ABSTRACT This paper considers turbulent damping of Alfvén waves in magnetized plasmas. We identify two cases of damping, one related to damping of cosmic-ray streaming instability, the other related ...to damping of Alfvén waves emitted by a macroscopic wave source, e.g., a stellar atmosphere. The physical difference between the two cases is that in the former case the generated waves are emitted with respect to the local direction of the magnetic field, and in the latter, waves are emitted with respect to the mean field. The scaling of damping is different in the two cases. We explore effects of turbulence in the regimes from sub-Alfvénic to super-Alfvénic to obtain analytical expressions for the damping rates and define the ranges of applicability of these expressions. In describing the damping of the streaming instability, we find that for sub-Alfvénic turbulence, the range of cosmic-ray energies influenced by weak turbulence is unproportionally large compared to the range of scales where weak turbulence is present. On the contrary, the range of cosmic-ray energies affected by strong Alfvénic turbulence is rather limited. A number of astrophysical applications of the process ranging from launching of stellar and galactic winds to propagation of cosmic rays in galaxies and clusters of galaxies is considered. In particular, we discuss how to reconcile the process of turbulent damping with the observed isotropy of the Milky Way cosmic rays.
Abstract
Radiative torques (RATs) or mechanical torques acting on irregular grains can induce the alignment of dust grains in respect to the alignment axis (AA), which can be either the direction of ...the magnetic field or the direction of the radiation. We show that carbonaceous grains can be aligned with their axes both parallel and perpendicular to the AA, and we explore the conditions where the particular mode of alignment takes place. We identify a new process of alignment of charged carbonaceous grains in a turbulent, magnetized interstellar medium with respect to an electric field. This field acts on grains accelerated in a turbulent medium and gyrorotating about a magnetic field. The electric field can also arise from the temporal variations of the magnetic field strength in turbulent, compressible media. The direction of the electric field is perpendicular to the magnetic field, and the carbonaceous grains precess in the electric field because of their electric moments. If this precession is faster than Larmor precession in the magnetic field, the alignment of such grains is with their long axes parallel to the magnetic field. We explore the parameter space for which the new mechanism aligns grains with long axes parallel to the magnetic field. We compare this mechanism with another process that provides the same type of alignment, namely, the RAT alignment of grains with insufficiently fast internal relaxation. We describe the conditions for which the particular mode of carbonaceous grain alignment is realized and discuss what information can be obtained by measuring the resulting polarization.
ABSTRACT By following the Kazantsev theory and taking into account both microscopic and turbulent diffusion of magnetic fields, we develop a unified treatment of the kinematic and nonlinear stages of ...a turbulent dynamo process, and we study the dynamo process for a full range of magnetic Prandtl number Pm and ionization fractions. We find a striking similarity between the dependence of dynamo behavior on Pm in a conducting fluid and (a function of ionization fraction) in a partially ionized gas. In a weakly ionized medium, the kinematic stage is largely extended, including not only exponential growth but a new regime of dynamo characterized by a linear-in-time growth of magnetic field strength, and the resulting magnetic energy is much higher than the kinetic energy carried by viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear stage is unaffected by microscopic diffusion processes and has a universal linear-in-time growth of magnetic energy with the growth rate as a constant fraction 3/38 of the turbulent energy transfer rate, showing good agreement with earlier numerical results. Applying the analysis to the first stars and galaxies, we find that the kinematic stage is able to generate a field strength only an order of magnitude smaller than the final saturation value. But the generation of large-scale magnetic fields can only be accounted for by the relatively inefficient nonlinear stage and requires longer time than the free-fall time. It suggests that magnetic fields may not have played a dynamically important role during the formation of the first stars.
Magnetic fields play a crucial role in various astrophysical processes, including star formation, accretion of matter, transport processes (e.g., transport of heat), and cosmic rays. One of the ...easiest ways to determine the magnetic field direction is via polarization of radiation resulting from extinction or/and emission by aligned dust grains. Reliability of interpretation of the polarization maps in terms of magnetic fields depends on how well we understand the grain-alignment theory. Explaining what makes grains aligned has been one of the big issues of the modern astronomy. Numerous exciting physical effects have been discovered in the course of research undertaken in this field. As both the theory and observations matured, it became clear that the grain-alignment phenomenon is inherent not only in diffuse interstellar medium or molecular clouds but also is a generic property of the dust in circumstellar regions, interplanetary space and cometary comae. Currently the grain-alignment theory is a predictive one, and its results nicely match observations. Among its predictions is a subtle phenomenon of radiative torques. This phenomenon, after having stayed in oblivion for many years after its discovery, is currently viewed as the most powerful means of alignment. In this article, I shall review the basic physical processes involved in grain alignment, and the currently known mechanisms of alignment. I shall also discuss possible niches for different alignment mechanisms. I shall dwell on the importance of the concept of grain helicity for understanding of many properties of grain alignment, and shall demonstrate that rather arbitrarily shaped grains exhibit helicity when they interact with gaseous and radiative flows.
Alignment of dust grains in astrophysical environments results in the polarization of starlight as well as the polarization of radiation emitted by dust. We demonstrate the advances in grain ...alignment theory that allow the use of linear and circular polarization to probe not only the magnetic field, but also dust composition, the dust environment, etc. We revisit the process of grain alignment by Radiative Torques (RATs) and focus on constraining magnetic susceptibility of grains via observations. We discuss the possibility of observational testing of the magnetic properties of grains as the alignment changes from being in respect to the magnetic field to being in respect to the radiation direction. This both opens a possibility of constraining the uncertain parameters of the RATs theory and provides a new way of measuring magnetic fields in the interstellar medium and circumstellar regions. We provide a detailed discussion of the precession induced both by the magnetic field and the anisotropic radiation and revisit a number of key processes related to magnetic response of the grains. We consider various effects that increase the rate of magnetic relaxation both in silicate and carbonaceous grains. In particular, we find a new relaxation process related to the change of the amplitude of internal magnetization within a wobbling triaxial grain and identify a range of grain sizes in which this effect can dominate the internal alignment of angular momentum within grain axes. We show that these relaxation processes significantly change the dynamics of grains in the presence of RATs. We apply our analysis for observed grain alignment in special environments to put constraints on the enhanced magnetic properties of dust grains in the cloud near supernovae, in cometary coma, and protoplanetary disks.
Grain alignment by radiative torques (RATs) has been extensively studied for various environment conditions, including interstellar medium, dense molecular clouds and accretion discs, thanks to ...significant progress in observational, theoretical and numerical studies. In this paper, we explore the alignment by RATs and provide quantitative predictions of dust polarization for a set of astrophysical environments that can be tested observationally. We first consider the alignment of grains in the local interstellar medium and compare predictions for linear polarization by aligned grains with recent observational data for nearby stars. We then revisit the problem of grain alignment in accretions discs by taking into account the dependence of RAT alignment efficiency on the anisotropic direction of radiation fields relative to magnetic fields. Moreover, we study the grain alignment in interplanetary medium, including diffuse Zodiacal cloud and cometary comae, and calculate the degree of circular polarization (CP) of scattered light arising from single scattering by aligned grains. We also discuss a new type of grain alignment, namely the alignment with respect to the ambient electric field instead of the alignment with the magnetic field. We show that this type of alignment can allow us to reproduce the systematic features of CP observed across a cometary coma. Our findings suggest that polarized Zodiacal dust emission may be an important polarized foreground component, which should be treated carefully in cosmic microwave background experiments.
The advancement of our understanding of MHD turbulence opens ways to develop new techniques to probe magnetic fields. In MHD turbulence, the velocity gradients are expected to be perpendicular to ...magnetic fields and this fact was used by González-Casanova & Lazarian to introduce a new technique to trace magnetic fields using velocity centroid gradients (VCGs). The latter can be obtained from spectroscopic observations. We apply the technique to GALFA-H i survey data and then compare the directions of magnetic fields obtained with our technique to the direction of magnetic fields obtained using PLANCK polarization. We find an excellent correspondence between the two ways of magnetic field tracing, which is obvious via the visual comparison and through the measuring of the statistics of magnetic field fluctuations obtained with the polarization data and our technique. This suggests that the VCGs have a potential for measuring of the foreground magnetic field fluctuations, and thus provide a new way of separating foreground and CMB polarization signals.
We study the alignment of irregular dust grains by mechanical torques due to the drift of grains through the ambient gas. We first calculate mechanical alignment torques (MATs) resulting from ...specular reflection of gas atoms for seven irregular shapes: one shape of mirror symmetry, three highly irregular shapes (HIS), and three weakly irregular shapes (WIS). We find that the grain with mirror symmetry experiences negligible MATs due to its mirror-symmetry geometry. Three HIS can produce strong MATs, which exhibit some generic properties as radiative torques (RATs), while three WIS produce less efficient MATs. We then study grain alignment by MATs for the different angles between the drift velocity and the ambient magnetic field, for paramagnetic and superparamagnetic grains assuming efficient internal relaxation. We find that for HIS grains, MATs can align subsonically drifting grains in the same way as RATs, with low-J and high-J attractors. For supersonic drift, MATs can align grains with low-J and high-J attractors, analogous to RAT alignment by anisotropic radiation. We also show that the joint action of MATs and magnetic torques in grains with iron inclusions can lead to perfect MAT alignment. Our results point out the potential importance of MAT alignment for HIS grains predicted by the analytical model of Lazarian & Hoang, although more theoretical and observational studies are required due to uncertainty in the shape of interstellar grains. We outline astrophysical environments where MAT alignment is potentially important.
We identify velocity channel map intensities as a new way to trace magnetic fields in turbulent media. This work makes use of both the modern theory of magnetohydrodynamic (MHD) turbulence, which ...predicts that magnetic eddies are aligned with the local direction of the magnetic field, and also the theory of spectral line position-position-velocity (PPV) statistics, which describes how velocity and density fluctuations are mapped onto PPV space. In particular, we use the fact that the fluctuations of the intensity of thin channel maps are mostly affected by the turbulent velocity, while the thick maps are dominated by density variations. We study how contributions of the fundamental MHD modes affect the Velocity Channel Gradients (VChGs), and demonstrate that the VChGs arising from Alfvén and slow modes are aligned perpendicular to the local direction of the magnetic field, while the VChGs produced by the fast mode are aligned parallel to the magnetic field. The dominance of Alfvén and slow modes in interstellar media will therefore allow reliable magnetic field tracing using the VChGs. We explore ways of identifying self-gravitating regions that do not require polarimetric information. In addition, we also introduce a new measure, termed "Reduced Velocity Centroids" (RVCGs), and compare its abilities with those of VChGs. We employed VChGs in analyzing GALFA 21 cm data and successfully compared the magnetic field directions with the Planck polarization observations. The applications of the suggested techniques include both tracing the magnetic field in diffuse interstellar media and star-forming regions, and removing the galactic foreground in the framework of cosmological polarization studies.
Diffusion of cosmic rays (CRs) is the key process for understanding their propagation and acceleration. We employ the description of spatial separation of magnetic field lines in magnetohydrodynamic ...turbulence in Lazarian & Vishniac to quantify the divergence of the magnetic field on scales less than the injection scale of turbulence and show that this divergence induces superdiffusion of CR in the direction perpendicular to the mean magnetic field. The perpendicular displacement squared increases, not as the distance x along the magnetic field, which is the case for a regular diffusion, but as the x super(3) for freely streaming CRs. The dependence changes to x super(3/2) for the CRs propagating diffusively along the magnetic field. In the latter case, we show that it is important to distinguish the perpendicular displacement with respect to the mean field and to the local magnetic field. We consider how superdiffusion changes the acceleration of CRs in shocks and show how it decreases efficiency of the CRs acceleration in perpendicular shocks. We also demonstrate that in the case when the small-scale magnetic field is generated in the pre-shock region, an efficient acceleration can take place for the CRs streaming without collisions along the magnetic loops.
PDF
