Cosmic ray-driven magnetohydrodynamic waves in magnetized self-gravitating dusty molecular clouds

The effects of cosmic ray (CR) diffusion and finite Larmor radius (FLR) corrections have been studied on the linear gravitational instability of thermally conducting plasmas typically in the H ii regions of molecular clouds. The hydrodynamic fluid–fluid approach is considered for interacting CRs with gravitating, magnetized, and thermally conducting gas in molecular clouds. The magnetohydrodynamic fluid model is formulated considering CR pressure gradients, CR diffusion, and radiative and FLR effects in terms of particle Larmor radius. The dispersion relation of the gravitational instability is analytically derived using the normal mode analysis, and the effects of CRs and FLR corrections have been discussed in longitudinal and transverse modes. It is observed that in the absence of CRs, the FLR effects (magnetic viscosity) reduce the growth rate for wavenumber smaller than a critical value, and above it gets increased. However, the growth rate is strongly suppressed in the presence of combined CRs and FLR effects. The individual behaviour of FLR effects is observed to destabilize the growth rate of the gravitational instability in the presence of CR effects. The CR pressure decreases the growth rates of the gravitational and thermal instabilities, whereas parallel CR diffusion enhances the growth rate of the gravitational instability. The Jeans length of the gravitating gas cloud gets increased due to an increase in the CR-to-gas pressure ratio. It is found that the gravitational collapse of the system is supported by high-energy (above knee) CR particles with the Larmor radii comparable to the cloud size. The present results have been applied to understand the role of CRs and FLR corrections on the gravitational collapse in the H ii regions of molecular clouds.

The interaction of two populations of highly energetic cosmic rays (CRs) and suprathermal kappa gas in the astrophysical systems manifests exciting features of low-frequency magnetohydrodynamic (MHD) waves and instabilities. Contrary to the previous works on waves and instability analysis in Maxwellian gas, this paper investigates the effects of suprathermal corrections on the CR driven MHD waves and gravitational (Jeans) instability using the kappa distribution function. The equation of state for a kappa gas, including spectral κ− index, is considered in the CR-plasma interactions using the hydrodynamic fluid–fluid approach. The modified dispersion properties of fast, slow, and pure Alfvén waves and Jeans instability have been discussed in a suprathermal gas in astrophysical environments. The suprathermal corrections enhance the phase speed of the fast mode of MHD waves which is found to be greater in the suprathermal gas (κ>3/2) and smaller in the Maxwellian gas (κ→∞). In the absence of CR diffusion, the Jeans instability criterion is modified due to the simultaneous presence of CR pressure and suprathermal corrections. However, in the presence of CR diffusion, only suprathermal corrections modify the Jeans instability criterion. The suprathermal gases with higher degrees of freedom require large values of the Jeans wavenumber to produce gravitational instability and make the system more unstable. The suprathermal corrections along with modified thermal speed stabilize the growth rate of Jean instability, supporting the gravitational collapse of non-thermal gas in astrophysical systems.

view Abstract Citations (33) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Acoustic Instability Driven by Cosmic-Ray Streaming Begelman, Mitchell C. ; Zweibel, Ellen G. Abstract We study the linear stability of compressional waves in a medium through which cosmic rays stream at the Alfven speed due to strong coupling with Alfven waves. Acoustic waves can be driven unstable by the cosmic-ray drift, provided that the streaming speed is sufficiently large compared to the thermal sound speed. Two effects can cause instability: (1) the heating of the thermal gas due to the damping of Alfven waves driven unstable by cosmic-ray streaming; and (2) phase shifts in the cosmic-ray pressure perturbation caused by the combination of cosmic-ray streaming and diffusion. The instability does not depend on the magnitude of the background cosmic-ray pressure gradient, and occurs whether or not cosmic-ray diffusion is important relative to streaming. When the cosmic-ray pressure is small compared to the gas pressure, or cosmic-ray diffusion is strong, the instability manifests itself as a weak overstability of slow magnetosonic waves. Larger cosmic-ray pressure gives rise to new hybrid modes, which can be strongly unstable in the limits of both weak and strong cosmic-ray diffusion and in the presence of thermal conduction. Parts of our analysis parallel earlier work by McKenzie & Webb (which were brought to our attention after this paper was accepted for publication), but our treatment of diffusive effects, thermal conduction, and nonlinearities represent significant extensions. Although the linear growth rate of instability is independent of the background cosmic-ray pressure gradient, the onset of nonlinear eff ects does depend on absolute value of DEL (vector differential operator) Pc. At the onset of nonlinearity the fractional amplitude of cosmic-ray pressure perturbations is delta PC/PC approximately (kL) -1 much less than 1, where k is the wavenumber and L is the pressure scale height of the unperturbed cosmic rays. We speculate that the instability may lead to a mode of cosmic-ray transport in which plateaus of uniform cosmic-ray pressure are separated by either laminar or turbulent jumps in which the thermal gas is subject to intense heating. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174519 Bibcode: 1994ApJ...431..689B Keywords: Acoustic Instability; Galactic Cosmic Rays; Magnetohydrodynamic Waves; Particle Acceleration; Sound Waves; Wave Propagation; Cosmology; Plasma Diffusion; Plasma Heating; Plasma Turbulence; Pressure Gradients; Propagation Velocity; Transport Theory; Astrophysics; ACCELERATION OF PARTICLES; ISM: COSMIC RAYS; INSTABILITIES; WAVES full text sources ADS |

Radiation pressure and galactic cosmic rays-driven gravitational instability in rotating and magnetized viscoelastic fluids

The generation of magnetohydrodynamic (MHD) waves and their instabilities are studied in galactic gaseous rotating plasmas with the effects of the magnetic field, the self gravity, the diffusion-convection of cosmic rays as well as the gas and cosmic-ray pressures. The coupling of the Jeans, Alfvén and magnetosonic waves, and the conditions of damping or instability are studied in three different cases, namely when the propagation direction is perpendicular, parallel and oblique to the static magnetic field, and are shown to be significantly modified by the effects of the Coriolis force due to the rotation of cosmic fluids and the cosmic-ray diffusion. The coupled modes can be damped or anti-damped depending on the wave number is above or below the Jeans critical wave number that is reduced by the effects of the Coriolis force and the cosmic-ray pressure. It is found that the deviation of the axis of rotation from the direction of the static magnetic field gives rise to the coupling between the Alfvén wave and the classical Jeans mode which otherwise results into the modified slow and fast Alfvén waves as well as the modified classical Jeans modes. Furthermore, due to the effects of the cosmic rays diffusion, there appears a new wave mode (may be called the fast Jeans mode) in the intermediate frequency regimes of the slow and fast Alfvén waves, which seems to be dispersionless in the long-wavelength propagation and has a lower growth rate of instability in the high density regimes of galaxies. The dispersion properties and the instabilities of different kinds of MHD waves reported here can play pivotal roles in the formation of various galactic structures at different length scales.

The gravitational (Jeans) instability of radiative quantum plasma including cosmic ray (CR) pressure and diffusion is theoretically investigated using a generalized hyperbolic magneto-hydrodynamic model. It concurrently includes the impacts of the finite electrical resistivity, the Hall parameter, and the Coriolis force. The application of normal mode technique yields a unique form of a generalized dispersion relation. This dispersion relation is further discussed in the different modes of propagation with the different axis of rotation along the direction of the magnetic field. It has been noted that all the considered parameters affect the system's growth rate in both directions, but the Hall parameter does not affect it in the transverse direction. We also explored that together with the CRs, the Hall parameter, resistivity, rotation, and quantum parameter suppressed the Jeans instability's growth rate. Thus, these parameters act as stabilizing agents to the instability. The study identified radiative instability and analyzed the impact of an arbitrary heat-loss function on the system. The current findings provide new theoretical support to the existing various astronomic observations on the cosmic plasma and in the development of unique galactic formations of distinct scale lengths.

Context. Ubiquitous vortex flows at the solar surface excite magnetohydrodynamic (MHD) waves that propagate to higher layers of the solar atmosphere. In the solar corona, these waves frequently encounter magnetic null points. The interaction of MHD waves with a coronal magnetic null in realistic 3D setups requires an appropriate wave identification method. Aims. We present a new MHD wave decomposition method that overcomes the limitations of existing wave identification methods. Our method allows for an investigation of the energy fluxes in different MHD modes at different locations of the solar atmosphere as waves generated by vortex flows travel through the solar atmosphere and pass near the magnetic null. Methods. We used the open-source MPI-AMRVAC code to simulate wave dynamics through a coronal null configuration. We applied a rotational wave driver at our bottom photospheric boundary to mimic vortex flows at the solar surface. To identify the wave energy fluxes associated with different MHD wave modes, we employed a wave decomposition method that is able to uniquely distinguish different MHD modes. Our proposed method utilizes the geometry of an individual magnetic field-line in the 3D space to separate the velocity perturbations associated with the three fundamental MHD waves. We compared our method with an existing wave decomposition method that uses magnetic flux surfaces instead. Over the selected flux surfaces, we calculated and analyzed the temporally averaged wave energy fluxes, as well as the acoustic and magnetic energy fluxes. Our wave decomposition method allowed us to estimate the relative strengths of individual MHD wave energy fluxes. Results. Our method for wave identification is consistent with previous flux-surface-based methods and provides the expected results in terms of the wave energy fluxes at various locations of the null configuration. We show that ubiquitous vortex flows excite MHD waves that contribute significantly to the Poynting flux in the solar corona. Alfvén wave energy flux accumulates on the fan surface and fast wave energy flux accumulates near the null point. There is a strong current density buildup at the spine and fan surface. Conclusions. The proposed method has advantages over previously utilized wave decomposition methods, since it may be employed in realistic simulations or magnetic extrapolations, as well as in real solar observations whenever the 3D fieldline shape is known. The essential characteristics of MHD wave propagation near a null – such as wave energy flux accumulation and current buildup at specific locations – translate to the more realistic setup presented here. The enhancement in energy flux associated with magneto-acoustic waves near nulls may have important implications in the formation of jets and impulsive plasma flows.

As the fundamental physical process with many astrophysical implications, the diffusion of cosmic rays (CRs) is determined by their interaction with magnetohydrodynamic (MHD) turbulence. We consider the magnetic mirroring effect arising from MHD turbulence on the diffusion of CRs. Due to the intrinsic superdiffusion of turbulent magnetic fields, CRs with large pitch angles that undergo mirror reflection, i.e., bouncing CRs, are not trapped between magnetic mirrors, but move diffusively along the turbulent magnetic field, leading to a new type of parallel diffusion, i.e., mirror diffusion. This mirror diffusion is in general slower than the diffusion of nonbouncing CRs with small pitch angles that undergo gyroresonant scattering. The critical pitch angle at the balance between magnetic mirroring and pitch-angle scattering is important for determining the diffusion coefficients of both bouncing and nonbouncing CRs and their scalings with the CR energy. We find nonuniversal energy scalings of diffusion coefficients, depending on the properties of MHD turbulence.

view Abstract Citations (73) References (86) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Magnetic Turbulence in Interstellar Clouds Myers, P. C. ; Khersonsky, V. K. Abstract Observations of diffuse, dark, and giant molecular clouds and their cores are analyzed to determine properties of their turbulent motions. Estimates of characteristic cloud internal density, external extinction, and external radiation field intensity are used to deduce the electron fraction chie due to both photoionization and cosmic rays. This ionization fraction exceeds that due to cosmic rays alone, by factors approximately 5 for dark cloud cores to approximately 4000 for giant molecular clouds with embedded OB stars. Estimates of characteristic cloud size, density, velocity dispersion, ionization fraction, and magnetic field strength then indicate that four diagnostic numbers exceed unity by a significant factor: the Reynolds number, the magnetic Reynolds number, the Hartmann number, and the 'wave coupling number,' or ratio of cloud size to minimum hydromagnetic wavelength. These results indicate that virtually all observed interstellar clouds have strong coupling between the magnetic field and the neutral gas, through ion-neutral collisions, even if the field is weaker than its equipartition value. This coupling allows energetically significant magnetohydrodynamic (MHD) waves to propagate above cutoff, so that MHD waves, chaotic motions, and clumpy density structure are probably more pervasive in interstellar clouds than would be expected from cosmic-ray ionization alone. This strong coupling implies that the timescale for ambipolar diffusion is at least approximately 107 yr for low-mass cores, and is at least approximately 108 yr for the gas around cores. These timescales may be too long for all of the mass in a low-mass core to condense via ambipolar diffusion. The observed velocity dispersion is strongly correlated with the estimated electron fraction, according to the power law nu approximately chie p, with p approximately = 0.3. This trend, and those already known among velocity dispersion, size, and density, suggest that increasing extinction may influence the structure of cloud density and velocity dispersion by driving a cycle of decreasing ionization, decreasing MHD wave activity, decreasing velocity dispersion, and increasing density. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175434 Bibcode: 1995ApJ...442..186M Keywords: Interstellar Magnetic Fields; Ionization; Magnetic Flux; Magnetohydrodynamic Waves; Molecular Clouds; Turbulence; Abundance; Ambipolar Diffusion; Density Distribution; Photoionization; Reynolds Number; Velocity Distribution; Astrophysics; ISM: CLOUDS; ISM: MAGNETIC FIELDS; ISM: MOLECULES; MAGNETOHYDRODYNAMICS: MHD; TURBULENCE; WAVES full text sources ADS | data products SIMBAD (7)

Context. An increasing number of observations have indicated the existence of slow diffusion phenomena in astrophysical environments, such as around the supernova remnants and pulsar γ-ray halos, where the diffusion coefficient of cosmic rays (CRs) near the source region is significantly smaller than that far away from the source region. The inhomogeneous diffusion indicates the existence of multiple diffusion mechanisms. Aims. Comparing the CR mirror diffusion with the scattering diffusion, our aim is to explore their diffusion characteristics in different magnetohydrodynamic (MHD) turbulence regimes and understand the effect of different MHD modes on mirror and scattering diffusion. Methods. We performed numerical simulations with the test particle method. Within the global frame of reference, we first measured parallel and perpendicular CR diffusion and then determined the mean free path of CRs with varying energies. Results. Our main results demonstrate that (1) CRs experience a transition from superdiffusion to normal diffusion; (2) mirror diffusion is more important than scattering diffusion in confining CRs; (3) CR diffusion strongly depends on the properties of MHD turbulence; and (4) magnetosonic and Alfvén modes respectively dominate the parallel and perpendicular diffusion of CR particles. Conclusions. The diffusion of CRs is a complex problem of mixing the mirror diffusion and scattering diffusion. The property of turbulent magnetic fields influences CR diffusion. The CR slow diffusion due to the presence of magnetic mirrors in turbulence has important implications for explaining observations near a CR source.

Heating of virialized gas by streaming cosmic rays (CRs) may be energetically important in galaxy haloes, groups, and clusters. We present a linear thermal stability analysis of plasmas heated by streaming CRs. We separately treat equilibria with and without background gradients, and with and without gravity. We include both CR streaming and diffusion along the magnetic-field direction. Thermal stability depends strongly on the ratio of CR pressure to gas pressure, which determines whether modes are isobaric or isochoric. Modes with $\boldsymbol {k \cdot B }\ne 0$ are strongly affected by CR diffusion. When the streaming time is shorter than the CR diffusion time, thermally unstable modes (with $\boldsymbol {k \cdot B }\ne 0$) are waves propagating at a speed ∝ the Alfvén speed. Halo gas in photoionization equilibrium is thermally stable independent of CR pressure, while gas in collisional ionization equilibrium is unstable for physically realistic parameters. In gravitationally stratified plasmas, the oscillation frequency of thermally overstable modes can be higher in the presence of CR streaming than the buoyancy/free-fall frequency. This may modify the critical tcool/tff at which multiphase gas is present. The criterion for convective instability of a stratified, CR-heated medium can be written in the familiar Schwarzschild form dseff/dz < 0, where seff is an effective entropy involving the gas and CR pressures. We discuss the implications of our results for the thermal evolution and multiphase structure of galaxy haloes, groups, and clusters.

Diffusion of cosmic rays in a multiphase interstellar medium swept-up by a supernova remnant blast wave

We investigate the possibility of cosmic ray (CR) confinement by charged dust grains through resonant drag instabilities (RDIs). We perform magnetohydrodynamic particle-in-cell simulations of magnetized gas mixed with charged dust and cosmic rays, with the gyro-radii of dust and GeV CRs on ∼au scales fully resolved. As a first study, we focus on one type of RDI wherein charged grains drift super-Alfvénically, with Lorentz forces strongly dominating over drag forces. Dust grains are unstable to the RDIs and form concentrated columns and sheets, whose scale grows until saturating at the simulation box size. Initially perfectly streaming CRs are strongly scattered by RDI-excited Alfvén waves, with the growth rate of the CR perpendicular velocity components equaling the growth rate of magnetic field perturbations. These rates are well-predicted by analytic linear theory. CRs finally become isotropized and drift at least at ∼vA by unidirectional Alfvén waves excited by the RDIs, with a uniform distribution of the pitch angle cosine μ and a flat profile of the CR pitch angle diffusion coefficient Dμμ around μ = 0, without the ‘90○ pitch angle problem.’ With CR feedback on the gas included, Dμμ decreases by a factor of a few, indicating a lower CR scattering rate, because the backreaction on the RDI from the CR pressure adds extra wave damping, leading to lower quasi-steady-state scattering rates. Our study demonstrates that the dust-induced CR confinement can be very important under certain conditions, e.g. the dusty circumgalactic medium around quasars or superluminous galaxies.

Coronal seismology, a new field of solar physics that emerged over the last 5 years, provides unique information on basic physical properties of the solar corona. The inhomogeneous coronal plasma supports a variety of magnetohydrodynamics (MHD) wave modes, which manifest themselves as standing waves (MHD oscillations) and propagating waves. Here, we briefly review the physical properties of observed MHD oscillations and waves, including fast kink modes, fast sausage modes, slow (acoustic) modes, torsional modes, their diagnostics of the coronal magnetic field, and their physical damping mechanisms. We discuss the excitation mechanisms of coronal MHD oscillations and waves: the origin of the exciter, exciter propagation, and excitation in magnetic reconnection outflow regions. Finally, we consider the role of coronal MHD oscillations and waves for coronal heating, the detectability of various MHD wave types, and we estimate the energies carried in the observed MHD waves and oscillations: Alfvénic MHD waves could potentially provide sufficient energy to sustain coronal heating, while acoustic MHD waves fall far short of the required coronal heating rates.

The regulation of the baryonic content in dwarf galaxies is a long-standing problem. Supernovae (SNe) are supposed to play a key role in forming large-scale galactic winds by removing important amounts of gas from galaxies. SNe are efficient accelerators of non-thermal particles, so-called cosmic rays (CRs), which can substantially modify the dynamics of the gas and conditions to form large-scale galactic winds. We investigate how CR injection by SNe impacts the star formation and the formation of large-scale winds in dwarf galaxies, and whether it can produce galaxy star-formation rates (SFR) and wind properties closer to observations. We ran CR magneto-hydrodynamical simulations of dwarf galaxies at high resolution (9 pc) with the adaptive mesh refinement codeRAMSES. Those disc galaxies are embedded in isolated halos of mass of 1010and 1011 M⊙, and CRs are injected by SNe. We included CR isotropic and anisotropic diffusion with various diffusion coefficients, CR radiative losses, and CR streaming. The injection of CR energy into the interstellar medium smooths out the highest gas densities, which reduces the SFR by a factor of 2–3. Mass outflow rates are significantly greater with CR diffusion, by 2 orders of magnitudes for the higher diffusion coefficients. Without diffusion and streaming, CRs are inefficient at generating winds. CR streaming alone allows for the formation of winds but which are too weak to match observations. The formation of galactic winds strongly depends on the diffusion coefficient: for low coefficients, CR energy stays confined in high density regions where CR energy losses are highest, and higher coefficients, which allow for a more efficient leaking of CRs out of dense gas, produce stronger winds. CR diffusion leads to colder and denser winds than without CRs, and brings outflow rates and mass loading factors much closer to observations.