Abstract
The bulk propagation speed of GeV-energy cosmic rays is limited by frequent scattering off hydromagnetic waves. Most galaxy evolution simulations that account for this confinement assume the ...gas is fully ionized and cosmic rays are well coupled to Alfvén waves; however, multiphase density inhomogeneities, frequently underresolved in galaxy evolution simulations, induce cosmic-ray collisions and ionization-dependent transport driven by cosmic-ray decoupling and elevated streaming speeds in partially neutral gas. How do cosmic rays navigate and influence such a medium, and can we constrain this transport with observations? In this paper, we simulate cosmic-ray fronts impinging upon idealized, partially neutral clouds and lognormally distributed clumps, with and without ionization-dependent transport. With these high-resolution simulations, we identify cloud interfaces as crucial regions where cosmic-ray fronts can develop a stairstep pressure gradient sufficient to collisionlessly generate waves, overcome ion–neutral damping, and exert a force on the cloud. We find that the acceleration of cold clouds is hindered by only a factor of a few when ionization-dependent transport is included, with additional dependencies on magnetic field strength and cloud dimensionality. We also probe how cosmic rays sample the background gas and quantify collisional losses. Hadronic gamma-ray emission maps are qualitatively different when ionization-dependent transport is included, but the overall luminosity varies by only a small factor, as the short cosmic-ray residence times in cold clouds are offset by the higher densities that cosmic rays sample.
Perspectives on magnetic reconnection Zweibel, Ellen G.; Yamada, Masaaki
Proceedings - Royal Society. Mathematical, physical and engineering sciences,
12/2016, Letnik:
472, Številka:
2196
Journal Article
Recenzirano
Odprti dostop
Magnetic reconnection is a topological rearrangement of magnetic field that occurs on time scales much faster than the global magnetic diffusion time. Since the field lines break on microscopic ...scales but energy is stored and the field is driven on macroscopic scales, reconnection is an inherently multi-scale process that often involves both magnetohydrodynamic (MHD) and kinetic phenomena. In this article, we begin with the MHD point of view and then describe the dynamics and energetics of reconnection using a two-fluid formulation. We also focus on the respective roles of global and local processes and how they are coupled. We conclude that the triggers for reconnection are mostly global, that the key energy conversion and dissipation processes are either local or global, and that the presence of a continuum of scales coupled from microscopic to macroscopic may be the most likely path to fast reconnection.
ABSTRACT
We investigate a mechanism for accelerating cool (104 K) clouds in the circumgalactic medium (CGM) with cosmic rays (CRs), possibly explaining some characteristics of observed high-velocity ...clouds (HVCs). Enforcing CRs to stream down their pressure gradient into a region of slow streaming speed results in significant build-up of CR pressure which can accelerate the CGM. We present the results of the first two-dimensional magnetohydrodynamic (MHD) simulations of such ‘CR bottlenecks,’ expanding on simpler simulations in 1D. Although much more investigation is required, we find two main results. First, radiative cooling in the interfaces of these clouds is sufficient to keep the cloud intact to CR wave heating. Secondly, cloud acceleration depends almost linearly with the injected CR flux at low values (comparable to that expected from a Milky Way-like star formation rate), but scales sublinearly at higher CR fluxes in 1D simulations. 2D simulations show hints of sublinear dependence at high CR fluxes but are consistent with pure linear dependence up to the CR fluxes tested. It may therefore be plausible to accelerate cool clouds in the CGM to speeds of hundreds of km s−1.
Abstract
In a seminal paper, Parker showed the vertical stratification of the interstellar medium (ISM) is unstable if magnetic fields and cosmic rays provide too large a fraction of pressure ...support. Cosmic ray acceleration is linked to star formation, so Parker’s instability and its nonlinear outcomes are a type of star formation feedback. Numerical simulations have shown the instability can significantly restructure the ISM, thinning the thermal gas layer and thickening the magnetic field and cosmic ray layer. However, the timescale on which this occurs is rather long (∼0.4 Gyr). Furthermore, the conditions for instability depend on the model adopted for cosmic ray transport. In this work, we connect the instability and feedback problems by examining the effect of a single, spatially and temporally localized cosmic ray injection on the ISM over ∼1 kpc
3
scales. We perform cosmic ray magnetohydrodynamic simulations using the
Athena++
code, varying the background properties, dominant cosmic ray transport mechanism, and injection characteristics between our simulation runs. We find robust effects of buoyancy for all transport models, with disruption of the ISM on timescales as short as 100 Myr when the background equilibrium is dominated by cosmic ray pressure.
The Parker instability, a Rayleigh-Taylor-like instability of thermal gas supported against gravity by magnetic fields and cosmic rays, is thought to be dynamically important for galaxy evolution, ...possibly promoting molecular cloud formation and the galactic dynamo. In previous work, we examined the effect of three different cosmic-ray transport models on the Parker instability: decoupled (γc = 0), locked to the thermal gas (γc = 4/3), and coupled to the gas with streaming by self-confinement. We expand on that work here by considering radiative cooling, a smooth gravitational potential, and simulations into the nonlinear regime. We determine that cosmic-ray transport away from compression points, whether by diffusion or streaming, is the largest driver of the instability. Heating due to cosmic-ray streaming is also destabilizing and especially affects the nonlinear regime. While cooling depressurizes the dense gas, streaming cosmic rays heat and inflate the diffuse extraplanar gas, greatly modifying the phase structure of the medium. In 3D, we find that the fastest growth favors short-wavelength modes in the horizontal direction perpendicular to the background magnetic field; this is imprinted on Faraday rotation measure maps that may be used to detect the Parker instability. The modifications to the Parker instability that we observe in this work have large implications for the structure and evolution of galaxies, and they highlight the major role that cosmic rays play in shaping their environments.
Galaxies at a Cosmic Ray Eddington Limit Heintz, Evan; Zweibel, Ellen G.
Astrophysical journal/The Astrophysical journal,
12/2022, Letnik:
941, Številka:
1
Journal Article
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Abstract
Cosmic rays have been shown to be extremely important in the dynamics of diffuse gas in galaxies, helping to maintain hydrostatic equilibrium, and serving as a regulating force in star ...formation. In this paper, we address the influence of cosmic rays on galaxies by re-examining the theory of a cosmic ray Eddington limit, first proposed by Socrates et al. and elaborated upon by Crocker et al. and Huang & Davis. A cosmic ray Eddington limit represents a maximum cosmic ray energy density above which the interstellar gas cannot be in hydrostatic equilibrium, resulting in a wind. In this paper, we continue to explore the idea of a cosmic ray Eddington limit by introducing a general framework that accounts for the circumgalactic environment and applying it to five galaxies that we believe to be a good representative sample of the star-forming galaxy population, using different cosmic ray transport models to determine what gives each galaxy the best chance to reach this limit. We show that, while an Eddington limit for cosmic rays does exist, for our five galaxies, the limit either falls at star formation rates that are much larger or gas densities that are much lower than each galaxy’s measured values. This suggests that cosmic ray pressure is not the main factor limiting the luminosity of starburst galaxies.
Recent studies have found that cosmic-ray transport plays an important role in feedback processes such as star formation and the launching of galactic winds. Although cosmic-ray buoyancy is widely ...held to be a destabilizing force in galactic disks, the effect of cosmic-ray transport on the stability of stratified systems has yet to be analyzed. We perform a stability analysis of a stratified layer for three different cosmic-ray transport models: decoupled (Classic Parker), coupled with γc = 4/3 but not streaming (Modified Parker), and finally coupled with streaming at the Alfvén speed. When the compressibility of the cosmic rays is decreased the system becomes much more stable, but the addition of cosmic-ray streaming to the Parker instability severely destabilizes it. Through comparison of these three cases and analysis of the work contributions for the perturbed quantities of each system, we demonstrate that cosmic-ray heating of the gas is responsible for the destabilization of the system. We find that a 3D system is unstable over a larger range of wavelengths than the 2D system. Therefore, the Parker instability with cosmic-ray streaming may play an important role in cosmic-ray feedback.
It has been known for half a century that the interstellar medium (ISM) of our Galaxy is structured on scales as small as a few hundred kilometers, more than 10 orders of magnitude smaller than ...typical ISM structures and energy input scales. In this review we focus on neutral and ionized structures on spatial scales of a few to ∼10
4
AU, which appear to be highly overpressured, as these have the most important role in the dynamics and energy balance of interstellar gas: the tiny scale atomic structures (TSASs) and extreme scattering events (ESEs) as the most overpressured example of the tiny scale ionized structures (TSISs). We review observational results and highlight key physical processes at AU scales. We present evidence for and against microstructures as part of a universal turbulent cascade and as discrete structures, and we review their association with supernova remnants, the Local Bubble, and bright stars. We suggest a number of observational and theoretical programs that could clarify the nature of AU structures. TSAS and TSIS probe spatial scales in the range of what is expected for turbulent dissipation scales and are therefore of key importance for constraining exotic and not-well-understood physical processes that have implications for many areas of astrophysics. The emerging picture is one in which a magnetized, turbulent cascade, driven hard by a local energy source and acting jointly with phenomena such as thermal instability, is the source of these microstructures.
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