Showing papers on "Radiative transfer published in 2006"

Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology-chemistry-aerosol model

Abstract: [1] A new fully coupled meteorology-chemistry-aerosol model is used to simulate the urban- to regional-scale variations in trace gases, particulates, and aerosol direct radiative forcing in the vicinity of Houston over a 5 day summer period. Model performance is evaluated using a wide range of meteorological, chemistry, and particulate measurements obtained during the 2000 Texas Air Quality Study. The predicted trace gas and particulate distributions were qualitatively similar to the surface and aircraft measurements with considerable spatial variations resulting from urban, power plant, and industrial sources of primary pollutants. Sulfate, organic carbon, and other inorganics were the largest constituents of the predicted particulates. The predicted shortwave radiation was 30 to 40 W m−2 closer to the observations when the aerosol optical properties were incorporated into the shortwave radiation scheme; however, the predicted hourly aerosol radiative forcing was still underestimated by 10 to 50 W m−2. The predicted aerosol radiative forcing was larger over Houston and the industrial ship channel than over the rural areas, consistent with surface measurements. The differences between the observed and simulated aerosol radiative forcing resulted from transport errors, relative humidity errors in the upper convective boundary layer that affect aerosol water content, secondary organic aerosols that were not yet included in the model, and uncertainties in the primary particulate emission rates. The current model was run in a predictive mode and demonstrates the challenges of accurately simulating all of the meteorological, chemical, and aerosol parameters over urban to regional scales that can affect aerosol radiative forcing.

Validation of a vector version of the 6S radiative transfer code for atmospheric correction of satellite data. Part I: Path radiance

Abstract: A vector version of the 6S (Second Simulation of a Satellite Signal in the Solar Spectrum) radiative transfer code (6SV1), which enables accounting for radiation polarization, has been developed and validated against a Monte Carlo code, Coulson's tabulated values, and MOBY (Marine Optical Buoy System) water-leaving reflectance measurements. The developed code was also tested against the scalar codes SHARM, DISORT, and MODTRAN to evaluate its performance in scalar mode and the influence of polarization. The obtained results have shown a good agreement of 0.7% in comparison with the Monte Carlo code, 0.2% for Coulson's tabulated values, and 0.001-0.002 for the 400-550 nm region for the MOBY reflectances. Ignoring the effects of polarization led to large errors in calculated top-of-atmosphere reflectances: more than 10% for a molecular atmosphere and up to 5% for an aerosol atmosphere. This new version of 6S is intended to replace the previous scalar version used for calculation of lookup tables in the MODIS (Moderate Resolution Imaging Spectroradiometer) atmospheric correction algorithm.

Multiple Scattering of Light by Particles: Radiative Transfer and Coherent Backscattering

Abstract: 1. Introduction 2. Maxwell's equations, electromagnetic waves, and Stokes parameters 3. Basic theory of electromagnetic scattering 4. Scattering by a fixed multi-particle group 5. Statistical averaging 6. Scattering by a single random particle 7. Single scattering by a small random particle group 8. Radiative transfer equation 9. Calculations and measurements of single-particle characteristics 10. Radiative transfer in plane-parallel scattering media 11. Macroscopically isotropic and mirror-symmetric scattering media 12. Radiative transfer in plane-parallel, microscopically isotropic and mirror-symmetric scattering media 13. Illustrative applications of radiative transfer theory 14. Coherent backscattering.

A Gray-Radiation Aquaplanet Moist GCM. Part I: Static Stability and Eddy Scale

Abstract: In this paper, a simplified moist general circulation model is developed and used to study changes in the atmospheric general circulation as the water vapor content of the atmosphere is altered. The key elements of the model physics are gray radiative transfer, in which water vapor and other constituents have no effect on radiative fluxes, a simple diffusive boundary layer with prognostic depth, and a mixed layer aquaplanet surface boundary condition. This GCM can be integrated stably without a convection parameterization, with large-scale condensation only, and this study focuses on this simplest version of the model. These simplifications provide a useful framework in which to focus on the interplay between latent heat release and large-scale dynamics. In this paper, the authors study the role of moisture in determining the tropospheric static stability and midlatitude eddy scale. In a companion paper, the effects of moisture on energy transports by baroclinic eddies are discussed. The authors ...

Monte Carlo radiative transfer in protoplanetary disks

Abstract: Aims.We present a new continuum 3D radiative transfer code, MCFOST, based on a Monte-Carlo method. MCFOST can be used to calculate (i) monochromatic images in scattered light and/or thermal emission; (ii) polarisation maps; (iii) interferometric visibilities; (iv) spectral energy distributions; and (v) dust temperature distributions of protoplanetary disks. Methods: .Several improvements to the standard Monte Carlo method are implemented in MCFOST to increase efficiency and reduce convergence time, including wavelength distribution adjustments, mean intensity calculations, and an adaptive sampling of the radiation field. The reliability and efficiency of the code are tested against a previously-defined benchmark, using a 2D disk configuration. No significant difference (no more than 10% and usually much less) is found between the temperatures and SEDs calculated by MCFOST and by other codes included in the benchmark. Results: . A study of the lowest disk mass detectable by Spitzer, around young stars, is presented and the colours of "representative" parametric disks compared to recent IRAC and MIPS Spitzer colours of solar-like young stars located in nearby star-forming regions.

Reevaluation of Mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data

Abstract: Modelling studies and satellite retrievals do not agree on the amplitude and/or sign of the direct radiative per- turbation from dust. Modelling studies have systematically overpredicted mineral dust absorption compared to estimates based upon satellite retrievals. In this paper we first point out the source of this discrepancy, which originates from the shortwave refractive index of dust used in models. The imag- inary part of the refractive index retrieved from AERONET over the range 300 to 700 nm is 3 to 6 times smaller than that used previously to model dust. We attempt to constrain these refractive indices using a mineralogical database and varying the abundances of iron oxides (the main absorber in the visible). We first consider the optically active min- eral constituents of dust and compute the refractive indices from internal and external mixtures of minerals with rela- tive amounts encountered in parent soils. We then compute the radiative perturbation due to mineral aerosols for inter- nally and externally mixed minerals for 3 different hematite contents, 0.9%, 1.5% and 2.7% by volume. These constant amounts of hematite allow bracketing the influence of dust aerosol when it is respectively an inefficient, standard and a very efficient absorber. These values represent low, central and high content of iron oxides in dust determined from the mineralogical database. Linke et al. (2006) determined inde- pendently that iron-oxides represent 1.0 to 2.5% by volume using x-Ray fluorescence on 4 different samples collected over Morocco and Egypt. Based upon values of the refrac- tive index retrieved from AERONET, we show that the best agreement between 440 and 1020 nm occurs for mineral dust internally mixed with 1.5% volume weighted hematite. This representation of mineral dust allows us to compute, using a general circulation model, a new global estimate of min- eral dust perturbation between -0.47 and -0.24 Wm 2 at the top of the atmosphere, and between -0.81 and -1.13 Wm 2 at the surface for both shortwave and longwave wavelengths. The anthropogenic dust fraction is thought to account for be- tween 10 and 50% of the total dust load present in the at- mosphere. We estimate a top of the atmosphere forcing be- tween -0.03 and -0.25 Wm 2 , which is markedly different that the IPCC range of -0.6 to +0.4 Wm 2 (IPCC, 2001). The 24-h average atmospheric heating by mineral dust dur- ing summer over the tropical Atlantic region (15 N-25 N; 45 W-15 W) is in the range +22 to +32 Wm 2 1 which compares well with the 30±4 Wm 2 1 measured by Li et al. (2004) over that same region. The refractive indices from Patterson et al. (1977) and from Volz (1973) overestimate by a factor of 2 the energy absorbed in the column during sum- mer over the same region. This discrepancy is due to too large absorption in the visible but we could not determine if this is linked to the sample studied by Patterson et al. (1997) or to the method used in determining the refractive index.

Ly alpha radiation from collapsing protogalaxies. I. Characteristics of the emergent spectrum

Abstract: We present Monte Carlo calculations of Ly alpha radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N(H) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.

The Observable Effects of a Photospheric Component on GRB and XRF Prompt Emission Spectrum

Abstract: A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes (XRFs) We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere We consider both the internal shock model and a "slow heating" model as possible dissipation mechanisms For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering This leads to a nearly flat energy spectrum (νFν ∝ ν0) above the thermal peak at ≈10-100 keV and below 10-100 MeV, for a wide range of optical depths 003 τγe 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process For higher values of the optical depth, a Wien peak is formed at 100 keV-1 MeV, and no higher energy component exists For any value of τγe, the number of pairs produced does not exceed the baryon-related electrons by a factor of larger than a few We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron-SSC model

Time-dependent Monte Carlo Radiative Transfer Calculations for Three-dimensional Supernova Spectra, Light Curves, and Polarization

Abstract: We discuss Monte Carlo techniques for addressing the three-dimensional time-dependent radiative transfer problem in rapidly expanding supernova atmospheres. The transfer code SEDONA has been developed to calculate the light curves, spectra, and polarization of aspherical supernova models. From the onset of free expansion in the supernova ejecta, SEDONA solves the radiative transfer problem self-consistently, including a detailed treatment of gamma-ray transfer from radioactive decay and with a radiative equilibrium solution of the temperature structure. Line fluorescence processes can also be treated directly. No free parameters need be adjusted in the radiative transfer calculation, providing a direct link between multidimensional hydrodynamic explosion models and observations. We describe the computational techniques applied in SEDONA and verify the code by comparison to existing calculations. We find that convergence of the Monte Carlo method is rapid and stable even for complicated multidimensional configurations. We also investigate the accuracy of a few commonly applied approximations in supernova transfer, namely, the stationarity approximation and the two-level atom expansion opacity formalism.

Simulating Cosmic Reionization at Large Scales I: the Geometry of Reionization

Abstract: We present the first large-scale radiative transfer simulat ions of cosmic reionization, in a simulation volume of (100 h 1 Mpc) 3 . This is more than a 2 orders of magnitude improvement over previous simulations. We achieve this by combining the results from extremely large, cosmological, N-body simulations with a new, fast and effici ent code for 3D radiative transfer, C 2 -Ray, which we have recently developed. These simulations allow us to do the first numerical studies of the large-scale structure of reionization w hich at the same time, and crucially, properly take account of the dwarf galaxy ionizing sources which are primarily responsible for reionization. In our realization, reionization starts around z � 21, and final overlap occurs by z � 11. The resulting electron-scattering optical depth is in goo d agreement with the firstyear WMAP polarization data. We show that reionization clearly proceeded in an inside-out fashion, with the high-density regions being ionized earli er, on average, than the voids. Ionization histories of smaller-size (5 to 10 comoving Mpc) subregions exabit a large scatter about the mean and do not describe the global reionization history well. This is true even when these subregions are at the mean density of the universe, which shows that small-box simulations of reionization have little predictive power for the evolut ion of the mean ionized fraction. The minimum reliable volume size for such predictions is � 30 Mpc. We derive the power-spectra of the neutral, ionized and total gas density fields and show t hat there is a significant boost of the density fluctuations in both the neutral and the ionized c omponents relative to the total at arcminute and larger scales. We find two populations of H II re gions according to their size, numerous, mid-sized (� 10 Mpc) regions and a few, rare, very large regions tens of Mpc in size. Thus, local overlap on fairly large scales of tens of Mp c is reached by z � 13, when our volume is only about 50% ionized, and well before the global overlap. We derive the statistical distributions of the ionized fraction and ionized gas densi ty at various scales and for the first time show that both distributions are clearly non-Gaussian. All these quantities are critical for predicting and interpreting the observational signals from reionization from a variety of observations like 21-cm emission, Ly-α emitter statistics, Gunn-Peterson optical depth and small-scale CMB secondary anisotropies due to patchy reionization.

Circumstellar Emission from Type Ib and Ic Supernovae

Abstract: The presumed Wolf-Rayet star progenitors of Type Ib/c supernovae have fast, low-density winds, and the shock waves generated by the supernova interaction with the wind are not expected to be radiative at typical times of observation. The injected energy spectrum of radio-emitting electrons typically has an observed index p = 3, which is suggestive of acceleration in cosmic-ray-dominated shocks. The early, absorbed part of the radio light curves can be attributed to synchrotron self-absorption, which leads to constraints on the magnetic field in the emitting region and on the circumstellar density. The range of circumstellar densities inferred from the radio emission is somewhat broader than that for Galactic Wolf-Rayet stars, if similar efficiencies of synchrotron emission are assumed in the extragalactic supernovae. For the observed and expected ranges of circumstellar densities to roughly overlap, a high efficiency of magnetic field production in the shocked region is required (B ≈ 0.1). For the expected densities around a Wolf-Rayet star, a nonthermal mechanism is generally required to explain the observed X-ray luminosities of Type Ib/c supernovae. Inverse Compton emission is a candidate for the emission, if the observations are near optical maximum. In other cases we suggest that the mechanism is X-ray synchrotron emission in a situation in which the shock wave is cosmic-ray-dominated so that the electron energy spectrum flattens at high energy. More comprehensive X-ray observations of a Type Ib/c supernova are needed to determine whether this suggestion is correct.

sunrise: polychromatic dust radiative transfer in arbitrary geometries

Abstract: This paper describes SUNRISE, a parallel, free Monte Carlo code for the calculation of radiation transfer through astronomical dust. SUNRISE uses an adaptive mesh refinement grid to describe arbitrary geometries of emitting and absorbing/scattering media, with spatial dynamical range exceeding 10 4 , and it can efficiently generate images of the emerging radiation at arbitrary points in space. In addition to the monochromatic radiative transfer typically used by Monte Carlo codes, SUNRISE is capable of propagating a range of wavelengths simultaneously. This 'polychromatic' algorithm gives significant improvements in efficiency and accuracy when spectral features are calculated. SUNRISE is used to study the effects of dust in hydrodynamic simulations of interacting galaxies, and the procedure for this is described. The code is tested against previously published results.

Interactive dust‐radiation modeling: A step to improve weather forecasts

Abstract: [1] Inclusion of mineral dust radiative effects could lead to a significant improvement in the radiation balance of numerical weather prediction models with subsequent improvements in the weather forecast itself. In this study the radiative effects of mineral dust have been fully incorporated into a regional atmospheric dust model. Dust affects the radiative fluxes at the surface and the top of the atmosphere and the temperature profiles at every model time step when the radiation module is processed. These changes influence the atmospheric dynamics, moisture physics, and near-surface conditions. Furthermore, dust emission is modified by changes in friction velocity and turbulent exchange coefficients; dust turbulent mixing, transport, and deposition are altered by changes in atmospheric stability, precipitation conditions, and free-atmosphere winds. A major dust outbreak with dust optical depths reaching 3.5 at 550 nm over the Mediterranean region on April 2002 is selected to assess the radiative dust effects on the atmosphere at a regional level. A strong dust negative feedback upon dust emission (35–45% reduction of the AOD) resulted from the smaller outgoing sensible turbulent heat flux decreasing the turbulent momentum transfer from the atmosphere and consequently dust emission. Significant improvements of the atmospheric temperature and mean sea-level pressure forecasts are obtained over dust-affected areas by considerably reducing both warm and cold temperature biases existing in the model without dust-radiation interactions. This study demonstrates that the use of the proposed model with integrated dust and atmospheric radiation represents a promising approach for further improvements in numerical weather prediction practice and radiative impact assessment over dust-affected areas.

VLIDORT: A linearized pseudo-spherical vector discrete ordinate radiative transfer code for forward model and retrieval studies in multilayer multiple scattering media

Abstract: We describe a new vector discrete ordinate radiative transfer model with a full linearization facility. The VLIDORT model is designed to generate simultaneous output of Stokes vector light fields and their derivatives with respect to any atmospheric or surface property. We develop new implementations for the linearization of the vector radiative transfer solutions, and go on to show that the complete vector discrete ordinate solution is analytically differentiable for a stratified multilayer multiply scattering atmospheric medium. VLIDORT will generate all output at arbitrary viewing geometry and optical depth. The model has the ability to deal with attenuation of solar and line-of-sight paths in a curved atmosphere, and includes an exact treatment of the single scatter computation. VLIDORT also contains a linearized treatment for non-Lambertian surfaces. A number of performance enhancements have been implemented, including a facility for multiple solar zenith angle output. The model has been benchmarked against established results in the literature.

Time Dependent Monte Carlo Radiative Transfer Calculations For 3-Dimensional Supernova Spectra, Lightcurves, and Polarization

Abstract: We discuss Monte-Carlo techniques for addressing the 3-dimensional time-dependent radiative transfer problem in rapidly expanding supernova atmospheres. The transfer code SEDONA has been developed to calculate the lightcurves, spectra, and polarization of aspherical supernova models. From the onset of free-expansion in the supernova ejecta, SEDONA solves the radiative transfer problem self-consistently, including a detailed treatment of gamma-ray transfer from radioactive decay and with a radiative equilibrium solution of the temperature structure. Line fluorescence processes can also be treated directly. No free parameters need be adjusted in the radiative transfer calculation, providing a direct link between multi-dimensional hydrodynamical explosion models and observations. We describe the computational techniques applied in SEDONA, and verify the code by comparison to existing calculations. We find that convergence of the Monte Carlo method is rapid and stable even for complicated multi-dimensional configurations. We also investigate the accuracy of a few commonly applied approximations in supernova transfer, namely the stationarity approximation and the two-level atom expansion opacity formalism.

Radiative transfer modeling of three-dimensional clumpy AGN tori and its application to NGC 1068

Abstract: Recent observations of NGC 1068 and other AGN support the idea of a geometrically and optically thick dust torus surrounding the central supermassive black hole and accretion disk of AGN. In type 2 AGN, the torus is seen roughly edge-on, leading to obscuration of the central radiation source and a silicate absorption feature near $10~{\rm\mu m}$. While most of the current torus models distribute the dust smoothly, there is growing evidence that the dust must be arranged in clouds. We describe a new method for modeling near- and mid-infrared emission of 3-dimensional clumpy tori using Monte Carlo simulations . We calculate the radiation fields of individual clouds at various distances from the AGN and distribute these clouds within the torus region. The properties of the individual clouds and their distribution within the torus are determined from a theoretical approach of self-gravitating clouds close to the shear limit in a gravitational potential. We demonstrate that clumpiness in AGN tori can overcome the problem of over-pronounced silicate features. Finally, we present model calculations for the prototypical Seyfert 2 galaxy NGC 1068 and compare them to recent high-resolution measurements. Our model is able to reproduce both the SED and the interferometric observations of NGC 1068 in the near- and mid-infrared.

Lyman α radiative transfer in a multiphase medium

Abstract: Hydrogen Lyman a (Lya) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lya radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lya resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lya surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption, N 0 , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths (EWs). Lya can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lya photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows. We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged.

How northern peatlands influence the Earth's radiative budget: Sustained methane emission versus sustained carbon sequestration

Abstract: [1] Northern peatlands sequester carbon and emit methane, and thus have both cooling and warming impacts on the climate system through their influence on atmospheric burdens of CO2 and CH4. These competing impacts are usually compared by the global warming potential (GWP) methodology, which determines the equivalent CO2 annual emission that would have the same integrated radiative forcing impact over a chosen time horizon as the annual CH4 emission. We use a simple model of CH4 and CO2 pools in the atmosphere to extend this analysis to quantify the dynamics, over years to millennia, of the net radiative forcing impact of a peatland that continuously emits CH4 and sequesters C. We find that for observed ratios of CH4 emission to C sequestration (roughly 0.1–2 mol mol−1), the radiative forcing impact of a northern peatland begins, at peatland formation, as a net warming that peaks after about 50 years, remains a diminishing net warming for the next several hundred to several thousand years, depending on the rate of C sequestration, and thereafter is or will be an ever increasing net cooling impact. We then use the model to evaluate the radiative forcing impact of various changes in CH4 and/or CO2 emissions. In all cases, the impact of a change in CH4 emissions dominates the radiative forcing impact in the first few decades, and then the impact of the change in CO2 emissions slowly exerts its influence.

Tropospheric emission spectrometer: retrieval method and error analysis

Abstract: We describe the approach for the estimation of the atmospheric state, e.g., temperature, water, ozone, from calibrated, spectral radiances measured from the Tropospheric Emission Spectrometer (TES) onboard the Aura spacecraft. The methodology is based on the maximum a posteriori estimate, which mathematically requires the minimization of the difference between observed spectral radiances and a nonlinear model of radiative transfer of the atmospheric state subject to the constraint that the estimated state must be consistent with an a priori probability distribution for that state. The minimization techniques employed here are based on the trust-region Levenberg-Marquardt algorithm. An analysis of the errors for this estimate include smoothing, random, spectroscopic, "cross-state", representation, and systematic errors. In addition, several metrics and diagnostics are introduced that assess the resolution, quality, and statistical significance of the retrievals. We illustrate this methodology for the retrieval of atmospheric and surface temperature, water vapor, and ozone over the Gulf of Mexico on November 3, 2004.

Thermal Microwave Radiation: Applications for Remote Sensing

Abstract: * Chapter 1: Radiative transfer and microwave radiometry * Chapter 2: Emission and spectroscopy of the clear atmosphere * Chapter 3: Emission and scattering by clouds and precipitation * Chapter 4: Surface emission * Chapter 5: Dielectric properties of natural media * Appendices

Wouthuysen-Field coupling strength and application to high-redshift 21-cm radiation

Abstract: The first ultraviolet sources in the universe are expected to have coupled the H I spin temperature to the gas kinetic temperature via scattering in the Lyα resonance (the 'Wouthuysen-Field effect'). By establishing an Hi spin temperature different from the temperature of the cosmic microwave background, the Wouthuysen-Field effect should allow observations of H I during the reionization epoch in the redshifted 21-cm hyperfine line. This paper investigates four mechanisms that can affect the strength of the Wouthuysen-Field effect that were not previously considered. (1) Photons redshifting into the Hi Lyman resonances may excite an H atom and result in a radiative cascade terminating in two-photon 2s 1/2 → 1s 1/2 emission, rather than always degrading to Lyα as usually assumed. (2) The fine structure of the Lyα resonance alters the photon frequency distribution and leads to a suppression of the scattering rate. (3) The spin-flip scatterings change the frequency of the photon and cause the photon spectrum to relax not to the kinetic temperature of the gas but to a temperature between the kinetic and spin temperatures, effectively reducing the strength of the Wouthuysen-Field coupling. (4) Near line centre, a photon can change its frequency by several times the line width in a single scattering event, thus potentially invalidating the usual calculation of the Lyα spectral distortion based on the diffusion approximation. It is shown that (1) suppresses the Wouthuysen-Field coupling strength by a factor of up to ∼2, while (2) and (3) are important only at low kinetic temperatures. Effect (4) has a ≤3 per cent effect for kinetic temperatures T k ≥ 2 K. In particular, if the pre-reionization intergalactic medium was efficiently heated by X-rays, only effect (1) is important. Fitting formulae for the Wouthuysen-Field coupling strength are provided for the range of T k ≥ 2 K and Gunn-Peterson optical depth 10 5 < TOP < 10 7 so that all of these effects can be easily incorporated into 21-cm codes.

Vertical Structure of Gas Pressure-dominated Accretion Disks with Local Dissipation of Turbulence and Radiative Transport

Abstract: We calculate the vertical structure of a local patch of an accretion disk in which heating by dissipation of MRI-driven MHD turbulence is balanced by radiative cooling. Heating, radiative transport, and cooling are computed self-consistently with the structure by solving the equations of radiation MHD in the shearing-box approximation. Using a fully three-dimensional and energy-conserving code, we compute the structure of this disk segment over a span of more than five cooling times. After a brief relaxation period, a statistically steady state develops. Measuring height above the midplane in units of the scale height predicted by a Shakura-Sunyaev model, we find that magnetic pressure causes the disk atmosphere to stretch upward, with the photosphere rising to 7H, in contrast to the 3H predicted by conventional analytic models. This more extended structure, as well as fluctuations in the height of the photosphere, may lead to departures from Planckian form in the emergent spectra. Dissipation is distributed across the region within 3H of the midplane but is very weak at greater altitudes. As a result, the temperature deep in the disk interior is less than that expected when all heat is generated in the midplane. With only occasional exceptions, the gas temperature stays very close to the radiation temperature, even above the photosphere. Because fluctuations in the dissipation are particularly strong away from the midplane, the emergent radiation flux can track dissipation fluctuations with a lag that is only 0.1-0.2 times the mean cooling time of the disk. Long-timescale asymmetries in the dissipation distribution can also cause significant asymmetry in the flux emerging from the top and bottom surfaces of the disk. Radiative diffusion dominates Poynting flux in the vertical energy flow throughout the disk.

Theoretical light curves for deflagration models of Type Ia supernova

Abstract: Aims. We present synthetic bolometric and broad-band UBVRI light curves of SNe Ia for four selected 3D deflagration models of thermonuclear supernovae. Methods. The light curves are computed with the 1D hydro code stella, which models (multi-group time-dependent) nonequilibrium radiative transfer inside SN ejecta. Angle-averaged results from 3D hydrodynamical explosion simulations with the composition determined in a nucleosynthetic postprocessing step served as the input to the radiative transfer model. Results. The predicted model UBV light curves do agree reasonably well with the observed ones for SNe Ia in the range of low to normal luminosities, although the underlying hydrodynamical explosion models produced only a modest amount of radioactive 56 Ni (i.e. ∼0.24–0.42 M� ) and relatively low kinetic energy in the explosion (less than 0.7 × 10 51 erg). The evolution of predicted B and ∞

Cosmological radiative transfer codes comparison project – I. The static density field tests

Abstract: Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H II region expansion; (2) H II region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form.

Magnetoacoustic Portals and the Basal Heating of the Solar Chromosphere

Abstract: We show that inclined magnetic field lines at the boundaries of large-scale convective cells (supergranules) provide "portals" through which low-frequency ( 5 mHz) acoustic waves, which are believed to provide the dominant source of wave heating of the chromosphere. This result opens up the possibility that low-frequency magnetoacoustic waves provide a significant source of energy for balancing the radiative losses of the ambient solar chromosphere.

High-Energy-Density Physics: Fundamentals, Inertial Fusion, and Experimental Astrophysics

Abstract: to High-Energy-Density Physics.- Descriptions of Fluids and Plasmas.- Properties of High-Energy-Density Plasmas.- Shocks and Rarefactions.- Hydrodynamic Instabilities.- Radiative Transfer.- Radiation Hydrodynamics.- Creating High-Energy-Density Conditions.- Inertial Confinement Fusion.- Experimental Astrophysics.- Relativistic High-Energy-Density Systems.