Spectral Line Radiation Research Articles

Updated Calculations of the Spectral-Line Radiation Force & Mass-Loss Rates for AGN Outflows

AbstractPhoton-driven flows have been studied for almost a century, and a quantitative description of the radiative forces on atoms and ions is important for understanding a wide variety of systems, including active galactic nuclei (AGN). The colloquially-termed “radiation pressure” of line-driven winds plays an important role in driving outflows in these environments. Quantifying the associated forces is crucial to understanding how these flows enable interactive mechanisms within these environments, such as AGN feedback. Here we provide new calculations of the dimensionless line strength parameter due to radiation driving. For representative AGN, we calculate the photoionization balance at each step along the line of sight (LOS) to the proposed wind-launching region above the accretion disk. We then use a recently compiled database of approximately 5.6 million spectral lines to compute the strength of the line-driving force on the gas and the mass-loss rates resulting from these outflows. We also introduce a “shielding factor’’ that increases the magnitude of the accretion disk column density prior to the launch radius. This shielding factor simulates a proposed inner “failed wind” region that is thought to shield the outflowing gas from becoming over-ionized by the central source. We also revisit and formalize the role of the commonly-used ionization parameter in setting the properties of the accelerating gas.

Structural, luminescence, and temperature sensing properties of the Er3+-doped germanate-tellurite glass by excitation at different wavelengths

Er3+-doped germanate-tellurite glasses were prepared by high-temperature melting method, in which structure, luminescent and, temperature-sensing properties were studied. The Raman spectrum indicates that the maximum phonon energy is low, which is 860 cm−1. The transmission spectrum indicates that the infrared transmissive range of glass is wide, visible, and infrared transmittance is high. Parameters of spectral line intensity Ωλ and radiation parameters were calculated based on Judd-Ofelt theory, which showed that the glass has large values of Ω2 and Ω6, and values of Ω2 and Ω6 decrease with increasing concentration of Er3+ ions. Strong up-conversion luminescence was obtained at 808, 980 and 1550 nmLD with wavelengths at 534, 553 and 668 nm, respectively, and strong down-conversion luminescence of 1.53 and 2.7 μm also were obtained. The up-conversion emission intensity under three-wavelength simultaneous excitation is significantly enhanced compared to monochromatic excitation. The emission intensity corresponding to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transition of Er3+ ions varied with temperature, and the fluorescence intensity ratio FIR of 556 nm and 524 nm increased with increasing temperature. In the temperature range from 330 to 610 K, the maximum sensitivity of glasses was 15.63 × 10−3 K−1 and 19.74 × 10−3 K−1 at T = 570 K, respectively under 808 and 980 nm LD excitation.

A method for calculating the radiation characteristics of nonequilibrium laboratory plasmas

One of the fundamentally important issues in the characterization of a dense high-temperature plasma is the description of its self-radiation aimed at determining the radiation losses and their spectral composition. In this paper, a new method is proposed for calculating the radiation characteristics of a high-temperature nonequilibrium plasma. The method is based on a collisional radiative model, which makes it possible to calculate the charge-state distribution in the plasma and the excited-state distribution of the plasma ions. The transfer equations are solved separately for the radiation with a continuous spectrum, combining the bremsstrahlung and recombination radiation spectra, and for the spectral line radiation. A feature of the method is that, when calculating the mean line absorption coefficients, dimensionless weighting coefficients are used, which make it possible to take into account the contribution of the strongest lines. The use of weighting coefficients has the result that even a one-group approximation yields good agreement with the numerical solution of the complete spectral problem.

The transfer of polarized radiation in resonance lines with partial frequency redistribution, J-state interference, and arbitrary magnetic fields

Aims. We present the theoretical framework and numerical methods we have implemented to solve the problem of the generation and transfer of polarized radiation in spectral lines without assuming local thermodynamical equilibrium, while accounting for scattering polarization, partial frequency redistribution (due to both the Doppler effect and elastic collisions), J-state interference, and hyperfine structure. The resulting radiative transfer code allows one to model the impact of magnetic fields of an arbitrary strength and orientation through the Hanle, incomplete Paschen-Back, and magneto-optical effects. We also evaluate the suitability of a series of approximations for modeling the scattering polarization in the wings of strong resonance lines at a much lower computational cost, which is particularly valuable for the numerically intensive case of three-dimensional radiative transfer. Methods. We examine the suitability of the considered approximations by using our radiative transfer code to model the Stokes profiles of the Mg II h & k lines and of the H I Lyman-α line in magnetized one-dimensional models of the solar atmosphere. Results. Neglecting Doppler redistribution in the scattering processes that are unperturbed by elastic collisions (i.e., treating them as coherent in the observer’s frame) produces a negligible error in the scattering polarization wings of the Mg II resonance lines and a minor one in the Lyman-α wings, although it is unsuitable to model the cores of these lines. For both lines, the scattering processes that are perturbed by elastic collisions only give a significant contribution to the intensity component of the emissivity. Neglecting collisional as well as Doppler redistribution (so that all scattering processes are coherent) represents a rough but suitable approximation for the wings of the Mg II resonance lines, but a very poor one for the Lyman-α wings. The magnetic sensitivity in the scattering polarization wings of the considered lines can be modeled by accounting for the magnetic field in only the ηI and ρV coefficients of the Stokes-vector transfer equation (i.e., using the zero-field expression for the emissivity).

Magnetic Field Diagnostics in the Solar Upper Atmosphere

The magnetic field is the main driver of the activity in the solar upper atmosphere, but its measurement is notoriously difficult. In order to determine the magnetic field in the chromosphere, transition region, and corona, we need to measure and interpret the polarization signals that the scattering of anisotropic radiation and the Hanle and Zeeman effects introduce in the emitted spectral line radiation. A number of recent advances have activated the development of this research field. ▪ The quantum theory of the generation and transfer of polarized radiation allows us to explain the polarization signals observed in chromospheric and coronal lines and to make successful predictions in unexplored spectral regions. ▪ The development of diagnostic techniques for the solar upper atmosphere has served to improve our empirical knowledge of the magnetic field in a variety of plasma structures, as well as to pave the way for their application to the unprecedented data that the new generation of solar telescopes are expected to provide. However, further improvements are required. ▪ The CLASP suborbital experiments have opened a new diagnostic window, namely ultraviolet (UV) spectropolarimetry as a tool for probing the magnetism and geometry of the upper chromosphere and transition region. A space telescope equipped with a UV spectropolarimeter would lead to major advances in our empirical understanding of solar magnetism.

Effect of the plasma self-radiation on the growth of thermal filamentation instabilities in imploding Z pinches

The development of thermal filamentation (TF) instabilities in a current-carrying plasma shell under the action of the plasma self-radiation was analyzed in terms of a small perturbation theory. A stationary collisional radiative model was used to calculate the parameters of the bremsstrahlung, recombination radiation, and spectral line radiation. It has been shown that radiative losses can either enhance or weaken the growth of TF instabilities. The pattern of the effect is governed by the dependence of the energy lost by the plasma due to radiation, Q Rad, on the plasma temperature T. If Q Rad increases slower than ∼T, the radiative losses enhance TF instabilities. In the opposite case, that is when Q Rad increases faster than ∼T, the radiative losses lead to suppression of TF instabilities. When the energy lost due to radiation is greater than the Joule energy input, TF instabilities can be completely stabilized due to radiation. The plasma parameter ranges for which stabilization of TF instabilities may occur due to radiation have been found for aluminum and argon.

Long Characteristics versus Short Characteristics in 3D Radiative Transfer Simulations of Polarized Radiation

Abstract We compare maps of scattering polarization signals obtained from three-dimensional (3D) radiation transfer calculations in a magnetoconvection model of the solar atmosphere using formal solvers based on the “short characteristics” (SC) and the “long characteristics” (LC) methods. The SC method requires less computational work, but it is known to introduce spatial blurring in the emergent radiation for inclined lines of sight. For polarized radiation this effect is generally more severe due to it being a signed quantity and to the sensitivity of the scattering polarization to the model’s inhomogeneities. We study the differences in the polarization signals of the emergent spectral line radiation calculated with such formal solvers. We take as a case study already published results of the scattering polarization in the Sr i 4607 Å line obtained with the SC method, demonstrating that in high-resolution grids it is accurate enough for that type of study. In general, the LC method is the preferred one for accurate calculations of the emergent radiation, which is the reason why it is now one of the options in the public version of the 3D radiative transfer code PORTA.

In-situ measurement of temperature and potassium concentration during the combustion of biomass pellets based on the emission spectrum

Here, a calibration method using the K concentration, temperature and the characteristic spectral line radiation intensity in flame is described. In addition, a radiation function model between these three variables, Iλ766T,CK=0.821·Ib(λ766,T)·CK, was established. A Y-type optical fiber was used to connect the spectral signals from the same direction to two spectrometers. The continuous spectra in the 500–800 nm band were used for multi-spectral temperature measurements. The combination of the measured temperature and the K characteristic spectral line radiation intensity at 766 nm was used for measurements of K concentration. The temperature and equivalent ratio of the heating atmosphere were 1300 K (±20 K) and 0.786, respectively. Compared with rice straw pellet, pine wood pellet had a lower K concentration and higher combustion temperature during the entire combustion process. The peak K release concentrations of the pine wood pellet and the rice straw pellet during the volatile combustion stage were 2.09 ppm and 10.03 ppm, respectively, and the peak flame temperatures were 1774 K and 1756 K, respectively. In the char combustion stage, the peak K release concentrations were 27.83 ppm and 110.43 ppm, respectively, and the peak surface temperatures were 1287 K and 1143 K, respectively. The established function model for measuring K concentration is also applicable to the combustion diagnosis of other K-rich fuels.

New explanation of opposite position angles in red and blue wings of spectral lines

ABSTRACTWe consider the emission of resonance line radiation from rotating circular accretion disc with the progressive increasing height (the inclined ring). Our theory can also be applied to the rotating spot-like sources of resonance radiation. We assume that the atmosphere of inclined ring is homogeneous. In this case, the every part of ring emits the radiation with the wave electric field oscillations perpendicular to plane $(\boldsymbol{nN^{\prime }})$, where ${\boldsymbol{n}}$ is the direction to a telescope and $\boldsymbol{N^{\prime }}$ is the normal to considered local surface of a ring. Geometrical consideration shows that the radiation polarization angle χ (frequently denoted as position angle or PA) depends on the inclination angle α of the ring relative to the central plane of accretion disc, the inclination angle θ of the central plane of accretion disc with the normal $\boldsymbol{N}$, and on azimuthal angle φ of radiating part on the ring. The right and left parts of a ring according to the symmetry plane $(\boldsymbol{nN})$ give rise to opposite polarization angles. For rotating accretion disc, due to Doppler’s effect, this means that polarization angles have opposite signs in the red and blue wings of emerging spectral line radiation. Such behaviour is observed in many objects.

Opacity effects on laser-produced plasma radiation sources

The escape of spectral line radiation from laser-produced plasma radiation sources with moderate opacity is examined using a simple model of emission with a planar geometry, constant source function, and an empirically determined optical depth. The model is applied to determine the radiation produced by laser irradiation of tin targets used as the source of radiation in extreme ultraviolet (EUV) lithography. Variations in emission relative to optically thin plasmas in agreement with previous experimental measurements of both the angular variation of the emission of EUV light at 13.5 nm and the effect of plasma opacity in reducing EUV emission at laser intensities above 1011 W cm−2 are found. The model is extended to predict optimum conditions for future lithography radiation sources at ≈6.7 nm.

Spectral Modeling of Hydrogen Radiation Emission in Magnetic Fusion Plasmas

Modeling of the spectral line and continuum radiation emitted by hydrogen isotopes in peripheral regions of magnetic fusion is presented through profiles of the Zeeman-Doppler broadened Hα/Dα line and those of the Stark broadened high-n Balmer lines extending beyond the series limit for recombining plasmas. The Hα/Dα line profiles should be modelled while accounting for several populations of neutrals to mimic real situations and analyze experimental data for isotopic ratio determination. On the other side, high-n Balmer lines of hydrogen are used for plasma electron density and temperature diagnostics. Moreover, modelling whole spectra including the continuum radiation contributes to the development of synthetic diagnostics for future magnetic fusion devices for which they can give predictive results through coupling to numerical simulation tools.

The effect of horizontal plasma inhomogeneities in 3D NLTE radiation transfer in stellar atmospheres

We aim to demonstrate the effect of atmospheric inhomogeneities on the emergent specific intensity and radiation flux of a spectral line radiation. We self-consistently solve the NLTE problem for a two-level atom in a 3D atmosphere using the Cartesian grid. For that purpose, we use the 3D radiative transfer code PORTA. By examining simple examples, we study cases where the temperature inhomogeneities in the atmosphere models lead to the modification of the emergent radiation. We show that specific temperature inhomogeneities in the model atmospheres influence the emerging radiation, and that interpretation of the stellar spectra based on a plane-parallel atmosphere models can lead to erroneous conclusions about the atmospheric structure.

CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region

Abstract The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometric complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Second, we introduce a statistical description of the solar atmosphere based on a 3D model derived from a state-of-the-art radiation magnetohydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

Continuum and Spectral Line Radiation from a Random Clumpy Medium

Abstract We present a formalism for continuum and line emission from random clumpy media together with its application to problems of current interest, including CO spectral lines from ensembles of clouds and radio emission from H ii regions, supernovae, and star-forming regions. For line emission, we find that the effects of clump opacity on observed line ratios can be indistinguishable from variations of intrinsic line strengths, adding to the difficulties in determining abundances from line observations. Our formalism is applicable to arbitrary distributions of cloud properties, provided the cloud volume filling factor is small; numerical simulations show it to hold up to filling factors of ∼10%. We show that irrespective of the complexity of the cloud ensemble, the radiative effect of clumpiness can be parameterized at each frequency by a single multiplicative correction to the overall optical depth; this multiplier is derived from appropriate averaging over individual cloud properties. Our main finding is that cloud shapes have only a negligible effect on radiation propagation in clumpy media; the results of calculations employing point-like clouds are practically indistinguishable from those for finite-sized clouds with arbitrary geometrical shapes.

Nonlocal electron kinetics and spectral line emission in the positive column of an argon glow discharge

Modern non-local electron kinetics theory predicts several interesting effects connected with spectral line emission from the positive column in the range of low and medium pressures and currents. Some theoretical works describe non-monotonic behavior of the radial profiles of line emission at intermediate pressures and currents between the validity ranges of the non-local and local approximation of the electron kinetics. Despite a great number of publications, there have been no systematic measurements attempting to confirm these theoretical predictions through experiments. In this work the radial profiles of the line emission from the positive column of an argon glow discharge have been measured with high spatial resolution and new effects caused by the narrowing and broadening of the spatial emission profiles with dependence on discharge conditions have been discovered. The effect of intensity maximum shift predicted by theory using a self-consistent model was not found in the experiment. The properties of the spectral line radiation are influenced by the peculiarities of the formation of the high-energy tail of the electron energy distribution function. An interpretation of the observed effects based on the non-local character of the electron kinetics in radially inhomogeneous fields is given. The obtained experimental data are compared with the results of calculations.

Formation of polarized spectral lines in atmospheres with horizontal inhomogeneities

AbstractWe study the problem of the generation and transfer of spectral line intensity and polarization in models of stellar atmospheres with horizontal plasma inhomogeneities. We solve the non-LTE radiative transfer problem in full 3D geometry taking into account resonant scattering polarization and its modification by magnetic fields via the Hanle effect. We show that horizontal fluctuations of the thermodynamical conditions of stellar atmospheres can have a significant impact on the linear polarization of the emergent spectral line radiation and its center-to-limb variation.

Radiation dynamics of low-energy laser plasma of CuSbSe2 polycrystalline

Oscillograms of the radiation of spectral lines of antimony and copper atoms from low-energy laser plasma of the polycrystalline CuSbSe2-based target at the distance of 1 mm from the surface are presented. The ion recombination time and the velocity of the laser plasma propagation are determined. The features of the time evolution of the radiation intensity of the spectral lines are analyzed.

NON-COHERENT CONTINUUM SCATTERING AS A LINE POLARIZATION MECHANISM

Line scattering polarization can be strongly affected by Rayleigh scattering by neutral hydrogen and Thompson scattering by free electrons. Often a continuum depolarization results, but the Doppler redistribution produced by the continuum scatterers, which are light (hence, fast), induces more complex interactions between the polarization in spectral lines and in the continuum. Here we formulate and solve the radiative transfer problem of scattering line polarization with non coherent continumm scattering consistently. The problem is formulated within the spherical tensor representation of atomic and light polarization. The numerical method of solution is a generalization of the Accelerated Lambda Iteration that is applied to both, the atomic system and the radiation field. We show that the redistribution of the spectral line radiation due to the non coherence of the continuum scattering may modify significantly the shape of the emergent fractional linear polarization patterns, even yielding polarization signals above the continuum level in intrinsically unpolarizable lines.

PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers

The interpretation of the intensity and polarization of the spectral line radiation produced in the atmosphere of the Sun and of other stars requires solving a radiative transfer problem that can be very complex, especially when the main interest lies in modeling the spectral line polarization produced by scattering processes and the Hanle and Zeeman effects. One of the difficulties is that the plasma of a stellar atmosphere can be highly inhomogeneous and dynamic, which implies the need to solve the non-equilibrium problem of the generation and transfer of polarized radiation in realistic three-dimensional (3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel radiative transfer code we have developed for the simulation of the spectral line polarization caused by scattering processes and the Hanle and Zeeman effects in 3D models of stellar atmospheres. The numerical method of solution is based on the non-linear multigrid iterative method and on a novel short-characteristics formal solver of the Stokes-vector transfer equation which uses monotonic B\'ezier interpolation. Therefore, with PORTA the computing time needed to obtain at each spatial grid point the self-consistent values of the atomic density matrix (which quantifies the excitation state of the atomic system) scales linearly with the total number of grid points. Another crucial feature of PORTA is its parallelization strategy, which allows us to speed up the numerical solution of complicated 3D problems by several orders of magnitude with respect to sequential radiative transfer approaches, given its excellent linear scaling with the number of available processors. The PORTA code can also be conveniently applied to solve the simpler 3D radiative transfer problem of unpolarized radiation in multilevel systems.

Microwave radiation generated by a high-power air explosion

The special features of the generation of microwave radiation by a high-power air explosion accompanied by ionizing radiation are discussed. Coherent pulse caused by asymmetry of the ejection of gamma quanta from a source, incoherent bremsstrahlung of a partially ionized plasma, incoherent radiation of the shock wave front, and radiation in spectral lines lying inside air transparency windows are considered.