Somef-values for transitions in ZrII,ZrIII and SrI

Transition probabilities were calculated for certain transitions in ZrII, ZrIII and SrI. For these allowed transitions Hartree-Fock wavefunctions were calculated for use in determining the absolute line strength. The methods developed by C. Froese-Fischer and Mayers and O'Brien for calculating the wavefunctions were used in determining those for the transitions in ZrII, ZrIII and SrI. Theoretical and experimentalf-values were compared in the case of SrI, and the results were in reasonable agreement for transition probabilities based on dipole velocity calculations.     

Oscillator strengths for optically allowed transitions in carbon-like ions

Using configuration interaction wavefunctions, the excitation thresholds for the twelve lowest terms arising from the configurations 1s ²2s ²2p ², 1s ²2s2p ³, and 1s ²2p ⁴ of Mgvii have been calculated. The same wavefunctions have been used to calculate the oscillator strengths for the optically allowed transitions in Mgvii. Combining these results with earlier published values for Oiii, Nev, Siix, Caxv, and Fexxi, oscillator strengths for other ions in the carbon isoelectronic sequence (F through Ni) have been predicted. The predicted values are found to be slightly lower when compared with the available published results.     

Log(gf) for singly-ionized elements of the iron group

Moderately accurate oscillator strengths are obtained by application of systematic corrections to Warner's-log(gf) for the once ionized elements of the iron group. Corrections are derived by comparison of Warner's values with those of Kurucz-Peytremann (Scii, Tiii,Vii, Crii, Mnii, Coii, and Niii) and Kurucz (Feii).K–P andK derived log(gf) by semi-empirical methods for many transitions using scaled Thomas-Fermi-Dirac potentials for the atoms of the elements from B to Ni. Although their individual values may be seriously in error, it has been shown that their mean scales are acceptable, being affected at most by rather small errors. It is known, for example, that when interacting terms are not properly accounted for in the semi-empirical method, very small values are derived for log(gf); these were dropped away in calculating mean corrections as they exhibited gross deviations.The relations loggf(KP)–loggf(W) vs , loggf(W), andE _u (energy of the upper level) have been investigated, and constant corrections, or weak dependences are found.Using Warner's corrected log(gf) for these elements, we have shown in a rather qualitative way that abundances consistent with the values accepted at present are found in the solar photosphere.Research supported in part by the SECYT.     

The forbidden transitions within 3s 2 3p 5 3d of Fe ix and Ni xi and 3s 2 3p 4 3d of Fe x and Ni xii

Improved level values for 3s ² 3p ⁵ 3d and 3s ² 3p ⁴ 3d of Fe ix and Fe x are derived from the observed forbidden lines with special regard to recent accurate measurements in the vacuum-ultraviolet part of the coronal spectrum. A procedure for estimating expected relative intensities is proposed and used for an additional check on the consistency of the identifications, which now comprise 12 lines of Fe IX and 11 lines of Fe x. Finally, by use of a suitable extrapolation technique the wavelengths of corresponding lines in nickel are predicted, which leads to some new identifications and a total of 6 identified lines of Ni xi and 4 of Ni xii.     

Intergalactic medium: X-ray background and compton parameter

A model of intergalactic medium heated by QSOs and cooled by the expansion of the universe and Compton cooling is studied in the framework of a Friedmann-Robertson-Walker universe. Cosmological evolution functions of the comoving density of QSO's as well as the case of no evolution are considered. The theoretical X-ray background spectrum (through thermal bremsstrahlung) and Comptony parameter are calculated including relativistic corrections in the electron-electron, electron-proton and electron-photon interactions. The observed X-ray background and the upper limit of the Compton parametery cobe given by the COBE satellite are used to adjust, for each value of reheating redshiftsz _c ranging from 0.1 to 5.0, the present values of the temperatureT ₀ and densityn ₀ of the intergalactic gas. Forz _c > 0.25, when the theoretical X-ray spectrum fits the observed one, the adjusted values ofT ₀ andn ₀ imply iny >y cobe. On the other hand, whenT ₀ andn ₀ are consistent withy cobe, the calculated X-ray spectrum is lower than the observed one. Unless 100% of the observed X-ray background is due to discrete sources and if the intergalactic medium contributes more than 2.5% to such background we come to the interesting result that the medium must have been heated atz _c < 1. In this case we shall have to explain the high energy rates necessary to heat the intergalactic medium. Forz _c 0.25, it is possible to find values ofT ₀ andn ₀ such that both the calculated X-ray background and the y parameter simultaneously reproduce the corresponding observed values. However, in this case, unless it could be shown to be otherwise by future observations or theoretical studies, it seems that the model of hot intergalactic medium is not plausible because of the high energies required to heat the intergalactic gas.     

The spectrum of the manganese-scandium star ϕ Herculis

Among manganese stars, Herculis is of great interest because of the unusual strength of the scandium lines. We present intensity measurements for numerous lines ofCii, Mgi, Mgii, Siii, Caii, Scii, Tiii, Crii, Mni, Mnii, Fei, Feii, Coii, Srii,Yii, and Zrii.     

Theoretical Neix line ratios compared to solar observations

R-matrix calculations of the 1¹S - 2³S and 1¹S - 2³P electron excitation rates in He - like Cv, Ovii, and Mgxi by Kingston and Tayal are used to interpolate results for Neix. Adoption of these in emission line strength calculations leads to values for the density-sensitiveR ratio very similar to those of Pradhanet al. and Wolfsonet al., although the temperature-sensitiveG ratios are approximately 10 to 20 % lower than those deduced by these authors. However the present theoretical value ofG at the temperature of maximum Neix emission,G(T _m) = 0.82, is in excellent agreement with those observed by the SMM and P78-1 satellites for the 1980, November5 flare (G = 0.83 ± 0.01) and nonflaring active regions (G = 0.80 ± 0.05), respectively.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Use of Al xii and Mg xi lines as solar plasma diagnostics

We present three sets of observations of n = 1 to n = 2 lines due to helium-like aluminium (Alxii), made during two solar flares (25 August, 1980 and 19 October, 1986), using the X-Ray Polychromator on the SMM satellite. The observed temperature-sensitive line ratio G is shown to be consistent with the close-coupling calculations of Keenan and McCann (1987), although the ratio R, which is both temperature and density-sensitive for lower-Z elements, is not sufficiently well determined from these data to say more than that the observed values of R are not inconsistent with the theoretical calculations. This region of the spectrum also includes the helium-like magnesium (Mgxi) 1¹ S - 3¹ P line, and it is shown that the ratio of this line to the Alxii resonance (1¹ S - 2¹ P) line is a more sensitive indicator of electron temperature than are the Alxii G and R ratios. We demonstrate that the three ratios may be used together in order to derive values of emission measure, electron temperature and electron density during these flares.     

The solar abundance of nickel from photospheric [Niii] lines

The detailed study of the possible presence of four [Niii] lines in solar absorption leads to an abundanceA _ni = logN _ni = 6.30±0.30 (in the usual scale where logN _H = 12.00), in agreement with the coronal and meteoritic values.On leave from Institut d'Astrophysique, Université de Liège.     

Emission lines from O IV as a plasma diagnostic

Using several density diagnostic Oiv theoretical line ratios and corresponding observed values for the same source by Sandlin et al. (1984) and Sandlin, Brueckner, and Tousey (1977), we find that an emitting region has a multidensity structure. We discuss several other line ratios for density measurement in sunspots, active regions, and flares.     

The Solar Spectral Irradiance as a Function of the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index for Atmosphere and Climate Modelling

We present a new method to reconstruct the solar spectrum irradiance in the Ly α – 400 nm region, and its variability, based on the Mg ii index and neutron-monitor measurements. Measurements of the solar spectral irradiance available in the literature have been made with different instruments at different times and different spectral ranges. However, climate studies require harmonised data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg ii index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg ii index. The set of coefficients expressing the spectral variability can be applied to the chosen reference spectrum to reconstruct the solar spectra within a given time frame or Mg ii index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades towards Ly α. In a further step, we extend our solar spectral-irradiance reconstruction back to the Maunder Minimum introducing the relationship between the Mg ii index and the neutron-monitor data. Consistent measurements of the Mg ii index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg ii index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this correlation can be applied to the past, we reconstruct the Mg ii index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar spectral-irradiance reconstruction back to that period. As there is no direct measurement of the spectral irradiance for this period we discuss this methodology in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate the solar irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This calibration can be re-assessed at any time, if necessary.     

Iron abundances in G dwarfs

High resolution coudé spectra in the blue and violet spectral regions have been obtained for five G dwarfs. Model atmospheres and spectrum synthesis of selected iron lines have been performed. Thegf values as well as other atomic parameters used in this analysis were previously tested by means of a solar model and the UtrechtSolar Atlas.Revised iron abundances have been derived. Some tentative evidence is presented that these abundances are related with the Wilson-Bappu width of Caii emissions.Based on observations obtained at the Observatoire de Haute Provence     

On the behaviour of the lines 2905 Siii and 2932 Mgii in the spectra of B-A stars

On the basis of theOrion-2 observational data it is shown that the ratio of the intensities of absorption lines 2905 (2904–2906) Siii and 2932 (2928–2936) Mgii varies strongly when we pass from B0 stars to A0. This circumstance may be used as a sensitive indicator for the determination of the effective temperatures of hot stars.     

Calculated profiles of H I lines of interest for solar plasma electric field measurements

We present calculated Stark-polarized line profiles for a number of H i lines observed in the visible and infrared emission spectrum of solar prominences and other limb activity. For use in measurements of possible electric fields in these structures, we also calculate curves giving the difference in line width between the 1/2 (I ± Q) profiles as a function of electric-field intensity. Our calculations take into account magnetic fields in these structures, and incorporate typical observed values of Doppler broadening. These calculations explicitly consider the H i fine structure neglected in previous work, and thus are more accurate in the range of low to intermediate electric-field intensity likely to be encountered in solar plasmas (E < 10³ V cm^–1). Our results enable us to compare behavior when E and B are parallel, or perpendicular. We draw particular attention to the high electric-field sensitivity of the transitions between high levels such as 12–8 and 15–9 in H i, observed in prominences at wavelengths around 11. Their sensitivity is roughly an order of magnitude larger than that of the high Paschen-series lines used in solar plasma electric field studies so far.     

Medium resolution EUV observations and network structure

Medium resolution observations have been used to find the fractional emitting area in three transition region lines. It is found that is given by DI _{mg x} ^k where k varies from 0.78 to 0.51 in the temperature range 2 × 10⁵ to 7 × 10⁵ K. The average emitting area in O vi deduced by this method is in good agreement with the results from ATM observations. The fractional emitting areas at different values of the Mg x intensity and at different temperatures are combined to find the variation of the areas with height. This variation is in good agreement with Giovanelli's model of the fractional area of cross-section of a magnetic tube of force in the transition region.     

Investigation of the Mg XII 8.42 Å doublet in solar flare spectra

The intensity ratio of the components of the Mg xii 8.42 Å (1s ² S _1/2 – 2p ² P _{1/2, 3/2}) doublet in solar flare spectra has been investigated using observations recorded from the Intercosmos 7 satellite. The observed values of the ratio fall within the interval 0.38–0.66 and have been compared with recent theoretical predictions based on an optically thin collisional-radiative model. It has been found that for the flare plasma the low values of the ratio cannot be explained since they fall below the smallest theoretical value. The highest values on the other hand require that an unacceptably high electron density be postulated. It is suggested that both high and low values may be caused by the resonance line scattering of the Mg xii quanta in the flare volume, provided that the volume is elongated and not spherical.The intensity of the nearby satellite lines is also investigated. Good agreement between the theoretical and observed intensities is found.     

Satellite lines in the solar X-ray spectrum

Observations of solar X-ray line emission using crystal spectrometers during a large chromospheric flare have provided a list of wavelengths with a precision of 0.003 Å in first order of diffraction and correspondingly better in higher orders. In addition to the resonance, intersystem (1 ¹ S ₀-2 ³ P ₁) and forbidden (1 ¹ S ₀-2 ³ S ₁) transitions of ions of the Hei isoelectronic sequence, we have recorded satellite lines arising from ions in the Lii, Bei and Bi isoelectronic sequences. These satellite features are most prominent in the iron spectrum. Apparent decreases in the ratio of forbidden and intersystem line intensities of Mgxi and Sixiii during the flare are used to derive electron densities possibly as high as 1 × 10¹³ cm^–3 in the Mgxi emitting region and 1 × 10¹⁴ cm^–3 in the Sixiii region during the event. A search for satellite lines on the long-wavelength side of the Lyman-alpha line of Hi-like ions has yielded no positive identifications.     

Density dependence of solar emission lines of boron-like ions

Steady state level density of 20 levels of boron-like ions, Mg viii and Si x, have been computed as a function of electron density and temperature. We have accounted for collisional and spontaneous radiative processes. Photo-excitation between the two levels of the ground term has also been considered. Using the computed level density, line intensities have been obtained as a function of electron density and temperature. In case of Mg viii, line intensity ratio I(430.47)/I(436.62) is found to be electron density sensitive. Likewise other pairs of lines namely, I(75.03)/I(74.86), I(315.02)/I(430.47), and I(315.02)/I(335.23) are also found to be density sensitive. Similar density sensitive line intensity ratios have been found for Si x. Absolute line fluxes from these ions at earth distance have been computed and are found to be comparable with values obtained, using various satellite and rocket measurements.Paper presented at the 5th Astronomical Society of India Meeting, held at Uttar Pradesh State Observatory, Naini Tal, India, 5–9 November, 1979.     

A comparison of theoretical O iii line strengths with XUV solar observations from the S082A instrument on board Skylab

Relative level populations in Oiii, determined using R-matrix calculations of electron impact excitation rates, are used to derive the theoretical emission line ratios R ₁ = I(525.80 Å)/I(599.62 Å), R ₂ = I(507.41 Å)/I(599.62 Å), R ₃ = I(507.71 Å)/I(599.62 Å), and R ₄ = I(508.18 Å)/I(599.62 Å). Electron temperatures deduced from the observed values of these ratios for several solar features obtained with the NRL S082A slitless spectrograph on board Skylab are in good agreement, and also compare favourably with that of maximum Oiii fractional abundance in ionisation equilibrium, logT _max = 4.96. These results provide experimental support for the accuracy of the atomic data adopted in the line ratio calculations.