TY - JOUR A1 - Maiti, Snehanshu A1 - Makwana, Kirit A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Cosmic-ray transport in magnetohydrodynamic turbulence JF - The astrophysical journal : an international review of spectroscopy and astronomical physics / part 1 N2 - This paper studies cosmic-ray (CR) transport in magnetohydrodynamic (MHD) turbulence. CR transport is strongly dependent on the properties of the magnetic turbulence. We perform test particle simulations to study the interactions of CR with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. Our results confirm that the fast modes dominate the CR propagation, whereas Alfven and slow modes are much less efficient and have shown similar pitch-angle scattering rates. We investigate the cross field transport on large and small scales. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by M-A(zeta) compared to the parallel diffusion coefficients, with zeta closer to 4 in Alfven modes than that in total turbulence, as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale, both the local and global magnetic reference frames are adopted. Superdiffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfven modes show clear Richardson diffusion in the local reference frame. The diffusion transitions smoothly from the Richardson's one with index 1.5 to normal diffusion as the particle mean free path decreases from lambda(parallel to) >> L to lambda(parallel to) << L, where L is the injection/coherence length of turbulence. Our results have broad applications to CRs in various astrophysical environments. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac46c8 SN - 1538-4357 VL - 926 IS - 1 PB - Institute of Physics Publ. CY - London ER - TY - JOUR A1 - Zhao, Siqi Q. A1 - Yan, Huirong A1 - Liu, Terry Z. A1 - Liu, Mingzhe A1 - Wang, Huizi T1 - Multispacecraft analysis of the properties of magnetohydrodynamic fluctuations in Sub-Alfvenic solar wind turbulence at 1 au JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present observations of three-dimensional magnetic power spectra in wavevector space to investigate the anisotropy and scalings of sub-Alfvenic solar wind turbulence at magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft. The magnetic power distributions are organized in a new coordinate determined by wavevectors ((kappa) over cap) and background magnetic field ((b) over cap (0)) in Fourier space. This study utilizes two approaches to determine wavevectors: the singular value decomposition method and multispacecraft timing analysis. The combination of the two methods allows an examination of the properties of magnetic field fluctuations in terms of mode compositions without any spatiotemporal hypothesis. Observations show that fluctuations (delta B-perpendicular to 1) in the direction perpendicular to (kappa) over cap and (b) over cap (0) prominently cascade perpendicular to (b) over cap (0), and such anisotropy increases with wavenumbers. The reduced power spectra of 6.8 11 follow Goldreich-Sridhar scalings: (P) over cap (k(perpendicular to)) proportional to k(perpendicular to)(-5/3) and (P) over cap (k(parallel to)) proportional to k(parallel to)(-2). In contrast, fluctuations within the (k) over cap(b) over cap (0) plane show isotropic behaviors: perpendicular power distributions are approximately the same as parallel distributions. The reduced power spectra of fluctuations within the (k) over cap(b) over cap (0) plane follow the scalings (P) over cap (k(perpendicular to)) proportional to k(perpendicular to)(-3/2) and (P) over cap (k(parallel to)) proportional to k(parallel to)(-3/2). Comparing frequency-wavevector spectra with theoretical dispersion relations of MHD modes, we find that delta B-perpendicular to 1 are probably associated with Alfven modes. On the other hand, magnetic field fluctuations within the (k) over cap(b) over cap (0) plane more likely originate from fast modes based on their isotropic behaviors. The observations of anisotropy and scalings of different magnetic field components are consistent with the predictions of current compressible MHD theory. Moreover, for the Alfvenic component, the ratio of cascading time to the wave period is found to be a factor of a few, consistent with critical balance in the strong turbulence regime. These results are valuable for further studies of energy compositions of plasma turbulence and their effects on energetic particle transport. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac822e SN - 0004-637X SN - 1538-4357 VL - 937 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Liu, Ruoyu A1 - Yan, Huirong T1 - On the unusually large spatial extent of the TeV nebula HESS J1825-137 BT - implication from the energy-dependent morphology JF - Monthly notices of the Royal Astronomical Society N2 - Deep observation of the High Energy Stereoscopic System (HESS) on the most extended pulsar wind nebula HESS J1825-137 reveals an enhanced energy-dependent morphology, providing useful information on the particle transport mechanism in the nebula. We find that the energy-dependent morphology is consistent with a diffusion-dominated transport of electrons/positrons. It provides an alternative possible interpretation for the unusually large spatial extent (i.e. greater than or similar to 100 pc) of the nebula, which could then be attributed to the diffusion of escaping electrons/positrons from a compact plerion. The influence of various model parameters on the energy-dependent extent of the nebula is studied in the diffusion-dominated scenario. We also show that the energy-dependent morphology of the nebula may also be used to study the spin-down history of the pulsar. KW - diffusion-radation mechanisms KW - non-thermal-pulsars KW - individual KW - HESS J1825-137 gamma-rays KW - general Y1 - 2020 U6 - https://doi.org/10.1093/mnras/staa911 SN - 0035-8711 SN - 1365-2966 VL - 494 IS - 2 SP - 2618 EP - 2627 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Makwana, Kirit D. A1 - Yan, Huirong T1 - Properties of magnetohydrodynamic modes in compressively driven plasma turbulence JF - Physical Review X N2 - We study properties of magnetohydrodynamic (MHD) eigenmodes by decomposing the data of MHD simulations into linear MHD modes-namely, the Alfven, slow magnetosonic, and fast magnetosonic modes. We drive turbulence with a mixture of solenoidal and compressive driving while varying the Alfven Mach number (M-A), plasma beta, and the sonic Mach number from subsonic to transsonic. We find that the proportion of fast and slow modes in the mode mixture increases with increasing compressive forcing. This proportion of the magnetosonic modes can also become the dominant fraction in the mode mixture. The anisotropy of the modes is analyzed by means of their structure functions. The Alfven-mode anisotropy is consistent with the Goldreich-Sridhar theory. We find a transition from weak to strong Alfvenic turbulence as we go from low to high M-A. The slow-mode properties are similar to the Alfven mode. On the other hand, the isotropic nature of fast modes is verified in the cases where the fast mode is a significant fraction of the mode mixture. The fast-mode behavior does not show any transition in going from low to high M-A. We find indications that there is some interaction between the different modes, and the properties of the dominant mode can affect the properties of the weaker modes. This work identifies the conditions under which magnetosonic modes can be a major fraction of turbulent astrophysical plasmas, including the regime of weak turbulence. Important astrophysical implications for cosmic-ray transport and magnetic reconnection are discussed. KW - Astrophysics KW - Plasma Physics Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevX.10.031021 SN - 2160-3308 VL - 10 IS - 3 PB - American Physical Society (APS) CY - College Park ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Polarization of submillimetre lines from interstellar medium JF - Monthly notices of the Royal Astronomical Society N2 - Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. KW - polarization KW - turbulence KW - H II regions KW - ISM: magnetic fields KW - photodissociation region (PDR) KW - submillimetre: ISM Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3164 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 2 SP - 2415 EP - 2420 PB - Oxford University Press CY - Oxford ER - TY - JOUR A1 - Lebiga, O. A1 - Santos-Lima, Reinaldo A1 - Yan, Huirong T1 - Kinetic-MHD simulations of gyroresonance instability driven by CR pressure anisotropy JF - Monthly notices of the Royal Astronomical Society N2 - The transport of cosmic rays (CRs) is crucial for the understanding of almost all high-energy phenomena. Both pre-existing large-scale magnetohydrodynamic (MHD) turbulence and locally generated turbulence through plasma instabilities are important for the CR propagation in astrophysical media. The potential role of the resonant instability triggered by CR pressure anisotropy to regulate the parallel spatial diffusion of low-energy CRs (less than or similar to 100 GeV) in the interstellar and intracluster medium of galaxies has been shown in previous theoretical works. This work aims to study the gyroresonance instability via direct numerical simulations, in order to access quantitatively the wave-particle scattering rates. For this, we employ a 1D PIC-MHD code to follow the growth and saturation of the gyroresonance instability. We extract from the simulations the pitch-angle diffusion coefficient D-mu mu produced by the instability during the linear and saturation phases, and a very good agreement (within a factor of 3) is found with the values predicted by the quasi-linear theory (QLT). Our results support the applicability of the QLT for modelling the scattering of low-energy CRs by the gyroresonance instability in the complex interplay between this instability and the large-scale MHD turbulence. KW - MHD KW - plasmas KW - turbulence KW - cosmic rays Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty309 SN - 0035-8711 SN - 1365-2966 VL - 476 IS - 2 SP - 2779 EP - 2791 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong A1 - Richter, Philipp T1 - The influence of atomic alignment on absorption and emission spectroscopy JF - Monthly notices of the Royal Astronomical Society N2 - Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in our Universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionization fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of Photodissociation regions in rho Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability. KW - magnetic fields KW - submillimetre: ISM KW - ultraviolet: ISM KW - ISM: abundances KW - ISM: lines and bands KW - techniques: spectroscopic Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1594 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3923 EP - 3935 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Xue, Rui A1 - Liu, Ruo-Yu A1 - Wang, Xiang-Yu A1 - Yan, Huirong A1 - Böttcher, Markus T1 - On the minimum jet power of TeV BL Lac objects in the p-gamma model JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We study the requirement of the jet power in the conventional p-gamma models (photopion production and Bethe-Heitler pair production) for TeV BL Lac objects. We select a sample of TeV BL Lac objects whose spectral energy distributions are difficult to explain by the one-zone leptonic model. Based on the relation between the p-gamma interaction efficiency and the opacity of gamma gamma absorption, we find that the detection of TeV emission poses upper limits on the p-gamma interaction efficiencies in these sources and hence minimum jet powers can be derived accordingly. We find that the obtained minimum jet powers exceed the Eddington luminosity of the supermassive black holes (SMBHs). Implications for the accretion mode of the SMBHs in these BL Lac objects and the origin of their TeV emissions are discussed. KW - galaxies: active KW - galaxies: jets KW - radiation mechanisms: non-thermal Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/aaf720 SN - 0004-637X SN - 1538-4357 VL - 871 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Liu, Ruo-Yu A1 - Yan, Huirong A1 - Wang, Xiang-Yu A1 - Shao, Shi A1 - Li, Hui T1 - Gamma-Ray production in the extended halo of the galaxy and possible implications for the origin of galactic cosmic rays JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Various studies have implied the existence of a gaseous halo around the Galaxy extending out to similar to 100 kpc. Galactic cosmic rays (CRs) that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale Galactic wind, can interact with the gas therein and produce gamma-rays via proton-proton collision. We calculate the CR distribution in the halo and the gamma-ray flux, and explore the dependence of the result on model parameters such as diffusion coefficient, CR luminosity, and CR spectral index. We find that the current measurement of isotropic gamma-ray background (IGRB) at less than or similar to TeV with the Fermi Large Area Telescope already approaches a level that can provide interesting constraints on the properties of Galactic CR (e.g., with CR luminosity L-CR <= 1041 erg s(-1)). We also discuss the possibilities of the Fermi bubble and IceCube neutrinos originating from the proton-proton collision between CRs and gas in the halo, as well as the implication of our results for the baryon budget of the hot circumgalactic medium of our Galaxy. Given that the isotropic gamma-ray background is likely to be dominated by unresolved extragalactic sources, future telescopes may extract more individual sources from the IGRB, and hence put even more stringent restrictions on the relevant quantities (such as Galactic CR luminosity and baryon budget in the halo) in the presence of a turbulent halo that we consider. KW - cosmic rays KW - Galaxy: halo KW - gamma rays: diffuse background KW - neutrinos Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/aaf567 SN - 0004-637X SN - 1538-4357 VL - 871 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Liu, Ruo-Yu A1 - Wang, Kai A1 - Xue, Rui A1 - Taylor, Andrew M. A1 - Wang, Xiang-Yu A1 - Li, Zhuo A1 - Yan, Huirong T1 - Hadronuclear interpretation of a high-energy neutrino event coincident with a blazar flare JF - Physical review : D, Particles, fields, gravitation, and cosmology N2 - Although many high-energy neutrinos detected by the IceCube telescope are believed to have an extraterrestrial origin, their astrophysical sources remain a mystery. Recently, an unprecedented discovery of a high-energy muon neutrino event coincident with a multiwavelength flare from a blazar, TXS 0506 + 056, shed some light on the origin of the neutrinos. It is usually believed that a blazar is produced by a relativistic jet launched from an accreting supermassive black hole (SMBH). Here, we show that the high-energy neutrino event can be interpreted by the inelastic hadronuclear interactions between the accelerated cosmic-ray protons in the relativistic jet and the dense gas clouds in the vicinity of the SMBH. Such a scenario only requires a moderate proton power in the jet, which could be much smaller than that required in the conventional hadronic model which instead calls upon the photomeson process. Meanwhile, the flux of the multiwavelength flare from the optical to gamma-ray band can be well explained by invoking a second radiation zone in the jet at a larger distance to the SMBH. In our model, the neutrino emission lasts a shorter time than the multiwavelength flare, so the neutrino event is not necessarily correlated with the flare, but it is probably accompanied by a spectrum hardening above a few giga-electron-volt (GeV). Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevD.99.063008 SN - 2470-0010 SN - 2470-0029 VL - 99 IS - 6 PB - American Physical Society CY - Melville ER -