful in understanding not only turbulent cascade, but its interaction with fast particles.
Spectrum and Anisotropy of Turbulence from Multi-Frequency Measurement of Synchrotron Polarization
Alex Lazarian,Dmitri Pogosyan
Physics , 2015,
Abstract: We consider turbulent synchrotron emitting media that also exhibits Faraday rotation and provide a statistical description of synchrotron polarization fluctuations. In particular, we consider these fluctuations as a function of the spatial separation of the direction of measurements and as a function of wavelength for the same line-of-sight. On the basis of our general analytical approach, we introduce several measures that can be used to obtain the spectral slopes and correlation scales of both the underlying magnetic turbulence responsible for emission and the spectrum of the Faraday rotation fluctuations. We show the synergetic nature of these measures and discuss how the study can be performed using sparsely sampled interferometric data. We also discuss how additional characteristics of turbulence can be obtained, including the turbulence anisotropy, the three dimensional direction of the mean magnetic field. We consider both cases when the synchrotron emission and Faraday rotation regions coincide and when they are spatially separated. Appealing to our earlier study in Lazarian and Pogosyan (2012) we explain that our new results are applicable to a wide range of spectral indexes of relativistic electrons responsible for synchrotron emission. We expect wide application of our techniques both with existing synchrotron data sets as well as with big forthcoming data sets from LOFAR and SKA.
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