arXiv subjects

Huirong Yan

Publications and source records attributed to Huirong Yan.

83 records · Page 5Linked to original sources

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

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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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 Acceleration by MHD Turbulence: Gyroresonance Mechanism

We discuss a new mechanism of dust acceleration that acts in a turbulent magnetized medium. The magnetohydrodynamic (MHD) turbulence includes both fluid motions and magnetic fluctuations. We show that while the fluid motions bring about grain motions through the drag, the electromagnetic fluctuations, can accelerate grains through resonant interactions. In this paper we calculate grain acceleration by gyroresonance in Cold Neutral Medium. We consider both incompressible and compressible MHD modes. We show that fast modes dominate grain acceleration. For the parameters chosen, fast modes render supersonic velocities to grains that may shatter grains and enable efficient absorption of heavy elements. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains get aligned with long axes perpendicular to the magnetic field.

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arXiv2002

The Properties of Compressible MHD and Cosmic Ray Transport

Turbulence is the most common state of astrophysical flows. In typical astrophysical fluids, turbulence is accompanied by strong magnetic fields, which has a large impact on the dynamics of the turbulent cascade. Recently, there has been a significant breakthrough on the theory of magnetohydrodynamic (MHD) turbulence. For the first time we have a scaling model that is supported by both observations and numerical simulations. We review recent progress in studies of both incompressible and compressible turbulence. We compare Iroshnikov-Kraichnan and Goldreich-Sridhar models, and discuss scalings of Alfven, slow, and fast waves. We discuss the implications of this new insight into MHD turbulence for cosmic ray transport.

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arXiv2002

Scattering of Cosmic Rays by Magnetohydrodynamic Interstellar Turbulence

Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call for substantial revisions in our understanding of cosmic ray transport. In this paper we use gyroresonance 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 conclude that the gyroresonance with fast modes is the dominant contribution to cosmic ray scattering for the typical interstellar conditions. In contrast to earlier studies, we find that Alfvenic and slow modes are inefficient because they are far from isotropy usually assumed.

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arXiv2002

Dust Motions Driven by MHD Turbulence

We discuss the relative grain motions due to MHD turbulence in interstellar medium. It has been known for decades that turbulent drag is an efficient way to induce grain relative motions. However, earlier treatments disregarded magnetic field and used Kolmogorov turbulence. Unlike hydro turbulence, MHD turbulence is anisotropic on small scales. Moreover, compressible modes are important for MHD and magnetic perturbations can directly interact with grains. We provide calculations of grain relative motion for realistic interstellar turbulence driving that is consistent with the velocity dispersions observed in diffuse gas and for realistic grain charging. We account for the turbulence cutoff arising from abmipolar drag. Our results on grain shattering are consistent with the customary accepted cutoff size. We obtain grain velocities for turbulence with parameters consistent with those in HI and dark clouds. These velocities are smaller than those in earlier papers, where MHD effects were disregarded. Finally, we consider grain velocities arising from photoelectric emission, radiation pressure and the thrust due to molecular hydrogen formation. These are lower than relative velocities induced by turbulence. We conclude that turbulence should prevent these mechanisms from segregating grains by size.

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arXiv2002

Grain Dynamics In Magnetized Interstellar Gas

Interstellar medium is turbulent and this induces relative motions of dust grains. We calculate relative velocities of charged grains in a partially ionized magnetized gas. We account for anisotropy of magnetohydrodynamic (MHD) turbulence, grain coupling with magnetic field, and the turbulence cutoff arising from the ambipolar drag. We obtain grain velocities for turbulence with parameters consistent with those in HI and dark clouds. Those velocities are smaller than those in earlier papers, where MHD effects were disregarded. Finally, we compare grain velocities arising from photoelectric emission, radiation pressure and thrusts from molecular hydrogen formation. We conclude that turbulence should prevent segregation of grains of different sizes.

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arXiv2001

Scaling of ISM Turbulence: Implications for HI

Galactic HI is a gas that is coupled to magnetic field because of its fractional ionization. Many properties of HI are affected by turbulence. Recently, there has been a significant breakthrough on the theory of magnetohydrodynamic (MHD) turbulence. For the first time in the history of the subject we have a scaling model that is supported by numerical simulations. We review recent progress in studies of both incompressible and compressible turbulence. We also discuss the new regime of MHD turbulence that happens below the scale at which conventional turbulent motions get damped by viscosity. The viscosity in the case of HI is being produced by neutrals and truncates the turbulent cascade at parsec scales. We show that below this scale magnetic fluctuations with a shallow spectrum persist and point out to a possibility of the resumption of the MHD cascade after ions and neutrals decouple. We discuss the implications of the new insight into MHD turbulence for cosmic ray transport and grain dynamics.

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