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    Cosmic rays (CRs) with energies ≪ TeV comprise a significant component of the interstellar medium (ISM). Major uncertainties in CR behaviour on observable scales (much larger than CR gyroradii) stem from how magnetic fluctuations scatter CRs in pitch angle. Traditional first-principles models, which assume these magnetic fluctuations are weak and uniformly scatter CRs in a homogeneous ISM, struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. We therefore explore a new category of ‘patchy’ CR scattering models, wherein CRs are pre-dominantly scattered by intermittent strong scattering structures with small volume-filling factors. These models produce the observed rigidity dependence with a simple size distribution constraint, such that larger scattering structures are rarer but can scatter a wider range of CR energies. To reproduce the empirically inferred CR scattering rates, the mean free path between scattering structures must be $\ell _{\rm mfp}\sim 10\, {\rm pc}$ at GeV energies. We derive constraints on the sizes, internal properties, mass/volume-filling factors, and the number density any such structures would need to be both physically and observationally consistent. We consider a range of candidate structures, both large scale (e.g. H ii regions) and small scale (e.g. intermittent turbulent structures, perhaps even associated with radio plasma scattering) and show that while many macroscopic candidates can be immediately ruled out as the primary CR scattering sites, many smaller structures remain viable and merit further theoretical study. We discuss future observational constraints that could test these models.

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    Active Galactic Nuclei (AGN) are believed to provide the energy that prevents runaway cooling of gas in the cores of galaxy clusters. However, how this energy is transported and thermalized throughout the Intracluster Medium (ICM) remains unclear. In recent work, we showed that streaming cosmic rays (CRs) destabilize sound waves in dilute ICM plasmas. Here, we show that CR streaming in the presence of gravity also destabilizes a pressure-balanced wave. We term this new instability the CR buoyancy instability (CRBI). In stark contrast to standard results without CRs, the pressure-balanced mode is highly compressible at short wavelengths due to CR streaming. Maximal growth rates are of order (pc/pg)β1/2ωff, where pc/pg is the ratio of CR pressure to thermal gas pressure, β is the ratio of thermal to magnetic pressure, and ωff is the free-fall frequency. The CRBI operates alongside buoyancy instabilities driven by background heat fluxes, i.e. the heat-flux-driven buoyancy instability (HBI) and the magneto-thermal instability (MTI). When the thermal mean free path lmfp is ≪ the gas scale height H, the HBI/MTI set the growth rate on large scales, while the CRBI sets the growth rate on small scales. Conversely, when lmfp ∼ H and (pc/pg)β1/2 ≳ 1, CRBI growth rates exceed HBI/MTI growth rates even on large scales. Our results suggest that CR-driven instabilities may be partially responsible for the sound waves/weak shocks and turbulence observed in galaxy clusters. CR-driven instabilities generated near radio bubbles may also play an important role redistributing AGN energy throughout clusters.

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    The nature of cosmic ray (CR) transport in the Milky Way remains elusive. The predictions of current microphysical CR transport models in magnetohydrodynamic (MHD) turbulence are drastically different from what is observed. These models usually focus on MHD turbulence with a strong guide field and ignore the impact of turbulent intermittency on particle propagation. This motivates our studying the alternative regime of large-amplitude turbulence with δB/B0 ≫ 1, in which intermittent small-scale magnetic field reversals are ubiquitous. We study particle transport in such turbulence by integrating trajectories in stationary snapshots. To quantify spatial diffusion, we use a set-up with continuous particle injection and escape, which we term the turbulent leaky box. We find that particle transport is very different from the strong guide-field case. Low-energy particles are better confined than high-energy particles, despite less efficient pitch-angle isotropization at small energies. In the limit of weak guide field, energy-dependent confinement is driven by the energy-dependent (in)ability to follow reversing magnetic field lines exactly and by the scattering in regions of ‘resonant curvature’, where the field line bends on a scale that is of the order of the local particle gyro-radius. We derive a heuristic model of particle transport in magnetic folds that approximately reproduces the energy dependence of transport found numerically. We speculate that CR propagation in the Galaxy is regulated by the intermittent field reversals highlighted here and discuss the implications of our findings for CR transport in the Milky Way.

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    Phenomenological models of cosmic ray (CR) transport in the Milky Way can reproduce a wide range of observations assuming that CRs scatter off of magnetic-field fluctuations with spectrum ∝ k−δ and δ ∼ [1.4, 1.67]. We study the extent to which such models can be reconciled with current microphysical theories of CR transport, specifically self-confinement due to the streaming instability and/or extrinsic turbulence due to a cascade of magnetohydrodynamic (MHD) fast modes. We first review why it is that on their own neither theory is compatible with observations. We then highlight that CR transport is a strong function of local plasma conditions in the multiphase interstellar medium, and may be diffusive due to turbulence in some regions and streaming due to self-confinement in others. A multiphase combination of scattering mechanisms can in principle reproduce the main trends in the proton spectrum and the boron-to-carbon ratio. However, models with a combination of scattering by self-excited waves and fast-mode turbulence require significant fine-tuning due to fast-mode damping, unlike phenomenological models that assume undamped Kolmogorov turbulence. The assumption that fast modes follow a weak cascade is also not well justified theoretically, as the weak cascade is suppressed by wave steepening and weak-shock dissipation even in subsonic turbulence. These issues suggest that there may be a significant theoretical gap in our understanding of MHD turbulence. We discuss a few topics at the frontier of MHD turbulence theory that bear on this (possible) gap and that may be relevant for CR scattering.

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  5. null (Ed.)
    ABSTRACT We argue that charged dust grains could significantly impact the confinement and transport of galactic cosmic rays. For sub-GeV to ∼103 GeV cosmic rays, small-scale parallel Alfvén waves, which isotropize cosmic rays through gyro-resonant interactions, are also gyro-resonant with charged grains. If the dust is nearly stationary, as in the bulk of the interstellar medium, Alfvén waves are damped by dust. This will reduce the amplitude of Alfvén waves produced by the cosmic rays through the streaming instability, thus enhancing cosmic ray transport. In well-ionized regions, the dust damping rate is larger by a factor of ∼10 than other mechanisms that damp parallel Alfvén waves at the scales relevant for ∼GeV cosmic rays, suggesting that dust could play a key role in regulating cosmic ray transport. In astrophysical situations in which the dust moves through the gas with super-Alfvénic velocities, Alfvén waves are rendered unstable, which could directly scatter cosmic rays. This interaction has the potential to create a strong feedback mechanism where dust, driven through the gas by radiation pressure, then strongly enhances the confinement of cosmic rays, increasing their capacity to drive outflows. This mechanism may act in the circumgalactic medium around star-forming galaxies and active galactic nuclei. 
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  6. null (Ed.)
    ABSTRACT We present a systematic shearing-box investigation of magnetorotational instability (MRI)-driven turbulence in a weakly collisional plasma by including the effects of an anisotropic pressure stress, i.e. anisotropic (Braginskii) viscosity. We constrain the pressure anisotropy (Δp) to lie within the stability bounds that would be otherwise imposed by kinetic microinstabilities. We explore a broad region of parameter space by considering different Reynolds numbers and magnetic-field configurations, including net vertical flux, net toroidal-vertical flux, and zero net flux. Remarkably, we find that the level of turbulence and angular-momentum transport are not greatly affected by large anisotropic viscosities: the Maxwell and Reynolds stresses do not differ much from the MHD result. Angular-momentum transport in Braginskii MHD still depends strongly on isotropic dissipation, e.g. the isotropic magnetic Prandtl number, even when the anisotropic viscosity is orders of magnitude larger than the isotropic diffusivities. Braginskii viscosity nevertheless changes the flow structure, rearranging the turbulence to largely counter the parallel rate of strain from the background shear. We also show that the volume-averaged pressure anisotropy and anisotropic viscous transport decrease with increasing isotropic Reynolds number (Re); e.g. in simulations with net vertical field, the ratio of anisotropic to Maxwell stress (αA/αM) decreases from ∼0.5 to ∼0.1 as we move from Re ∼ 103 to Re ∼ 104, while 〈4$\pi$Δp/B2〉 → 0. Anisotropic transport may thus become negligible at high Re. Anisotropic viscosity nevertheless becomes the dominant source of heating at large Re, accounting for ${\gtrsim } 50 {{\ \rm per\ cent}}$ of the plasma heating. We conclude by briefly discussing the implications of our results for radiatively inefficient accretion flows on to black holes. 
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  7. ABSTRACT 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. 
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