arXiv subjects

A. Lazarian

Publications and source records attributed to A. Lazarian.

At least 253 records · Page 14Linked to original sources

arXiv2005

Astrophysical Implications of Turbulent Reconnection: from cosmic rays to star formation

Turbulent reconnection allows fast magnetic reconnection of astrophysical magnetic fields. This entails numerous astrophysical implications and opens new ways to approach long standing problems. I briefly discuss a model of turbulent reconnection within which the stochasticity of 3D magnetic field enables rapid reconnection through both allowing multiple reconnection events to take place simultaneously and by restricting the extension of current sheets. In fully ionized gas the model in Lazarian and Vishniac 99 predicts reconnection rates that depend only on the intensity of turbulence. In partially ionized gas a modification of the original model in Lazarian, Vishniac and Cho 04 predicts the reconnection rates that, apart from the turbulence intensity depend on the degree of ionization. In both cases the reconnection may be slow and fast depending on the level of turbulence in the system. As the result, the reconnection gets bursty, which provides a possible explanation to Solar flares and possibly to gamma ray busts. The implications of the turbulent reconnection model have not been yet studied in sufficient detail. I discuss first order Fermi acceleration of cosmic ray that takes place as the oppositely directed magnetic fluxes move together. This acceleration would work in conjunction with the second order Fermi acceleration that is caused by turbulence in the reconnection region. In partially ionized gas the stochastic reconnection enables fast removal of magnetic flux from star forming molecular clouds.

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arXiv2005

Velocity centroids as tracers of the turbulent velocity statistics

We use the results of magnetohydrodynamic (MHD) simulations to emulate spectroscopic observations and use maps of centroids to study their statistics. In order to assess under which circumstances the scaling properties of the velocity field can be retrieved from velocity centroids, we compare two point statistics (structure functions and power-spectra) of velocity centroids with those of the underlying velocity field and analytic predictions presented in a previous paper (Lazarian & Esquivel 2003). We tested a criterion for recovering velocity spectral index from velocity centroids derived in our previous work, and propose an approximation of the early criterion using only the variances of ``unnormalized'' velocity centroids and column density maps. It was found that both criteria are necessary, however not sufficient to determine if the centroids recover velocity statistics. Both criteria are well fulfilled for subsonic turbulence. We find that for supersonic turbulence with sonic Mach numbers > 2.5 centroids fail to trace the spectral index of velocity. Asymptotically, however, we claim that recovery of velocity statistics is always possible provided that the density spectrum is steep and the observed inertial range is sufficiently extended. In addition, we show that velocity centroids are useful for anisotropy studies and determining the direction of magnetic field, even if the turbulence is highly supersonic, but only if it is sub-Alfvenic. This provides a tool for mapping the magnetic field direction, and testing whether the turbulence is sub-Alfvenic or super-Alfvenic.

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arXiv2005

Grain Alignment by Radiation in Dark Clouds and Cores

We study alignment of grains by radiative torques. We found steep rise of radiative torque efficiency as grain size increases. This allows larger grains that are known to exist within molecular clouds to be aligned by the attenuated and reddened interstellar radiation field. In particular, we found that, even deep inside giant molecular clouds, e.g. at optical depths corresponding to less than Av of 10 large grains can still be aligned by radiative torque. This means that, contrary to earlier claims, far-infrared/submillimeter polarimetry provides a reliable tool to study magnetic fields of pre-stellar cores. Our results show that the grain size distribution is important for determining the relation between the degree of polarization and intensity.

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arXiv2005

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 Alfvén waves. It is noteworthy that originally such models were advocated for the case of low-Mach, nearly incompressible turbulence.

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arXiv2005

Magnetically limited X-ray filaments in young SNR

We discuss the damping of strong magnetic turbulence downstream of the forward shock of young supernova remnants (SNR). We find that strong magnetic fields, that have been produced by the streaming instability in the upstream region of the shock, or by other kinetic instabilities at the shock, will be efficiently reduced, so the region of enhanced magnetic field strength would typically have a thickness of the order l_d=(10^{16}-10^{17}) cm. The non-thermal X-ray filaments observed in young SNR are thus likely limited by the magnetic field and not by the energy losses of the radiating electrons. Consequently the thickness of the filaments would not be a measure of the magnetic field strength and claims of efficient cosmic-ray acceleration on account of a run-away streaming instability appear premature.

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arXiv2005

Optical polarization from aligned atoms as a new diagnostic of Astrophysical magnetic fields

Population of levels of the hyperfine and fine split ground state of an atom is affected by radiative transitions induced by anisotropic radiation flux. Such aligned atoms precess in the external magnetic field and this affects properties of polarized radiation arising from both scattering and absorption by atoms. As the result the degree of light polarization depends on the direction of the magnetic field. This provides a perspective tool for studies of astrophysical magnetic fields using optical and UV polarimetry. We provide calculations for several atoms and ions that can be used to study magnetic fields in interplanetary medium, interstellar medium, circumstellar regions and quasars.

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arXiv2004

Spurious contribution to CR scattering calculations

The quasilinear theory for cosmic ray propagation is a well known and widely accepted theory. In this paper, we discuss the different contributions to the pitch-angle Fokker-Planck coefficient from large and small scales for slab geometry using the damping model of dynamical turbulence. These examinations will give us a hint on the limitation range where quasilinear approximation is a good approximation.

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arXiv2004

Magnetic turbulence in compressible fluids

We present high resolution numerical simulations of compressible magnetohydrodynamic (MHD) turbulence. We concentrate on studies of spectra and anisotropy of velocity and density. We describe a technique of separating different magnetohydrodynamic (MHD) modes (slow, fast and Alfven) and apply it to our simulations. For the mildly supersonic case, velocity and density show anisotropy. However, for the highly supersonic case, we observe steepening of the slow mode velocity spectrum and isotropization of density. Our present studies show that the spectrum of density gets substantially {\it flatter} than the Kolmogorov one.

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arXiv2004

Alignment of Dust in Molecular Clouds

Polarimetry is one of the most informative techniques of studying magnetic fields in molecular clouds. How reliable the interpretation of the polarization maps in terms of magnetic fields is the issue that the grain alignment theory addresses. We show that grain alignment involves several processes acting simultaneously, but on different time-scales. We explain that rotating dust grains get substantial magnetic moment that allows them precess fast about magnetic field lines. As the result, grains preserve their orientation to magnetic field when the magnetic field direction fluctuates. We point out to the importance of internal alignment, i.e. the process forces grain axes to be aligned in respect to the grain angular momentum. We show that subtle quantum effects, in particular relaxation related to nuclear magnetic moments of atoms composing the grain, brings to live complex grain motions, e.g. flips. These flips substantially alter the dynamics of grain and limit the applicability of earlier theories that did not account for them. We also briefly review basic physical processes involved in the alignment of grain angular momentum in respect to interstellar magnetic field. We claim that the bulk of existing observational data is consistent with the radiative torque alignment mechanism. In particular, we show that large grains that are known to exist in the cores of molecular clouds may be aligned by the attenuated external interstellar radiation field.

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arXiv2004

Obtaining Statistics of Turbulent Velocity from Astrophysical Spectral Line Data

Turbulence is a crucial component of dynamics of astrophysical fluids dynamics, including those of ISM, clusters of galaxies and circumstellar regions. Doppler shifted spectral lines provide a unique source of information on turbulent velocities. We discuss Velocity-Channel Analysis (VCA) and its offspring Velocity Coordinate Spectrum (VCS) that are based on the analytical description of the spectral line statistics. Those techniques are well suited for studies of supersonic turbulence. We stress that a great advantage of VCS is that it does not necessary require good spatial resolution. Addressing the studies of mildly supersonic and subsonic turbulence we discuss the criterion that allows to determine whether a traditional tool for such a research, namely, Velocity Centroids are dominated by density or velocity. We briefly discuss the use of higher order correlations as the means to study intermittency of turbulence. We discuss observational data available and prospects of the field.

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arXiv2004

Cosmic Ray Scattering and Streaming in Compressible Magnetohydrodynamic Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for revisions in the picture of cosmic ray transport. In this paper we use recently obtained scaling laws for MHD modes to obtain the scattering frequency for cosmic rays. Using quasilinear theory we calculate gyroresonance with MHD modes (Alfvénic, slow and fast) and transit-time damping (TTD) by fast modes. We provide calculations of cosmic ray scattering for various phases of interstellar medium with realistic interstellar turbulence driving that is consistent with the velocity dispersions observed in diffuse gas. We account for the turbulence cutoff arising from both collisional and collisionless damping. We obtain analytical expressions for diffusion coefficients that enter Fokker-Planck equation describing cosmic ray evolution. We obtain the scattering rate and show that fast modes provide the dominant contribution to cosmic ray scattering for the typical interstellar conditions in spite of the fact that fast modes are subjected to damping. We determine how the efficiency of the scattering depends on the characteristics of ionized media, e.g. plasma $β$. We calculate the range of energies for which the streaming instability is suppressed by the ambient MHD turbulence.

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arXiv2004

Dust Dynamics in Compressible MHD Turbulence

We calculate the relative grain-grain motions arising from interstellar magnetohydrodynamic (MHD) turbulence. The MHD turbulence includes both fluid motions and magnetic fluctuations. While the fluid motions accelerate grains through hydro-drag, the electromagnetic fluctuations accelerate grains through resonant interactions. We consider both incompressive (Alfvén) and compressive (fast and slow) MHD modes and use descriptions of MHD turbulence obtained in Cho & Lazarian (2002). Calculations of grain relative motion are made for realistic grain charging and interstellar turbulence that is consistent with the velocity dispersions observed in diffuse gas, including cutoff of the turbulence from various damping processes. We show that fast modes dominate grain acceleration, and can drive grains to supersonic velocities. Grains are also scattered by gyroresonance interactions, but the scattering is less important than acceleration for grains moving with sub-Alfvénic velocities. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains will be aligned with long axes perpendicular to the magnetic field. We compare grain velocities arising from MHD turbulence with those arising from photoelectric emission, radiation pressure and H$_{2}$ thrust. We show that for typical interstellar conditions turbulence should prevent these mechanisms from segregating small and large grains. Finally, gyroresonant acceleration is bound to preaccelerate grains that are further accelerated in shocks. Grain-grain collisions in the shock may then contribute to the overabundance of refractory elements in the composition of galactic cosmic rays.

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arXiv2004

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.

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arXiv2004

The Anisotropy of Electron MHD Turbulence

We present numerical studies of 3-dimensional electron magnetohydrodynamic (EMHD) turbulence. We investigate cascade timescale and anisotropy of freely decaying strong EMHD turbulence with zero electron skin depth. Cascade time scales with $k^{-4/3}$. Our numerical results clearly show scale-dependent anisotropy. We discuss that the observed anisotropy is consistent with $k_{\|} \propto k_{\perp}^{1/3}$, where $k_{\|}$ and $k_{\perp}$ are wave numbers parallel and perpendicular to (local) mean magnetic field, respectively.

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arXiv2004

Velocity Modification of Power Spectrum from Absorbing Medium

Quantitative description of the statistics of intensity fluctuations within spectral line data cubes introduced in our earlier work is extended to the absorbing media. A possibility of extracting 3D velocity and density statistics from both integrated line intensity as well as from the individual channel maps is analyzed. We find that absorption enables the velocity effects to be seen even if the spectral line is integrated over frequencies. This regime that is frequently employed in observations is characterized by a non-trivial relation between the spectral index of velocities and the spectral index of intensity fluctuations. For instance when density is dominated by fluctuations at large scales, i.e. when correlations scale as r^{-γ}, γ<0, the intensity fluctuations exhibit a universal spectrum of fluctuations ~K^{-3} over a range of scales. When small scale fluctuations of density contain most of the energy, i.e. when correlations scale as r^{-γ}, γ>0, the resulting spectrum of the integrated lines depends on the scaling of the underlying density and scales as K^{-3+γ}. We show that if we take the spectral line slices that are sufficiently thin we recover our earlier results for thin slice data without absorption. As the result we extend the Velocity Channel Analysis (VCA) technique to optically thick lines enabling studies of turbulence in molecular clouds. In addition, the developed mathematical machinery enables a quantitative approach to solving other problems that involved statistical description of turbulence within emitting and absorbing gas.

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arXiv2003

Cosmic Ray Scattering by Compressible Magnetohydrodynamic Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in the picture of cosmic ray transport. In this paper we use recently obtained scaling laws for MHD modes to calculate the scattering frequency for cosmic rays in the ISM. We consider gyroresonance with MHD modes (Alfvenic, slow and fast) and transit-time damping (TTD) by fast modes. We provide calculations of cosmic ray scattering for various phases of interstellar medium with realistic interstellar turbulence driving that is consistent with the velocity dispersions observed in diffuse gas. We account for the turbulence cutoff arising from both collisional and collisionless damping. We obtain analytical expressions for diffusion coefficients that enter Fokker-Planck equation describing cosmic ray evolution. We calculate the scattering rate and parallel spatial diffusion coefficients of cosmic rays for both Alfvenic and fast modes. We conclude that fast modes provides the dominant contribution to cosmic ray scattering for the typical interstellar conditions in spite of the fact that fast modes are subjected to damping. We show that the efficiency of the scattering depends on the plasma beta since it determines the damping of the fast modes. We also show that the streaming instability is modified in the presence of turbulence.

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arXiv2003

Translational Velocities and Rotational Rates of Interstellar Dust Grains

Interstellar dust grains exhibit complex dynamics which is essential for understanding many key interstellar processes that involve dust, including grain alignment, grain growth, grain shattering etc. Grain rotational and translational motions are affected not only by gaseous collisions, but also by interactions with ions, photons, magnetic fields etc. Some of those interactions, e.g. interactions of ions with the dipole electric moment of dust grains, require the quantum nature of the process to be accounted for. Similarly, coupling of rotational and vibrational degrees of freedom in a grain happens due to relaxation processes, among which the process related to nuclear spins frequently is the dominant one. This coupling modifies substantially both the dynamics of rotational and translational motions by inducing grain flips. The flipping averages our certain systematic torques and forces that act on grains. As the rate of flipping is larger for smaller grains, these grains can be ``thermally trapped'', i.e. rotate at a thermal rate in spite of the presence of systematic torques. Moreover, a subset of small grains with high dipole moments may rotate at subthermal rates due to high damping arising from grain emission in microwave range of frequencies. Translational dynamics of grains is mostly dominated by grain interactions with magnetohydrodynamic turbulence. Efficient turbulent mixing of dust grains limits the degree to which grains of different sizes may be segregated in space.

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arXiv2003

Grain Alignment in Molecular Clouds

One of the most informative techniques of studying magnetic fields in molecular clouds is based on the use of starlight polarization and polarized emission arising from aligned dust. How reliable the interpretation of the polarization maps in terms of magnetic fields is the issue that the grain alignment theory addresses. I briefly review basic physical processes involved in grain alignment.

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