Non-local Thermodynamic Equilibrium Effects Research Articles

Fast forward radiative transfer modeling of 4.3 μm nonlocal thermodynamic equilibrium effects for infrared temperature sounders

Retrieval algorithms for downlooking infrared sounders typically avoid using channels from the 4.3 μm CO2 region that probe the mid‐ and upper‐atmosphere due to very high altitude Non Local Thermodynamic Equilibrium (NLTE) emission, which can add as much as 10 K to the measured daytime brightness temperatures (BT). In this paper we report a fast radiative transfer model for a nadir sounding instrument (AIRS) that includes the effects of NLTE, allowing the retrieval algorithm to use many short wave CO2 channels for upper‐air soundings. Model biases and standard deviations are very similar for both day and night. This work allows an infrared sounder to probe the upper atmosphere much more completely using only short wave 4.3–4.5 μm channels.

A non-LTE study of neutral and singly-ionized calcium in late-type stars

Aims. Non-local thermodynamical equilibrium (NLTE) line formation for neutral and singly-ionized calcium is considered through a range of spectral types when the Ca abundance varies from the solar value down to [Ca/H] = -5. We evaluate the influence of departures from LTE on Ca abundance determinations and inspect the possibility of using Cal / Ca II line-strength ratios as indicators of surface gravity for extremely metal-poor stars. Methods. A comprehensive model atom for Cal and Call is presented. Accurate radiative and electron collisional atomic data are incorporated. The role of inelastic collisions with hydrogen atoms in the statistical equilibrium of Ca I/II is estimated empirically from inspection of their different influences on the Cal and Call lines in selected stars with well determined stellar parameters and high-quality observed spectra. Results. The dependence of NLTE effects on the atmospheric parameters is discussed. Departures from LTE significantly affect the profiles of Cal lines over the whole range of stellar parameters being considered. However, at [Ca/H] ≥ -2, NLTE abundance correction of individual lines have a low absolute value due to the different influence of NLTE effects on line wings and the line core. At lower Ca abundances, NLTE leads to systematically depleted total absorption in the line and positive abundance corrections, exceeding +0.5 dex for Cal λ4226 at [Ca/H] = -4.9. In contrast, the NLTE effects strengthen the Call lines and lead to negative abundance corrections. NLTE corrections are small, <0.02 dex, for the Call resonance lines, and they grow in absolute value with decreasing Ca abundance for the IR lines of multiplet 3d-4p, exceeding 0.4 dex in the metal-poor models with [Fe/H] < -3. As a test and first application of the Ca I/II model atom, Ca abundances are determined on the basis of plane-parallel LTE model atmospheres for the Sun, Procyon (F IV-V), and seven metal-poor stars, using high S/N and high-resolution spectra at visual and near-IR wavelengths. Lines of Ca I and Ca II give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from Cal and Call lines) is log e Ca.⊙ = 6.38 ± 0.06. For Procyon, the mean Ca abundance from Cal lines is markedly subsolar, [Ca/H] = -0.14 ± 0.03. All metal-poor stars within our sample show an overabundance of calcium relative to iron with [Ca/Fe] abundance ratios of 0.26 to 0.46 that are typical of the halo population. The W(Ca I4226) / W(Ca II8498) equivalent width ratio is predicted to be sensitive to surface gravity for extremely metal-poor stars, while this is not the case for the ratio involving the Ca II resonance line(s).

Modeling of EUV emission from xenon and tin plasma sources for nanolithography

Over the last decade there has been a major effort devoted to the development of efficient extreme UV sources designed for nanolithography, operating in the 13.5-nm range. Possible sources include laser-produced plasmas and discharge-produced plasmas. This paper, devoted to the modeling of such emission, emphasizes the atomic physics effects and particularly the effects of configuration interaction. Two types of theoretical approaches are presented, one involving the detailed computation with the parametric potential code H ullac, the other based on the superconfiguration code SCO. Computations of emission spectra in xenon and tin are presented. The possible influence of non-local thermodynamic equilibrium (NLTE) effects is investigated using populations given by the simple collisional-radiative formulas from Colombant and Tonon. Convergence to LTE is analyzed in the tin case.

Retrieval of stratospheric NOx from 5.3 and 6.2 μm nonlocal thermodynamic equilibrium emissions measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat

We present the first global observations of stratospheric NOx(= NO + NO2) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat during 24 July, 18 to 27 September, and 11 to 13 October 2002. Volume mixing ratio profiles of both NOx species were derived from MIPAS limb emission spectra by means of an innovative retrieval scheme under consideration of nonlocal thermodynamic equilibrium (non‐LTE) effects. In the quiescent atmosphere, the estimated accuracy of retrieved NOx at the altitude of its stratospheric mixing ratio maximum at 35–40 km is around 1–2 ppbv, and the vertical resolution is around 3.5–6.5 km at altitudes between 20 and 50 km. In order to correctly consider NO2 non‐LTE effects in the retrievals, the photochemical excitation rate of NO2(v3 &gt; 0) vibrational states was derived from NO2(002→001) emissions and was found to be about 50 times smaller than previously estimated from Limb Infrared Monitor of the Stratosphere (LIMS) measurements. The NOx partitioning of the retrieved data is in excellent agreement with steady state photochemistry, which confirms predicted stratospheric NO(v &gt; 0) non‐LTE population enhancements. The retrieved NOx abundances are also consistent with Halogen Occultation Experiment (HALOE) NOx observations.

Effects of non-local thermodynamic equilibrium conditions on numerical simulations of inertial confinement fusion plasmas

Effects of non-local thermodynamic equilibrium (non-LTE) condition on emission and hydrodynamics of typical inertial confinement fusion (ICF) plasmas are studied. The average degree of ionization at high temperatures is seen to be much lower compared to the values obtained from Thomas-Fermi scaling or Saha equation for high-Z element like gold. LTE and non-LTE predictions for emitted radiation from laser-driven gold foil are compared with the experimental results and it is seen that non-LTE simulations show a marked improvement over LTE results. The effects of one group and multigroup, LTE and non-LTE approximations of radiation transport on hydrodynamic parameters are studied for laser-driven aluminium and gold foils. It is further seen that non-LTE and multigroup effects play an important role in predicting conversion efficiency of laser light to X-rays.

Sensitivity of trace gas abundances retrievals from infrared limb emission spectra to simplifying approximations in radiative transfer modelling

The accuracy of trace gas abundances retrieved from spectrally resolved infrared limb emission measurements relies, among other things, on the appropriate modelling of radiative transfer through the actual atmosphere. We quantify the mapping of several commonly applied simplifications in radiative transfer modelling on the trace gas abundances retrieval error at the example of the Michelson interferometer for passive atmospheric sounding (MIPAS)/environmental satellite (ENVISAT) space borne Fourier transform infra-red limb emission experiment. The Karlsruhe optimized and precise radiative transfer algorithm (KOPRA) which was used as a tool for this study will be introduced. KOPRA supports accurate modelling of the particular instrument requirements of MIPAS and the observation scenarios during the ENVISAT mission, in particular with respect to its viewing direction and its altitude coverage of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficient accuracy in modelling of field-of-view and apodisation effects, and disregarding individual profiles of isotopic species play the key roles in radiative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noise.

The Non-local Thermodynamical Equilibrium Effects on Opacity**The project supported by the Science and Technology Funds of China Academy of Engineering Physics (990221)

Based on the detailed configuration accounting (DCA) model, a method is developed to include the resonant photoionization and the excitation-autoionization in the non-local thermodynamical equilibrium (NLTE) average atom (AA) model. Using this new model, the mean charge states and the opacity are calculated for NLTE high- plasmas and compared with other results. The agreement with AA model is poor at low electron density. The present results agree well with those of DCA model within 10%. The calculations show that the NLTE effects on opacity are strong.

Massive Multispecies, Multilevel Non‐LTE Model Atmospheres for Novae in Outburst

We have used our PHOENIX multipurpose model atmosphere code to calculate atmospheric models that represent novae in the optically thick wind phases of their outburst. We have improved the treatment of non-local thermodynamic equilibrium (NLTE) effects by expanding the number of elements that are included in the calculations from 15 to 19 and the number of ionization stages from 36 to 87. The code can now treat a total of 10,713 levels and 102,646 lines in NLTE. Al, P, K, and Ni are included for the first time in the NLTE treatment, and most elements now have at least the lowest six ionization stages included in the NLTE calculation. We have investigated the effects of expanded NLTE treatment on the chemical concentration of astrophysically significant species in the atmosphere, the equilibrium structure of the atmosphere, and the emergent flux distribution. Although we have found general qualitative agreement with previous, more limited NLTE models, the expanded NLTE treatment leads to significantly different values for the size of many of the NLTE deviations. In particular, for the hottest model presented here (Teff = 35,000 K), for which NLTE effects are largest, we find that the expanded NLTE treatment reduces the NLTE effects for these important variables: H I concentration, pressure structure, and emergent far-UV flux. Moreover, we find that the addition of new NLTE species may greatly affect the concentration of species that were already treated in NLTE, so that, generally, all species that contribute significantly to the e- reservoir or to the total opacity, or whose line spectrum overlaps or interlocks with that of a species of interest, must be treated in NLTE to ensure an accurate result for any particular species.

Formation of L[CLC]i[/CLC] [CSC]i[/CSC] Lines in Photospheric Granulation

The possibility of significant systematic errors due to the use of one-dimensional homogeneous atmospheres in lithium-abundance determinations of cool stars motivates a study of non-local thermodynamic equilibrium (NLTE) effects on Li I line formation in a three-dimensional solar granulation simulation snapshot. The NLTE effect on the equivalent width of the 671 nm resonance line is small in one-dimensional models or in integrated light from the granulation model. The line-strength variations over the granulation pattern are, however, markedly different in NLTE compared with LTE—observations of this may provide diagnostics to NLTE effects. The effects of horizontal photon exchange found in the granulation model are moderate and due entirely to bound-bound processes; ultraviolet overionization is unimportant.

Non-local thermodynamic equilibrium effects in modelling of Supernovae near maximum light

Non-local thermodynamic equilibrium effects in modelling of Supernovae near maximum light

Non-LTE effects on the strength of the Lyman edge in quasar accretion disks

We have calculated UV/EUV (300 A which is less than or equal to lambda which is less than or equal to 1500 A) continuous energy distributions of accretion disks in the centers of active galactic nuclei (AGNs) for disk luminosities in the range 0.1 L(sub Edd) less than or equal to L(sub acc) less than 1.0 L(sub Edd) and central masses ranging from 10(exp 8) solar mass to 10(exp 9) solar mass. The vertical gas pressure structure of the disk and the disk height are obtained analytically; the temperature stratification and the resulting continuum radiation fields are calculated numerically. We have included non-Local Thermodynamic Equilibrium (LTE) effects of both the ionization equilibrium and the level populations of hydrogen and helium. We show that these non-LTE effects reduce the strength of the Lyman edge when comapred to the LTE case. In non-LTE we find that the edge can be weakly in emission or absorption for disks seen face-on, depending on the disk parameters.

Ground‐based microwave observations of ozone in the upper stratosphere and mesosphere

A 9‐month‐long series of measurements of ozone in the upper stratosphere and mesosphere is reported. The measurements are presented as monthly averages of profiles in blocks of roughly 20 min local time and as night‐to‐day ratios. An error analysis predicts accuracies of 5–26% for the monthly profiles and 2.5–9% for the ratios. The data are compared to historical data from SME and limb infrared monitor of the stratosphere (LIMS), and it is shown how to remove the effect of different vertical resolution from the comparisons. The microwave data typically agree to better than 10% with SME and nighttime LIMS ozone at all altitudes below the 0.1‐mbar surface. Comparison of the microwave night‐to‐day ratio with the corresponding ratio from LIMS suggests that nonlocal thermodynamic equilibrium effects in the LIMS daytime data exceed 10% at all pressures less than or equal to 1 mbar.

Effects of Non-Local Thermodynamic Equilibrium (NLTE) on Molecular Opacities

Molecules increase in importance in stellar atmospheres as the temperature falls along the spectral sequence for all luminosity classes. Among their many roles molecular features provide the basis for spectral classification (temperature and chemical composition) in M, S, and C stars; they constitute important opacity sources across the spectrum from the ultraviolet to the infrared; they influence the energy budget and hence the kinetic temperature in the outer photospheric layers; they may contribute significantly to the radiative levitation of the atmosphere; they form masers; they may cause or facilitate mass loss; and they are indicative of physical conditions and processes from the photosphere to the outer circumstellar envelope.

Nonlocal thermodynamic equilibrium effects of vibrationally excited NO

The vibrational distribution of NO(X 2π) from the reaction O(3P)+NO2(X 2A1)→NO(X 2π,v″)+O2 was studied at room temperature in a fast-flow system. Oxygen atoms were produced via the microwave-discharge dissociation of molecular oxygen with argon as the carrier gas. The reaction time between the NO2 molecules and the oxygen atoms was kept at ∼3 ms to minimize vibrational energy quenching. NO(X) radicals were detected via laser-induced fluorescence of the A 2Σ+←X 2π electronic transition. The relative vibrational population ratio of NO in the ground electronic state was determined to be 87.4±2.6% and 12.6±2.7% in the v″=0 and v″=1 levels, respectively. No higher vibrational states were observed. The NO vibrational population ratio measured in this study correlates well with the earlier observation of high vibrational excitation in O2.

Middle and upper atmosphere pressure‐temperature profiles and the abundances of CO2 and CO in the upper atmosphere from ATMOS/Spacelab 3 observations

Based on a new approach, profiles of kinetic temperature and atmospheric pressure between 20 and 116 km altitude and CO2 and CO volume mixing ratios between 70 and 116 km have been derived from the ∼0.01‐cm−1 resolution infrared solar occultation spectra recorded by the atmospheric trace molecular spectroscopy (ATMOS) Fourier transform spectrometer during the Spacelab 3 shuttle mission (April 29 to May 6, 1985). The physical model and CO2 profile results have been obtained from simultaneous multiscan least squares fitting of microwindows containing CO2 lines with temperature‐dependent and temperature‐independent intensities and N2 lines with temperature‐independent intensities. The analysis included the retrieval of tangent height separations between spectra in the lower stratosphere, where independent information shows drifts of the sun tracker field of view on the solar disk. The physical model results are compared with climatological profiles, lower thermospheric temperatures computed with the MSISE‐90 model, and other data. The CO2 retrievals indicate a nearly constant volume mixing ratio of 320 ± 35 ppmv (parts per million, 10−6, by volume) in the 70‐to 90‐km altitude region with a rapid decline in the CO2 volume mixing ratio beginning between 90 and 100 km; at 116 km the CO2 volume mixing ratio has declined to about 70 ppmv and is equal to the CO volume mixing ratio, which has been derived from fits to the (1‐0) CO vibration‐rotation band. The absorption by the v2 + v3 ‐ v2 band of CO2 has been analyzed to quantify nonlocal thermodynamic equilibrium (non‐LTE) effects in the upper mesosphere and lower thermosphere. Up to an altitude of 100 km, the results indicate that the vibrational temperature of the v2 state of CO2 is within 5 K of the kinetic temperature. At higher altitudes, non‐LTE effects are observable with the vibrational temperature of the v2 state cooler than the kinetic temperature by 40 K (48°S) and 70 K (30°N) at 112 km. Above 112 km, lines of the v2 + v3 ‐ v2 band are too weak to be observed in the ATMOS spectra.

Nonlocal thermodynamic equilibrium effects in stratospheric HF by collisional energy transfer from electronically excited O_2 and implications for infrared remote sensing

A possible nonlocal thermodynamic equilibrium (non-LTE) effect involving stratospheric HF arising from the direct photochemical excitation of vibrationally excited HF by collisional energy transfer from electronically excited O(2) [O(2)((1)Delta), O(2)((1)Sigma)] is presented. Although this non-LTE effect is smaller than one associated with the direct solar excitation of both HF(nu= 1) and HF(nu= 2), calculations show that inclusion of the mechanism proposed here into retrieval algorithms is necessary if correct daytime upper stratosphere HF profiles are to be inferred in future infrared thermal emission measurements. Accurate determinations of the state-to-state rate constants of the reactions involved in these excitation processes and of the quenching rates of vibrationally excited HF are needed if IR thermal emission techniques are to be accurately employed in the measurement of stratospheric HF.

Vertical, meridional, seasonal, and local time dependence of non-LTE effects in stratospheric NO and implications for infrared remote sensing

Further calculations have been carried out on possible non-local thermodynamic equilibrium (non-LTE) effects previously suggested for stratospheric nitric oxide (NO) associated with the direct photochemical production of vibrationally excited NO by the processes NO 2 + hν → NO(ν) + O O + NO 2 → NO(ν) + O 2 . The new calculations, which make use of improved calculations of the NO vibrational state distribution from NO 2 photolysis, are carried out as a function of altitude and latitude for a variety of seasons and local times. Non-LTE effects on the order of 30% for ν = 1 are obtained, maximizing in the middle stratosphere over the equator. For ν = 2, enhancements in the population by a factor of 3000 over that expected in LTE are obtained. The calculated enhancements are functions of latitude for both ν = 1 and 2. For ν = 1 there is only very small seasonal and local time variation; larger variations were obtained for ν = 2. The results of these calculations suggest that incorporation of this non-LTE effect into the retrieval algorithm for NO from infrared thermal emission measuring instruments on the Upper Atmosphere Research Satellite will need to be done carefully if correct distributions and variations of NO with altitude, latitude, season, and local time are to be obtained. The sensitivity of the calculated non-LTE effects to various assumed parameters, especially of the rate of quenching of NO by O 2, is reviewed.

Nonlocal thermodynamic equilibrium effects in stratospheric NO and implications for infrared remote sensing

It is shown that the vibrational state population of stratospheric nitric oxide (NO) could be substantially different from that expected on the basis of local thermodynamic equilibrium (LTE). Deviations from LTE may arise because stratospheric NO can be photochemically produced from NO(2) with several vibrational quanta. Model calculations suggest that the population of NO(upsilon = l) could be some 30% above that expected from LTE at 30 km with smaller enhancements above and below. Substantially larger enhancements are predicted for NO(upsilon = 2). This result is shown to have important implications for NO determination by remote sensing of IR emission. Data needed for the quantification of these effects are enumerated.

The aluminum I autoionization doublet in the quiet solar spectrum

Observations are presented of the Al I autoionization doublet 1932 A and 1936 A in the quiet solar spectrum, obtained from the NRL slit spectrograph aboard Skylab and from the University of Hawaii Echelle Rocket Spectrograph. The observed profiles are compared with theoretical spectra computed for the Harvard Smithsonian Reference Atmosphere and the Vernazza, Avrett and Loeser (1976) solar models. It is found that nonlocal thermodynamic equilibrium effects are important in the line-formation problem and the synthetic spectra are in good agreeement with the data.

On the formation of Fe II lines in stellar spectra. I - Solar spatial intensity variation of lambda 3969.4

We employ high-spatial-resolution solar observations of the weak Fe II lambda3969.4 line to study non-local thermodynamic equilibrium (NLTE) effects in Fe II line formation. This line is superposed onthe wing of the Ca II H line, which raises its height of formation. The line shows extraordinary spatial intensity variations, including emission features whose contrast increases toward the limb. Observed profiles of the Fe II resonance lines in he UV are used to define toward the limb. Observed profiles of the Fe II resonance lines in the UV are used to define formation parameters in a 15-level atomic model computation, which shows that Fe II subordinate lines are generally formed out of local thermodynamic equilibrium (LTE) as a result of pumping by UV line-wing photons from the deep photosphere. For the lambda3969.4 line, this pumping results in large sensitivity to the atmospheric structure in layers deeper than the layer of formation of the H-wing background intenisty. We discuss the absence of intense emission cores in the Fe II resonance lines, the effects of partially coherent scattering, and the effects of chromospheric and photosheric inhomogeneities. We find that emission of lambda3969.4 provides a diagnostic of the inhomogeneous structure of the deep photosphere,more » for the Sun and for late-type stars.« less