Application of Spectroscopic Diagnostics to Early Observations with the SOHO Coronal Diagnostic Spectrometer

The Coronal Diagnostic Spectrometer (CDS) has as a scientific goal the determination of the physical parameters of the solar plasma using spectroscopic diagnostic techniques. Absolute intensities and intensity ratios of the EUV spectral emission lines can be used to obtain information on the electron density and temperature structure, element abundances, and dynamic nature of different features in the solar atmosphere. To ensure that these techniques are accurate it is necessary to interface solar analysis programs with the best available atomic data calculations. Progress is reported on this work in relation to CDS observations.     

将来最准确最稳定的钟——空间微重力钟

介绍了目前最准确的原子钟,描述了将来最准确、最稳定的钟——空间微重力钟,以及高精度空间组合钟的预期性能,还介绍了一些正在实施的高精度空间钟计划。     

Non-LTE sodium abundance in galactic thick- and thin-disk red giants

The non-LTE sodium abundance has been determined from the Na I 6154 and 6161 Å lines for 38 thin-disk stars (15 of them are Ba II stars), 15 thick-disk stars, 13 Hercules-stream stars, and 13 stars that cannot be attributed neither to the thick Galactic disk nor to the thin one. The Na I model atom has been constructed using the most accurate present-day atomic data. For the Na I 6154 and 6161 Å lines, the non-LTEabundance corrections are from ?0.06 to ?0.24 dex, depending on the stellar parameters. No differences in [Na/Fe] abundance between the thick and thin disks have been detected; the derived ratios are close to the solar ones. The existence of a [Na/Fe] overabundance in the Ba II stars has been confirmed. The Hercules-stream stars exhibit nearly solar [Na/Fe] ratios. The results obtained can be used to test the sodium nucleosynthesis models.     

The Xray-EUV spectrum of optically thin plasmas

The new observations of the Extreme Ultraviolet Explorer Spectrometers on EUVE and the future high resolution observation by Coronal Diagnostic Spectrometer (CDS) and SUMER on SOHO have suggested the revision of the Xray-EUV spectral code of the authors (1990) for optically thin plasmas. More accurate atomic data computations are now available and several lines have been added. Work is in progress to update the Xray-EUV code following the suggestions of the reviewers of the workshop on Atomic data assessment for SOHO held in Abingdon (March 1992). Special care is given to the highly ionized iron lines in the EUV spectral region.     

Diagnostics for regions of formation of atomic lines in stellar atmospheres

We present a procedure to make direct diagnostics of the physical conditions of the regions of formation of the atomic spectral lines in stellar atmospheres using the atomic line widths at half maximum and the number of lines visible of a given atomic series in the observed stellar spectra. This is accomplished using the theoretical widths at half maximum of the atomic lines induced by the broadening produced by thermal energy fluctuations and considering the maximum number of levels that exist in those atoms under the physical conditions of the given system. The procedure is easy to use in any application. As an example we apply the procedure to the observed Lyman lines of hydrogen in the ultraviolet of some stars.     

Empirical NLTE analyses of solar spectral lines

A method is suggested for empirical NLTE analyses of solar spectral lines. Special depthdependent departure coefficients (x) are introduced and the formulas are given for further application. From test calculations it is shown that the separation of the departure coefficients for the upper and the lower level from each other and from the uncertainties in several input parameters (oscillator strength times abundance, turbulent velocities and damping constant) is greatly facilitated when spectral lines are analysed on the disk, around the limb, as well as in the flash spectrum. Therefore it is necesssary that all accessible line data are used, from accurate line profiles to equivalent widths or integrated intensities. To reduce the number of independent variables the analysis should include many multiplets between a few spectroscopic terms.We discuss the advantages of translating the departure coefficients into a set of temperatures. Some published LTE and NLTE analyses are compared in terms of these temperatures. The results do not show inexplicable contradictions. For several atomic spectra the departures from LTE seem large enough to be established quantitatively by the detailed method suggested in this paper.     

Influence of Inelastic Collisions with Hydrogen Atoms on Non-LTE Oxygen Abundance Determinations

We present the results of our modeling of the O I line formation under non-LTE conditions in the atmospheres of FG stars. The statistical equilibrium of O I has been calculated using Barklem’s quantum-mechanical rates of inelastic collisions with hydrogen atoms. We have determined the non-LTE oxygen abundance from atomic O I lines for the Sun and 46 FG stars in a wide metallicity range, ?2.6 < [Fe/H] < 0.2. The application of accurate atomic data has led to an increase in the departures from LTE and a decrease in the oxygen abundance compared to the use of Drawin’s theoretical approximation. The change in the non-LTE abundance from the infrared O I 7771-5 Å triplet lines is 0.11 dex for solar atmospheric parameters and diminishes in absolute value with decreasing metallicity. We have revised the [O/Fe]–[Fe/H] relationship derived by us previously. The change in [O/Fe] is small in the [Fe/H] range from ?1.5 to 0.2. For stars with [Fe/H] < ?1 the [O/Fe] ratio has increased so that [O/Fe] = 0.60 at [Fe/H] = ?0.8 and rises to [O/Fe] = 0.75 at [Fe/H] = ?2.6.     

Comments on the observability of coronal variations

We discuss the observable variability of spectral lines in the soft X-ray and XUV region. Rapid variability of coronal emission, both in flaring and non-flaring structures has been reported and is particularly prominent when high spatial resolution is available. Examination of the ionization and recombination time-scales for the formation and removal of ions with prominent solar emission lines shows that, even though ionization equilibrium generally prevails, the observable variability time-scales are often limited by these atomic processes, independent of the physical process which is causing the change in the solar atmosphere. Rapid heating can lead to an initial freezing-in of abundances of some ions; observations of at least one low- and one high-excitation line from such an ion would permit studies of the time evolution of the emission measure and temperature. In a very limited number of cases, rapid cooling leads to freezing-in of the abundance of an ion and observations of a low-excitation line of this ion will not yield accurate information about the thermal evolution. Thus, future observations of Mgx 609 Å should be augmented by simultaneous observation at another wavelength, such as 63 Å. In addition, with the ability to produce images in isolated spectral lines it becomes possible to select those for which rapid variability is observable, such asOvii, rather than lines which were selected on the basis of previous hardware constraints, such asOvii.     

Atomic line broadening by thermal energy fluctuations in stellar atmospheres and plasma diagnostics

A new method for finding the line widths of atomic lines produced by thermal energy fluctuations in a gaseous system is developed assuming that the atomic linear density change in the energy of the levels is equal to the linear density of the energy fluctuations per degree of freedom. A formula is derived for the atomic line widths that depends on temperature, the cubic root of the total number density of particles in the system, and on the sum of the squares of the principal quantum numbers of the states that participate in the transitions that produce the lines. The calculated widths agree well with the published experimental and theoretical values. This formula will be useful for directly diagnosing the physical state of stellar atmospheres and plasmas.     

Collisional broadening and shift of the alkali resonance lines

We use the 6-8-12 interatomic potential, with a suitable scaling for the relevant atomic radii, to reproduce the measured broadening and shift of the alkali resonance lines perturbed by noble gases, at temperatures of 500 K.Then, using the fit to the data with helium and neon as perturbers, we compute new values for the broadening and shift of the alkali resonance lines due to atomic hydrogen, at the temperature of 5000 K, which are of interest in the analysis of solar and stellar spectra, and discuss the reliability of these results.     

The molecular spectrum of sunspots

The sunspot spectrum shows many molecular lines, of which some have been identified. These lines are formed farther out in the sunspot atmosphere than the atomic lines or the continuum, and are thus useful for probing the outer layers. Photographic spectrograms were obtained for three different sunspots under carefully selected seeing conditions, showing molecular absorption lines due to MgH, CaH, and TiO. Analysis led to estimates of the effective rotational temperatures of each; in the case of CaH, no definite conclusions could be drawn. Predicted rotational temperatures and observational f-values were calculated on the basis of model umbral atmospheres due to several authors. A new model was derived from the molecular lines measured here, and shown to differ widely from previous models. The usefulness of photographic spectra for this purpose is seriously questioned, and suggestions are made for new observations.     

On the effective temperature of large sunspot umbra using CrH sextets electronic state molecular lines

A high-resolution spectrum clearly shows the presence of A ⁶Σ⁺–X ⁶Σ⁺(0,0;0,1;1,0;2,0;2,1) and (2,2) bands system of CrH molecular lines in the spectral range 10,000 cm⁻¹ to 14,050 cm⁻¹. At least 3928 lines of the six bands, accounting for 57% of the lines registered in the laboratory can be identified with certainty in the sunspot spectrum. Most of the lines are found blended with TiO, CaH, MgH and other atomic species. These molecular lines are typically much more temperature sensitive than atomic lines, which make them ideal, complementary tool for studying cool stellar atmospheres as well as the internal structure of sunspots. Equivalent width for an adequate number of well identified molecular lines of these bands using the Gaussian-profile approximation method versus rotational quantum number J has been used to determine the effective rotational temperature of the CrH molecule. The range of effective rotational temperature value obtained from these bands is 1766 K to 2442 K. This range agrees well with the effective rotational temperatures derived for other molecules in sunspot umbrae.     

Hyperfine structure and interstellar curves of growth

The effects of the hyperfine structure (HFS) that is present in some interstellar absorption lines are investigated in the case of a single absorbing cloud. If the respective total equivalent widths of two or more unresolved HFS multiplets measured in relatively low-resolution spectra are analysed specifically by means of a curve of growth, the column density N (X) and the linewidth parameter b (X) inferred for absorbing species X in the cloud will generally be in error if the HFS is ignored. The fundamental physical effect is the reduced line saturation that arises because the total column density is divided among the HFS levels of the ground atomic level, each of which generally gives rise to an HFS line at a different wavelength. For nuclear spins   I = 3/2  and   I = 5/2  , theoretical curves of growth are calculated for some of the resonance lines of some alkali atoms, for each of four illustrative choices of the parameter  α=Δ/ b   , the ratio of the HFS splitting in the ground atomic level to the linewidth. Applications of the results to interstellar absorption by Na  i , K  i and Al  iii are emphasized. HFS is, fortunately, unimportant for most interstellar lines, however. Among the 35 elements that have been detected in diffuse clouds via interstellar absorption in the ultraviolet/optical spectral region, the most abundant isotope of each of 25 of these shows no HFS, because   I = 0  or, in the relevant ground atomic level,   J = 0  .     

Approximating the X‐ray spectrum emitted from astrophysical charge exchange

Charge exchange (CX), both onto ions in the solar wind and potentially in other astrophysical contexts, can create X‐ray emission lines largely indistinguishable from those created in collisional or photoionized plasmas. The prime distinguishing characteristic is in the distinctly different line ratios generated by the CX process. A complete astrophysical model of the process would require a vast number of atomic calculations; we describe here an approximate approach that will allow astronomers to evaluate the likely contribution of CX to an observed spectrum. The method relies upon an approximate calculation of the CX cross section paired with detailed atomic structure calculations used to determine the emission lines. Simulated spectra based on observed solar wind CX data are shown for both current (Suzaku) and near‐term (Astro‐H) missions (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Line formation in magnetic fields

Hyder (1968) has suggested that longitudinal magnetograph measurements of weak magnetic fields in prominences have underestimated field strengths as the result of zero-field levelcrossing interference (the Hanle effect). Hyder (1968) also suggested that resonance polarization effects have sometimes led to errors in measurements of the transverse component of magnetic fields. Stenflo (1969) has pointed out some errors in Hyder's paper, while contending that the Hanle effect is implicitly included in current theories of line formation in the presence of Zeeman splitting.In the present Note these questions are re-examined using the results of a density matrix treatment of absorption, emission, and scattering processes. The basic conclusions are as follows: (1) Longitudinal magnetograph measurements using optically thin lines are not influenced by the Hanle effect. (2) Although present theories of line formation in magnetic fields do not include the Hanle effect, this omission is generally unimportant for lines formed in the photosphere and lower chromosphere due to rapid collisional depolarization of atomic levels. (3) For the same reason, other resonance polarization effects are probably too small to cause significant errors in magnetograph measurements of all but the very weakest magnetic fields, when photospheric and lower chromospheric lines are used. (4) By contrast, the general phenomenon of atomic level polarization is quite important in most prominences. As emphasized by Hyder, extreme care must be used in selecting lines for magnetograph studies of solar magnetic fields.     

Real-time Solution of Atomic Clock Operating Parameters Based on Adaptive Kalman Filtering

The real-tine and accurate knowledge of the operation of an atomic clock is of great significance to satellite navigation and time keeping. In the light of the problems which exist in the real-time solution of the atomic clock operating parameters by means of the standard Kalman filtering, the method for applying the adaptive Kalman filtering model based on the prediction residual to the real-time solution of the clock parameters is proposed. An adaptive factor is constructed by the model based on the standard Kalman filtering model so that the accurate and reliable estimates of the operating parameters may be given rather fast after the frequency modulation or phase modulation of the atomic clock. Finally, in combination with the 3 computational examples of the work on the navigation and time keeping, the effectiveness of the method is verified.     

Recombination of helium-like ions – I. Photoionization cross-sections and total recombination and cooling coefficients for atomic helium

A new calculation of photoionization cross-sections is described for the ground and excited states of atomic helium up to principal quantum number n =25 and angular momentum quantum number l =5. These cross-sections are used to calculate total recombination and cooling coefficients for atomic helium for electron temperatures given by log( T )=1(0.2)4.4. A comparison of the threshold photoionization cross-sections obtained here with extrapolations of the highly accurate bound–bound oscillator strength calculations by Drake shows that the new calculations are in error by no more than 1 per cent. The accuracy of the photoionization cross-sections used by previous workers to derive recombination coefficients is also discussed.     

The N ii spectrum of the Orion nebula

The predicted emission spectrum of N  ii is compared with observations of permitted lines in the Orion nebula. Conventional nebular models show that the intensities of the more intense lines can be explained by fluorescence of starlight absorption with a N abundance that is consistent with forbidden lines. Lines excited mostly by recombination, on the other hand, predict high N abundances. The effects of stellar and nebular parameters and of the atomic data on the predicted intensities are examined.     

Global atomic oxygen density derived from OGO-6 1304 Å airglow measurements

The OGO-6 UV photometer experiment measured the atomic oxygen OI 1304 Å triplet in the Earth's dayglow between 400 and 1100 km. We have analyzed the data for the period 15 September–25 October 1969 by obtaining best-fit models in which the 1304 Å emission is excited by solar resonance scattering and photoelectron excitation. Provided the excitation processes are specified, we find a unique relationship between the vertical column density of atomic oxygen and the zenith 1304 Å intensity. This is essentially independent of the atmospheric temperature. Because of the large numerical uncertainties, the excitation sources are determined from the 1304 Å data and quiet-time in situ measurements of atomic oxygen density. They are found to be in good agreement with recent solar measurements of the 1304 Å lines and with calculations of the photoelectron excitation source. The deduced variations of atomic oxygen column densities over the daytime atmosphere are found to agree well with the Jacchia 1971 models. During the geomagnetic storm, the column density generally increased above a fixed altitude. However, the latitudinal dependence is complex. Following the strong geomagnetic activity between 15 September and 1 October, depletions in atomic oxygen are observed. At times, there is evidence of high-altitude transport of atomic oxygen from high latitude to low latitude.