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Ions and neutrals in the interstellar medium play a key role in the dynamics of magnetohydrodynamic (MHD) turbulence, but challenging to study. In this study, we investigate the damping of MHD turbulence using 3D two-fluid simulations generated with the AthenaK code. Specifically, we examine the density, velocity, and magnetic field statistics of the two-fluid turbulence. Our results demonstrate that when ions and neutrals are strongly coupled, the velocity statistics resemble those of single-fluid MHD turbulence. However, when neutrals begin to decouple from ions, turbulence in both neutrals and ions is damped, resulting in steep kinetic energy spectra compared to Kolmogorov-type turbulence. We attribute the damping of turbulence in neutrals to their local coupling with ions, caused by local variations in the ionization fraction and Alfv\'en speed. The neutral-ion decoupling scale is not fixed but extends to a range of values. After neutrals completely decouple from ions, the neutrals have a Kolmogorov-type kinetic energy spectrum, while the ions' spectrum remains steep. We find that ion and neutral densities can be different when their coupling is weak but velocity statistics remain similar, indicating that densities are more sensitive to neutral-ion decoupling than velocities. The density structures of ions are filamentary, while those of neutrals are clumpy. Using the probability distribution function of logarithmic mass density, we find density fluctuations in ions can be enhanced when neutral and ions are weakly coupled. We confirm that the magnetic field spectrum can be steep due to the damping of MHD turbulence by neutral-ion collision., Comment: 17 pages, 14 figures, submitted to MNRAS

Propagation of cosmic rays (CRs) in turbulent and magnetized astrophysical media is a long-standing problem that requires both understanding of the properties of turbulent magnetic fields and their interaction with energetic particles. This review focuses on selected recent theoretical findings made based on the progress in understanding and simulating magnetohydrodynamic (MHD) turbulence. In particular, we address the problem of perpendicular and parallel propagation of CRs and identify the conditions when the perpendicular propagation is superdiffusive and diffusive. For the parallel diffusion, we discuss the problems of the traditionally used diffusion mechanism arising from pitch angle scattering and the possible solutions provided by the recently identified ``mirror diffusion" in the presence of turbulent magnetic mirrors., 21 pages, 4 figures, submitted to Frontiers in Astronomy and Space Sciences

The ubiquitous interstellar turbulence regulates star formation and the scaling relations between the initial velocity differences and the initial separations of stars. We propose that the formation of wide binaries with initial separations $r$ in the range $\sim 10^3~\text{AU} \lesssim r \lesssim 10^5$ AU is a natural consequence of star formation in the turbulent interstellar medium. With the decrease of $r$, the mean turbulent relative velocity $v_\text{tur}$ between a pair of stars decreases, while the largest velocity $v_\text{bon}$ at which they still may be gravitationally bound increases. When $v_\text{tur} < v_\text{bon}$, a wide binary can form. In this formation scenario, we derive the eccentricity distribution $p(e)$ of wide binaries for an arbitrary relative velocity distribution. By adopting a turbulent velocity distribution, we find that wide binaries at a given initial separation generally exhibit a superthermal $p(e)$. This provides a natural explanation for the observed superthermal $p(e)$ of the wide binaries in the Solar neighborhood., 7 pages, 4 figures, submitted to ApJL

In a weakly compressible high-$β$ medium, pitch-angle scattering and the associated scattering acceleration of cosmic rays (CRs) by anisotropic Alfvén and slow modes of magnetohydrodynamic (MHD) turbulence is inefficient. To tap the energy from magnetic compressions for efficient particle acceleration, a diffusion mechanism that can effectively confine particles in space without causing their trapping or pitch-angle isotropization is needed. We find that the mirror diffusion in MHD turbulence recently identified in Lazarian and Xu (2021) satisfies all the above conditions and serves as a promising diffusion mechanism for efficient acceleration of CRs via their stochastic non-resonant interactions with magnetic compressions/expansions. The resulting mirror acceleration is dominated by the slow-mode eddies with their lifetime comparable to the mirror diffusion time of CRs. Consequently, we find that the acceleration time of mirror acceleration is independent of the spatial diffusion coefficient of CRs. The mirror acceleration brings new life for the particle acceleration in a weakly compressible/incompressible medium and has important implications for studying CR re-acceleration in the high-$β$ intracluster medium., 10 pages, 2 figures, submitted to ApJ

Delayed radio flares of optical tidal disruption events (TDEs) indicate the existence of non-relativistic outflows accompanying TDEs. The interaction of TDE outflows with the surrounding circumnuclear medium creates quasi-perpendicular shocks in the presence of toroidal magnetic fields. Because of the large shock obliquity and large outflow velocity, we find that the shock acceleration induced by TDE outflows generally leads to a steep particle energy spectrum, with the power-law index significantly larger than the "universal" index for a parallel shock. The measured synchrotron spectral indices of recently detected TDE radio flares are consistent with our theoretical expectation. It suggests that the particle acceleration at quasi-perpendicular shocks can be the general acceleration mechanism accounting for the delayed radio emission of TDEs., 8 pages, 5 figures; accepted for publication in The Astrophysical Journal

Alfv��nic component of MHD turbulence damps Alfv��nic waves. The consequences of this effect are important for many processes, from cosmic ray (CR) propagation to launching outflows and winds in galaxies and other magnetized systems. We discuss the differences in the damping of the streaming instability by turbulence and the damping of a plane parallel wave. The former takes place in the system of reference aligned with the local direction of magnetic field along which CRs stream. The latter is in the reference frame of the mean magnetic field and traditionally considered in plasma studies. We also compare the turbulent damping of streaming instability with ion-neutral collisional damping, which becomes the dominant damping effect at a sufficiently low ionization fraction. Numerical testing and astrophysical implications are also discussed., 19 pages, 3 figures, accepted for publication in Frontiers in Astronomy and Space Sciences

Understanding the physics of how stars form is a highly-prioritized goal of modern Astrophysics, in part because star formation is linked to both galactic dynamics on large scales and to the formation of planets on small scales. It is well-known that stars form from the gravitational collapse of molecular clouds, which are in turn formed out of the turbulent interstellar medium. Star formation is highly inefficient, with one of the likely culprits being the regulation against gravitational collapse provided by magnetic fields. Measurement of the polarized emission from interstellar dust grains, which are partially aligned with the magnetic field, provides a key tool for understanding the role these fields play in the star formation process. Over the past decade, much progress has been made by the most recent generation of polarimeters operating over a range of wavelengths (from the far-infrared through the millimeter part of the spectrum) and over a range of angular resolutions (from less than an arcsecond through fractions of a degree). Future developments in instrument sensitivity for ground-based, airborne, and space-borne polarimeters operating over range of spatial scales are critical for enabling revolutionary steps forward in our understanding of the magnetized turbulence from which stars are formed., Submitted to the Astro2020 Decadal Survey (with a few updated citations)

The Probe of Inflation and Cosmic Origins (PICO) is an imaging polarimeter that will scan the sky for 5 years in 21 frequency bands spread between 21 and 799 GHz. It will produce full-sky surveys of intensity and polarization with a final combined-map noise level of 0.87 $\mu$K arcmin for the required specifications, equivalent to 3300 Planck missions, and with our current best-estimate would have a noise level of 0.61 $\mu$K arcmin (6400 Planck missions). PICO will either determine the energy scale of inflation by detecting the tensor to scalar ratio at a level $r=5\times 10^{-4}~(5\sigma)$, or will rule out with more than $5\sigma$ all inflation models for which the characteristic scale in the potential is the Planck scale. With LSST's data it could rule out all models of slow-roll inflation. PICO will detect the sum of neutrino masses at $>4\sigma$, constrain the effective number of light particle species with $\Delta N_{\rm eff}, Comment: Probe class mission study submitted to NASA and 2020 Decadal Panel. Executive summary: 2.5 pages; Science: 28 pages; Total: 50 pages, 36 figures

This study is motivated by recent observations on the ubiquitous interstellar density filaments and guided by the modern theory of magnetohydrodynamic (MHD) turbulence. The interstellar turbulence shapes the observed density structure. The perpendicular turbulent mixing, as the fundamental dynamics of MHD turbulence, naturally entails parallel filaments presented in both the diffuse medium and molecular clouds (MCs). The minimum width is determined by the perpendicular neutral-ion decoupling scale in partially ionized media. Differently, dense perpendicular filaments arise in highly supersonic turbulence in MCs as a result of shock compression. Their width specifically depends on the turbulence properties. We demonstrate that different alignments of filaments with respect to the magnetic field originate from the varying turbulence properties in the multi-phase interstellar medium., 6 pages, submitted to ApJ

Astrophysical fluids are turbulent, magnetized and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening in different ranges of scales. A significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of the MHD turbulence physics facilitates the development of various theoretical domains, including the damping process for dissipating MHD turbulence and the dynamo process for generating MHD turbulence with many important astrophysical implications. In this paper, we review some important findings from our recent theoretical works to demonstrate the interconnection between the properties of MHD turbulence and those of turbulent dynamo in a partially ionized gas. We also briefly exemplify some new tentative studies on how the revised basic processes influence the associated outstanding astrophysical problems in, such as, magnetic reconnection, cosmic ray scattering, magnetic field amplification in both the early and the present-day universe., Comment: 25 pages, 9 figures, published in New Journal of Physics