Cosmic Ray Streaming Research Articles

Bell Instability–mediated Diffusive Shock Acceleration at Supernova Blast Wave Shock Propagating in the Interstellar Medium

Supernova blast wave shock is a very important site of cosmic-ray acceleration. However, the detailed physical process of acceleration, in particular, nonlinear interplay between cosmic-ray streaming and magnetic field amplification, has not been studied under a realistic environment. In this paper, using a unique and novel numerical method, we study cosmic-ray acceleration at supernova blast wave shock propagating in the interstellar medium with well-resolved magnetic field amplification by nonresonant hybrid instability (or Bell instability). We find that the magnetic field is mildly amplified under typical interstellar medium conditions that leads to maximum cosmic-ray energy ≃30 TeV for supernova remnants with age ≃1000 yr consistent with gamma-ray observations. The strength of the amplified magnetic field does not reach the so-called saturation level because the cosmic-ray electric current toward the shock upstream has a finite spatial extent, by which Bell instability cannot experience many e-folding times.

Effect of cosmic rays and ionizing radiation on observational ultraviolet plasma diagnostics in the circumgalactic medium

ABSTRACT The relevance of some galactic feedback mechanisms, in particular cosmic-ray (CR) feedback and the hydrogen ionizing radiation field, has been challenging to definitively describe in a galactic context, especially far outside the galaxy in the circumgalactic medium (CGM). Theoretical and observational uncertainties prevent conclusive interpretations of multiphase CGM properties derived from ultraviolet (UV) diagnostics. We conduct three-dimensional magnetohydrodynamic simulations of a section of a galactic disc with star formation and feedback, including radiative heating from stars, a UV background, and CR feedback. We utilize the temperature phases present in our simulations to generate Cloudy models to derive spatially and temporally varying synthetic UV diagnostics. We find that radiative effects without additional heating mechanisms are not able to produce synthetic diagnostics in the observed ranges. For low CR diffusivity $\kappa _{\rm {cr}}=10^{28} \rm {cm}^2 \rm {s}^{-1}$, CR streaming heating in the outflow helps our synthetic line ratios roughly match observed ranges by producing transitional temperature gas (T ∼ 105–106 K). High CR diffusivity $\kappa _{\rm {cr}}=10^{29} \rm {cm}^2 \rm {s}^{-1}$, with or without CR streaming heating, produced transitional temperature gas. The key parameter controlling the production of this gas phase remains unclear, as the different star formation history and outflow evolution itself influences these diagnostics. Our work demonstrates the use of UV plasma diagnostics to differentiate between galactic/circumgalactic feedback models.

The impact of cosmic rays on thermal and hydrostatic stability in galactic haloes

ABSTRACT We investigate how cosmic rays (CRs) affect thermal and hydrostatic stability of circumgalactic (CGM) gas, in simulations with both CR streaming and diffusion. Local thermal instability can be suppressed by CR-driven entropy mode propagation, in accordance with previous analytic work. However, there is only a narrow parameter regime where this operates, before CRs overheat the background gas. As mass dropout from thermal instability causes the background density and hence plasma β ≡ Pg/PB to fall, the CGM becomes globally unstable. At the cool disc-to-hot−halo interface, a sharp drop in density boosts Alfven speeds and CR gradients, driving a transition from diffusive to streaming transport. CR forces and heating strengthen, while countervailing gravitational forces and radiative cooling weaken, resulting in a loss of both hydrostatic and thermal equilibrium. In lower β haloes, CR heating drives a hot, single-phase diffuse wind with velocities v ∝ (theat/tff)−1, which exceeds the escape velocity when theat/tff ≲ 0.4. In higher β haloes, where the Alfven Mach number is higher, CR forces drive multi-phase winds with cool, dense fountain flows and significant turbulence. These flows are CR dominated due to ‘trapping’ of CRs by weak transverse B-fields, and have the highest mass loading factors. Thus, local thermal instability can result in winds or fountain flows where either the heat or momentum input of CRs dominates.

Deciphering the radio–star formation correlation on kpc scales

The relation between the resolved star formation rate (SFR) per unit area and the nonthermal radio continuum emission is studied in 21 Virgo cluster galaxies and the two nearby spiral galaxies, NGC 6946 and M 51. For the interpretation and understanding of our results, we used a 3D model where star formation, 2D cosmic-ray (CR) propagation, and the physics of synchrotron emission are included. Based on the linear correlation between the SFR per unit area and the synchrotron emission and its scatter, radio-bright and radio-dim regions can be robustly defined for our sample of spiral galaxies. We identified CR diffusion or streaming as the physical causes of radio-bright regions of unperturbed symmetric spiral galaxies as NGC 6946. The enhanced magnetic field in the region of interstellar medium (ISM) compression via ram pressure is responsible for the southwestern radio-bright region in NGC 4501. We identified the probable causes of radio-bright regions in several galaxies as CR transport, via either gravitational tides (M 51) or galactic winds (NGC 4532) or ram pressure stripping (NGC 4330 and NGC 4522). Three galaxies are overall radio dim: NGC 4298, NGC 4535, and NGC 4567. Based on our model of synchrotron-emitting disks, we suggest that the overall radio-dim galaxies have a significantly lower magnetic field than expected by equipartition between the magnetic and turbulent energy densities. We suggest that this is linked to the difference between the timescales of the variation in the SFR and the small-scale dynamo. In NGC 4535, shear motions increase the total magnetic field strength via the induction equation, which leads to enhanced synchrotron emission with respect to the SFR in an otherwise radio-dim galactic disk. Radio-bright regions frequently coincide with asymmetric ridges of polarized radio continuum emission, and we found a clear albeit moderate correlation between the polarized radio continuum emission and the radio/SFR ratio. When compression or shear motions of the ISM are present in the galactic disk, the radio-bright regions are linked to the commonly observed asymmetric ridges of polarized radio continuum emission and represent a useful tool for the interaction diagnostics. The magnetic field is enhanced (as observed in NGC 4535 and NGC 4501) and ordered by these ISM compression and shear motions. Whereas the enhancement of the magnetic field is rather modest and does not significantly influence the radio-SFR correlation, the main effect of ISM compression and shear motions is the ordering of the magnetic field, which significantly affects the CR transport. Cosmic-ray energy losses and transport also affect the spectral index, which we measured between 4.85 and 1.4 GHz. The influence of CR losses and transport on the spectral index distribution with respect to the synchrotron/SFR ratio is discussed with the help of model calculations. Based on our results, we propose a scenario for the interplay between star formation, CR electrons, and magnetic fields in spiral galaxies.

A new buoyancy instability in galaxy clusters due to streaming cosmic rays

ABSTRACT 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.

Turbulent Reacceleration of Streaming Cosmic Rays

Subsonic, compressive turbulence transfers energy to cosmic rays (CRs), a process known as nonresonant reacceleration. It is often invoked to explain the observed ratios of primary to secondary CRs at ∼GeV energies, assuming wholly diffusive CR transport. However, such estimates ignore the impact of CR self-confinement and streaming. We study these issues in stirring box magnetohydrodynamic (MHD) simulations using Athena++, with field-aligned diffusive and streaming CR transport. For diffusion only, we find CR reacceleration rates in good agreement with analytic predictions. When streaming is included, reacceleration rates depend on plasma β. Due to streaming-modified phase shifts between CR and gas variables, they are slower than canonical reacceleration rates in low-β environments like the interstellar medium but remain unchanged in high-β environments like the intracluster medium. We also quantify the streaming energy-loss rate in our simulations. For sub-Alfvénic turbulence, it is resolution dependent (hence unconverged in large-scale simulations) and heavily suppressed compared to the isotropic loss rate v A · ∇P CR/P CR ∼ v A/L 0, due to misalignment between the mean field and isotropic CR gradients. Unlike acceleration efficiencies, CR losses are almost independent of magnetic field strength over β ∼ 1–100 and are, therefore, not the primary factor behind lower acceleration rates when streaming is included. While this paper is primarily concerned with how turbulence affects CRs, in a follow-up paper we consider how CRs affect turbulence by diverting energy from the MHD cascade, altering the pathway to gas heating and steepening the turbulent spectrum.

Standard self-confinement and extrinsic turbulence models for cosmic ray transport are fundamentally incompatible with observations

ABSTRACT Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR scattering from magnetic fluctuations. In the ISM, for CRs with energies ∼MeV-TeV, these fluctuations are usually attributed either to ‘extrinsic turbulence’ (ET) – a cascade from larger scales – or ‘self-confinement’ (SC) – self-generated fluctuations from CR streaming. Using simple analytic arguments and detailed ‘live’ numerical CR transport calculations in galaxy simulations, we show that both of these, in standard form, cannot explain even basic qualitative features of observed CR spectra. For ET, any spectrum that obeys critical balance or features realistic anisotropy, or any spectrum that accounts for finite damping below the dissipation scale, predicts qualitatively incorrect spectral shapes and scalings of B/C and other species. Even if somehow one ignored both anisotropy and damping, observationally required scattering rates disagree with ET predictions by orders of magnitude. For SC, the dependence of driving on CR energy density means that it is nearly impossible to recover observed CR spectral shapes and scalings, and again there is an orders-of-magnitude normalization problem. But more severely, SC solutions with super-Alfvénic streaming are unstable. In live simulations, they revert to either arbitrarily rapid CR escape with zero secondary production, or to bottleneck solutions with far-too-strong CR confinement and secondary production. Resolving these fundamental issues without discarding basic plasma processes requires invoking different drivers for scattering fluctuations. These must act on a broad range of scales with a power spectrum obeying several specific (but plausible) constraints.

Cosmic rays and thermal instability in self-regulating cooling flows of massive galaxy clusters

One of the key physical processes that helps prevent strong cooling flows in galaxy clusters is the continued energy input from the central active galactic nucleus (AGN) of the cluster. However, it remains unclear how this energy is thermalised so that it can effectively prevent global thermal instability. One possible option is that a fraction of the AGN energy is converted into cosmic rays (CRs), which provide non-thermal pressure support, and can retain energy even as thermal energy is radiated away. By means of magneto-hydrodynamical simulations, we investigate how CR injected by the AGN jet influence cooling flows of a massive galaxy cluster. We conclude that converting a fraction of the AGN luminosity as low as 10% into CR energy prevents cooling flows on timescales of billion years, without significant changes in the structure of the multi-phase intra-cluster medium. CR-dominated jets, by contrast, lead to the formation of an extended, warm central nebula that is supported by CR pressure. We report that the presence of CRs is not able to suppress the onset of thermal instability in massive galaxy clusters, but CR-dominated jets do significantly change the continued evolution of gas as it continues to cool from isobaric to isochoric. The CR redistribution in the cluster is dominated by advection rather than diffusion or streaming, but the heating by CR streaming helps maintain gas in the hot and warm phase. Observationally, self-regulating, CR-dominated jets produce a γ-ray flux in excess of current observational limits, but low CR fractions in the jet are not ruled out.

Biermann battery powered by resistive heating induced by cosmic ray streaming

ABSTRACT It is recently proposed that cosmic rays generate a seed magnetic field in the early Universe. In this paper, we propose another generation mechanism of magnetic fields by cosmic rays, which is the Biermann battery driven by resistive heating induced by the streaming of cosmic rays. This mechanism is dominant in small-scale, low-temperature, and strongly ionized regions, compared with other previously proposed mechanisms. Because cosmic rays are expected to be accelerated after the death of the first stars, this mechanism can work during structure formation in the early Universe. We show that it makes the seed magnetic field with sufficient strength for the subsequent dynamo to amplify it to the micro Gauss level in the current galaxies.

Turbulent Cosmic Ray–Mediated Shocks in the Hot Ionized Interstellar Medium

The structure of shocks and turbulence are strongly modified during the acceleration of cosmic rays (CRs) at a shock wave. The pressure and the collisionless viscous stress decelerate the incoming thermal gas and thus modify the shock structure. A CR streaming instability ahead of the shock generates the turbulence on which CRs scatter. The turbulent magnetic field in turn determines the CR diffusion coefficient and further affects the CR energy spectrum and pressure distribution. The dissipation of turbulence contributes to heating the thermal gas. Within a multicomponent fluid framework, CRs and thermal gas are treated as fluids and are closely coupled to the turbulence. The system equations comprise the gas dynamic equations, the CR pressure evolution equation, and the turbulence transport equations, and we adopt typical parameters for the hot ionized interstellar medium. It is shown that the shock has no discontinuity but possesses a narrow but smooth transition. The self-generated turbulent magnetic field is much stronger than both the large-scale magnetic field and the preexisting turbulent magnetic field. The resulting CR diffusion coefficient is substantially suppressed and is more than three orders smaller near the shock than it is far upstream. The results are qualitatively consistent with certain observations.

Damping of Alfvén Waves in MHD Turbulence and Implications for Cosmic Ray Streaming Instability and Galactic Winds

Alfvénic component of MHD turbulence damps Alfvénic 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.

Toward First-principles Characterization of Cosmic-Ray Transport Coefficients from Multiscale Kinetic Simulations

Abstract A major uncertainty in understanding the transport and feedback of cosmic rays (CRs) within and beyond our Galaxy lies in the unknown CR scattering rates, which are primarily determined by wave–particle interaction at microscopic gyroresonant scales. The source of the waves for the bulk CR population is believed to be self-driven by the CR streaming instability (CRSI), resulting from the streaming of CRs downward a CR pressure gradient. While a balance between driving by the CRSI and wave damping is expected to determine wave amplitudes and hence the CR scattering rates, the problem involves significant scale separation with substantial ambiguities based on quasi-linear theory (QLT). Here we propose a novel “streaming box” framework to study the CRSI with an imposed CR pressure gradient, enabling first-principles measurement of the CR scattering rates as a function of environmental parameters. By employing the magnetohydrodynamic particle-in-cell method with ion–neutral damping, we conduct a series of simulations with different resolutions and CR pressure gradients and precisely measure the resulting CR scattering rates in steady state. The measured rates show scalings consistent with QLT, but with a normalization smaller by a factor of several than typical estimates based on the single-fluid treatment of CRs. A momentum-by-momentum treatment provides better estimates when integrated over momentum but is also subject to substantial deviations, especially at small momentum. Our framework thus opens up the path toward providing comprehensive subgrid physics for macroscopic studies of CR transport and feedback in broad astrophysical contexts.

Momentum deposition of supernovae with cosmic rays

ABSTRACT The cataclysmic explosions of massive stars as supernovae are one of the key ingredients of galaxy formation. However, their evolution is not well understood in the presence of magnetic fields or cosmic rays (CRs). We study the expansion of individual supernova remnants (SNRs) using our suite of 3D hydrodynamical (HD), magnetohydrodynamical (MHD) and CRMHD simulations generated using ramses. We explore multiple ambient densities, magnetic fields, and fractions of supernova energy deposited as CRs (χCR), accounting for CR anisotropic diffusion and streaming. All our runs have comparable evolutions until the end of the Sedov-Taylor phase. However, our CRMHD simulations experience an additional CR pressure-driven snowplough phase once the CR energy dominates inside the SNR. We present a model for the final momentum deposited by supernovae that captures this new phase: $p_{\rm SNR} = 2.87\times 10^{5} (\chi _{\text{CR}} + 1)^{4.82}\left(\frac{n}{\text{cm}^{-3}}\right)^{-0.196} M_{\odot }$ km s−1. Assuming a 10 per cent fraction of SN energy in CRs leads to a 50 per cent boost of the final momentum, with our model predicting even higher impacts at lower ambient densities. The anisotropic diffusion of CRs assuming an initially uniform magnetic field leads to extended gas and CR outflows escaping from the supernova poles. We also study a tangled initial configuration of the magnetic field, resulting instead in a quasi-isotropic diffusion of CRs and earlier momentum deposition. Finally, synthetic synchrotron observations of our simulations using the polaris code show that the local magnetic field configuration in the interstellar medium modifies the overall radio emission morphology and polarization.

The physics of galactic winds driven by cosmic rays I: Diffusion

ABSTRACT The physics of Cosmic ray (CR) transport remains a key uncertainty in assessing whether CRs can produce galaxy-scale outflows consistent with observations. In this paper, we elucidate the physics of CR-driven galactic winds for CR transport dominated by diffusion. A companion paper considers CR streaming. We use analytic estimates validated by time-dependent spherically symmetric simulations to derive expressions for the mass-loss rate, momentum flux, and speed of CR-driven galactic winds, suitable for cosmological-scale or semi-analytic models of galaxy formation. For CR diffusion coefficients κ ≳ r0ci, where r0 is the base radius of the wind and ci is the isothermal gas sound speed, the asymptotic wind energy flux is comparable to that supplied to CRs, and the outflow rapidly accelerates to supersonic speeds. By contrast, for κ ≲ r0ci, CR-driven winds accelerate more slowly and lose most of their energy to gravity, a CR analogue of photon-tired stellar winds. Given CR diffusion coefficients estimated using Fermi gamma-ray observations of pion decay, we predict mass-loss rates in CR-driven galactic winds of the order of the star formation rate for dwarf and disc galaxies. The dwarf galaxy mass-loss rates are small compared to the mass-loadings needed to reconcile the stellar and dark matter halo mass functions. For nuclear starbursts (e.g. M82, Arp 220), CR diffusion and pion losses suppress the CR pressure in the galaxy and the strength of CR-driven winds. We discuss the implications of our results for interpreting observations of galactic winds and for the role of CRs in galaxy formation.

The physics of galactic winds driven by cosmic rays – II. Isothermal streaming solutions

ABSTRACT We use analytical calculations and time-dependent spherically symmetric simulations to study the properties of isothermal galactic winds driven by cosmic rays (CRs) streaming at the Alfvén velocity. The simulations produce time-dependent flows permeated by strong shocks; we identify a new linear instability of sound waves that sources these shocks. The shocks substantially modify the wind dynamics, invalidating previous steady state models: the CR pressure pc has a staircase-like structure with dpc/dr ≃ 0 in most of the volume, and the time-averaged CR energetics are in many cases better approximated by pc ∝ ρ1/2, rather than the canonical pc ∝ ρ2/3. Accounting for this change in CR energetics, we analytically derive new expressions for the mass-loss rate, momentum flux, wind speed, and wind kinetic power in galactic winds driven by CR streaming. We show that streaming CRs are ineffective at directly driving cold gas out of galaxies, though CR-driven winds in hotter ISM phases may entrain cool gas. For the same physical conditions, diffusive CR transport (Paper I) yields mass-loss rates that are a few-100 times larger than streaming transport, and asymptotic wind powers that are a factor of ≃4 larger. We discuss the implications of our results for galactic wind theory and observations; strong shocks driven by CR-streaming-induced instabilities produce gas with a wide range of densities and temperatures, consistent with the multiphase nature of observed winds. We also quantify the applicability of the isothermal gas approximation for modelling streaming CRs and highlight the need for calculations with more realistic thermodynamics.

Cosmic-Ray Transport in Simulations of Star-forming Galactic Disks

Abstract Cosmic-ray transport on galactic scales depends on the detailed properties of the magnetized, multiphase interstellar medium (ISM). In this work, we postprocess a high-resolution TIGRESS magnetohydrodynamic simulation modeling a local galactic disk patch with a two-moment fluid algorithm for cosmic-ray transport. We consider a variety of prescriptions for the cosmic rays, from a simple, purely diffusive formalism with constant scattering coefficient, to a physically motivated model in which the scattering coefficient is set by the critical balance between streaming-driven Alfvén wave excitation and damping mediated by local gas properties. We separately focus on cosmic rays with kinetic energies of ∼1 GeV (high-energy) and ∼30 MeV (low energy), respectively important for ISM dynamics and chemistry. We find that simultaneously accounting for advection, streaming, and diffusion of cosmic rays is crucial for properly modeling their transport. Advection dominates in the high-velocity, low-density hot phase, while diffusion and streaming are more important in higher-density, cooler phases. Our physically motivated model shows that there is no single diffusivity for cosmic-ray transport: the scattering coefficient varies by four or more orders of magnitude, maximal at density n H ∼ 0.01 cm−3. The ion-neutral damping of Alfvén waves results in strong diffusion and nearly uniform cosmic-ray pressure within most of the mass of the ISM. However, cosmic rays are trapped near the disk midplane by the higher scattering rate in the surrounding lower-density, higher-ionization gas. The transport of high-energy cosmic rays differs from that of low-energy cosmic rays, with less effective diffusion and greater energy losses for the latter.

A consistent reduced-speed-of-light formulation of cosmic ray transport valid in weak- and strong-scattering regimes

ABSTRACT We derive a consistent set of moment equations for cosmic ray (CR)-magnetohydrodynamics, assuming a gyrotropic distribution function (DF). Unlike previous efforts, we derive a closure, akin to the M1 closure in radiation hydrodynamics (RHD), that is valid in both the nearly isotropic DF and/or strong-scattering regimes, and the arbitrarily anisotropic DF or free-streaming regimes, as well as allowing for anisotropic scattering and transport/magnetic field structure. We present the appropriate two-moment closure and equations for various choices of evolved variables, including the CR phase space DF f, number density n, total energy e, kinetic energy ϵ, and their fluxes or higher moments, and the appropriate coupling terms to the gas. We show that this naturally includes and generalizes a variety of terms including convection/fluid motion, anisotropic CR pressure, streaming, diffusion, gyro-resonant/streaming losses, and re-acceleration. We discuss how this extends previous treatments of CR transport including diffusion and moment methods and popular forms of the Fokker–Planck equation, as well as how this differs from the analogous M1-RHD equations. We also present two different methods for incorporating a reduced speed of light (RSOL) to reduce time-step limitations: In both, we carefully address where the RSOL (versus true c) must appear for the correct behaviour to be recovered in all interesting limits, and show how current implementations of CRs with an RSOL neglect some additional terms.

Influence of Ion–Neutral Damping on the Cosmic-Ray Streaming Instability: Magnetohydrodynamic Particle-in-cell Simulations

Abstract We explore the physics of the gyroresonant cosmic-ray streaming instability (CRSI) including the effects of ion–neutral (IN) damping. This is the main damping mechanism in (partially ionized) atomic and molecular gas, which are the primary components of the interstellar medium (ISM) by mass. Limitation of CRSI by IN damping is important in setting the amplitude of Alfvén waves that scatter cosmic rays (CRs) and control galactic-scale transport. Our study employs the magnetohydrodynamic (MHD)–particle-in-cell hybrid fluid-kinetic numerical technique to follow linear growth as well as post-linear and saturation phases. During the linear phase of the instability—where simulations and analytical theory are in good agreement—IN damping prevents wave growth at small and large wavelengths, with the unstable bandwidth lower for higher IN collision rates ν in. Purely MHD effects during the post-linear phase extend the wave spectrum toward larger k. In the saturated state, the CR distribution evolves toward greater isotropy (lower streaming velocity) by scattering off of Alfvén waves excited by the instability. In the absence of low-k waves, CRs with sufficiently high momentum are not isotropized. The maximum wave amplitude and rate of isotropization of the distribution function decrease at higher ν in. When the IN damping rate approaches the maximum growth rate of CRSI, wave growth and isotropization are suppressed. Implications of our results for CR transport in partially ionized ISM phases are discussed.

Particle acceleration and magnetic field amplification in massive young stellar object jets

ABSTRACT Synchrotron radio emission from non-relativistic jets powered by massive protostars has been reported, indicating the presence of relativistic electrons and magnetic fields of strength ∼0.3–5 mG. We study diffusive shock acceleration and magnetic field amplification in protostellar jets with speeds between 300 and 1500 km s−1. We show that the magnetic field in the synchrotron emitter can be amplified by the non-resonant hybrid (Bell) instability excited by the cosmic ray streaming. By combining the synchrotron data with basic theory of Bell instability we estimate the magnetic field in the synchrotron emitter and the maximum energy of protons. Protons can achieve maximum energies in the range 0.04–0.65 TeV and emit γ rays in their interaction with matter fields. We predict detectable levels of γ rays in IRAS 16547−5247 and IRAS 16848−4603. The γ ray flux can be significantly enhanced by the gas mixing due to Rayleigh–Taylor instability. The detection of this radiation by the Fermi satellite in the GeV domain and the forthcoming Cherenkov Telescope Array at higher energies may open a new window to study the formation of massive stars, as well as diffusive acceleration and magnetic field amplification in shocks with velocities of about 1000 km s−1.

Cosmic-ray current-driven instabilities – revisiting environmental conditions

ABSTRACT The growth of magnetohydrodynamic fluctuations relevant to cosmic-ray (CR) confinement in and near their sources, and the effects of local plasma conditions is revisited. We consider cases where CRs penetrate a medium which may contain a fraction of neutral particles, and explore the possible effects of high-order CR anisotropies. An algorithm for calculating the dispersion relation for arbitrary distributions, and anisotropies is presented, and a general solution for power-law cosmic-ray distributions is provided. Implications for the resulting instabilities near to strong Galactic CR sources are discussed. We argue that CR streaming in weakly ionized plasmas eliminates the need for the existence of an evanescent band in the dispersion relation, a conclusion which may be confirmed by gamma-ray observations. The necessity for additional multiscale numerical simulations is highlighted, as understanding the non-linear behaviour is crucial.