EMISSION LINES OF Ni XVIII IN THE SOLAR EUV SPECTRUM

Using electron excitation rates calculated with the R-matrix code, theoretical Nixviii electron-temperature-sensitive emission line ratios are presented for R ₁ = I(220.41 Å)/I(320.56 Å) , R ₂ = I(233.79 Å)/I(320.56 Å) , and R ₃ = I(220.41 Å)/I(292.00 Å) . A comparison of these with observational data for two solar flares, obtained by the Naval Research Laboratory's S082A slitless spectrograph on board Skylab, reveals good agreement between theory and observation for R ₁ and R ₂ in two spectra, which provides limited support for the accuracy of the atomic data adopted in the analysis. However, several of the measured ratios are much larger than theory predicts, which is probably due mainly to saturation of the strong 292.00 and 320.56 Å lines on the photographic film used to record the S082A data. A comparison of our line ratio calculations with active region observations made by the Solar EUV Rocket Telescope and Spectrograph (SERTS) indicate that a feature at 236.335 Å, identified as the Nixviii 3p ² P _3/2 - 3d ² D _3/2 transition in the SERTS data, is actually the Arxiii 2s ²2p ^{2 3} P ₀ - 2s2p ^{3 3} D ₁ line. The potential usefulness of the Nixviii line ratios as electron temperature diagnostics for the solar corona is briefy discussed.     

Theoretical Siiv line ratios compared to extreme ultraviolet solar observations

New theoretical electron temperature sensitive emission line ratios in Siiv involving the 3d ² D – 3p ² P and 4s ² S – 3p ² P multiplets at 1125 and 816 Å, respectively, are derived using recent R-matrix electron excitation rate calculations. A comparison of these with observational data for a solar active region at the limb obtained with the Harvard S-055 spectrometer on board Skylab reveals that there is good agreement between theory and observation for ratios that include the ² D _{3/2, 5/2} – ² P _3/2 transition at 1128.3 Å. This is in contrast to the findings of Keenan, Dufton, and Kingston (1986) and provides support for the atomic data adopted in the calculations. However, the ² D _3/2 – ² P _1/2 line at 1122.5 Å appears to be severely blended, as suggested previously by Burton and Ridgeley (1970) and Feldman and Doschek (1977), as it leads to electron temperature estimates that differ significantly from that expected in ionisation equilibrium. The fact that the I(1122.5 Å)/I(1128.3 Å) intensity ratios determined from several flare spectra are closer to theory than that for the active region indicates that the blending is probably due to species with relatively low ionization potentials, as noted by Flower and Nussbaumer (1975). Electron temperatures deduced for a sunspot are much lower than that predicted from ionisation balance calculations, in agreement with earlier results, and imply that a cooling flow may be present.     

The identification of Feix and Nixi in the solar corona

The levels of the configuration 3s ²3 _p ⁵3d of Fe ix have been experimentally determined from their combinations with 3s3 _p ⁶3d ³ D in the region 300–400 Å. Wavelengths can now be accurately predicted for all transitions within 3s ²3 _p 53^d, and eight of these can be identified with coronal lines from 2042 to 4585 Å. Also, identifications of solar lines from 171 to 245 Å with electric-dipole and magnetic-quadrupole transitions to the ground state, 3s ²3p ^{6 1} S, are confirmed and extended. Solar identifications with corresponding transitions in Ni xi, both within 3s ²3 _p ⁵3d and to the ground state, are proposed on the basis of a short extrapolation.     

A comparison of theoretical Sv emission line strengths with extreme ultraviolet observations of a sunspot

Electron impact excitation rates for transitions in the S v ion, calculated with theR-matrix code, are used to derive the electron temperature sensitive emission line ratiosR ₁ =I(854.8 Å)/I(786.9 Å),R ₂ =I(852.2 Å)/I(786.9 Å),R ₃ =I(849.2 Å)/I(786.9 Å), andR ₄ =I(1199.1 Å)/I(786.9 Å), which are found to be significantly different from previous estimates. A comparison of the present results with observational data for a sunspot obtained with the Harvard S-055 spectrometer on boardSkylab reveals generally good agreement between theory and experiment, except in the case ofR ₁, which is probably due to blending in the 854.8 Å feature. The possible effects of Lyman continuum absorption on the observed line ratios is briefly discussed.     

Si III electron temperature diagnostics for the solar transition region

R-matrix calculations of electron impact excitation rates for transitions in Si iii are used to derive the electron-density-sensitive emission line ratios R ₁ = I(1113.2 Å)/I(1206.3 Å), R ₂ = I(1298.9 Å)/I(1206.3 Å), and R ₃ = I(1296.7 Å)/I(1206.3 Å). A comparison of these with observational data for several solar features obtained with the Harvard S-055 spectrometer on board Skylab reveals that theory and experiment are compatible if the electron temperature of the Si iii emitting region of the solar atmosphere is log T _e = 4.5, but not if log T _e = 4.7. The implication of the choice of a lower temperature on the electron energy distribution function is also briefly discussed.     

The identification of new forbidden coronal lines in the solar EUV spectrum

Identifications are proposed for twenty of the twenty-eight coronal lines observed in the spectra obtained during a rocket flight into the path of the 7 March, 1970 solar eclipse. The methods by which the lines have been identified are discussed. Most of the lines identified are from forbidden transitions between levels in the ground 2p ⁿ and 3p ⁿ configurations in high ions of magnesium, silicon, sulphur, iron, and nickel. The temperature range represented is from 6.9 × 10⁵ K to 2.5 × 10⁶ K. The classification of three lines of Fexii and two of Nixiv has led to a revised identification for the near ultraviolet ² D _3/2-² P _1/2 transition in Fe xii. This transition can be identified with the line at 3072 Å rather than that at 3021 Å as previously suggested in the literature.     

Multiresolution wavelet analysis of SUMER/SOHO observations in a solar prominence

We have studied through a multiresolution wavelet analysis the oscillations in a limb prominence. Intensity fluctuations in time and height corresponding to different lines of Siiv and Oiv observed with SUMER on board SOHO have been analyzed in the wavelet bands of J₃= 1 min 36 s to 3 min 12 s and J₄=3 min 12 s to 6 min 24 s. For all species, oscillations in the J₄ band were dominant. We found relevant differences between the behavior of line D₁ (1393.76 Å) corresponding to Siiv and the set D₂ (1401.16 Å), D₃ (1404.81 Å), D₄ (1402.77 Å) corresponding to Oiv, Oiv and Siiv respectively. We also report the identification of a pulse in the intensity of the line D₁ that appears in the range of 15–20 min. This disturbance seems to travel with a speed of about 170 km s^–1.     

Effect of fluctuations in Doppler width on the center of strong absorption lines

Stochastic temperatures and turbulence are characterized by average velocities u _th and < u _turb  > ≡ u ₀ and fluctuations u￠_th {u'_{th}} and u′ (<u′ > = 0). Thus, the Doppler width of a line also has a fluctuating component Dl￠_D \Delta {\lambda '_D} . Observed spectra correspond to the radiative flux averaged over time and over a star’s surface, <H_λ>. Usually, only the average velocities u _th and u ₀ are taken into account in photospheric models and these yield the Doppler width Dl_D⁽⁰⁾ \Delta \lambda_D^{(0)} of a line in the customary way. The fluctuations Dl￠_D \Delta {\lambda '_D} mean that near a line center the average absorption coefficient < α_λ > is larger than the usual α_λ, which depends only on the average velocities u _th and u ₀. This enhances the absorption line near the center and is not explained by the photospheric models. This new statistical effect depends on the wavelength of the line. A comparison of observed lines with model profiles yields an estimate for the average level of fluctuations in the Doppler width, h = á | Dl￠_D | ñ

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.     

The ultraviolet absorption band at 2175 Å: Correlations with other interstellar features

The hump in the ultraviolet part of the interstellar extinction curve is interpreted as a broad diffuse absorption band. Its equivalent width is estimated for 36 stars by means of OAO-2 data. The equivalent widths are correlated with the following parameters: colour excessE(B-V), colour excessE(B – V), depth of the band m _max, equivalent widths of the diffuse bands at 5780 and 6284 Å, and the column density of neutral hydrogenN _HI. The physical parameters half-width and oscillator strength of the band at 2175 Å are estimated.     

The abundance of chlorine in the Sun

The Cli line 8375.943 (4s ⁴P_5/2 – 4p ⁴D7/2) is identified in the solar spectrum. This is the first identification of a chlorine line in solar spectrum. The measured equivalent width (W = 0.8 mÅ) corresponds to an abundance log N(Cl) = 5.65 on the scale log N(H) = 12.00.     

A discussion on the maximum aperture of a ground-based optical telescope

Because of the limitations both from the slit width and the slit length of the spectrograph used in high resolution work, the efficiency of a telescope, with an aperture between D₀ = 9m and D′₀ = 55m, will be no better than that of a telescope with aperture D₀. D₀ can be identified as the maximum aperture of a ground-based telescope.     

Theoretical emission line strengths for Ne VII compared to EUV solar observations

Theoretical electron-temperature-sensitive Ne vii emission line ratios, calculated using accurate R-matrix electron impact excitation rates, are presented for R ₁ = I(895.2 Å)/I(465.2 Å), R ₂ = I(561.7 Å)/I(465.2 Å) and R ₃ = I(564.5 Å)/I(465.2 Å). A comparison of these with observational data for several solar features obtained with the Harvard S-055 spectrometer on board Skylab reveals good agreement between theory and experiment. This provides observational support for the accuracy of the atomic physics adopted in the calculations, and the methods employed in the derivation of the theoretical diagnostics.     

Forbidden sulphur I lines in the solar spectrum

The [SI] lines are due to transitions within the 3s²3p⁴ ground configuration of neutral sulphur. The results are presented from a search for the [Si] lines in the Fraunhofer spectrum. Two identifications are proposed with faint features in the Fraunhofer spectrum: ¹D₂-¹S₀ at = 7725.02 Å and ³P₂-¹D₂ at = 10821.23 Å. Their measured equivalent widths are shown to confirm the value for the solar abundance of sulphur, logN _S = 7.21 (in the scale logN _H = 12.00), which is derived from the permitted high-excitation lines. These lines give the first convincing identification of [Si] lines in an astrophysical source.     

On the excitation of lower levels of singlet helium in quiescent prominences

The paper deals with the excitation of the helium singlet level 2¹ P in the homogeneous and filamentary models of quiescent prominences with following parameters: the optical thickness at the limit of helium Lyman continuum _1c ^M = 0.1–100, T _e = 7000 K, n _e = 5 × 10¹⁰ cm^–3. Assuming a model He atom with seven discrete levels (1¹ S, 2³ S, 2¹ S, 2³ P, 2¹ P, 3³ D, 3¹ D) and the continuum the steady state equations for the levels 2³ S, 2¹ P and the continuum have been solved together with the radiative transfer equations for the line 584 Å and the continuum 504 Å. The variations with depth of the functions n ₂ ³ _S /n ₁ ¹ _S (₁ c), n ₂ ¹ _P /n ₁ ¹ _S (_1c ), and n ⁺ _He n _e /n ₁ ¹ S(_1c ) as well as the intensities of the triplet (D₃, 10830 Å) and singlet (16678, 20581 Å) lines have been calculated. Comparison with observations leads to the following conclusions: (1) The line intensities calculated for filamentary models of prominences agree better with observations than those for homogeneous ones. (2) The helium level 2¹ P is excited by diffuse field 584 Å being formed by recombinations and spontaneous transitions 2¹ P – 1¹ S and escaping from the prominence into the space between the filaments and to the surface. (3) Underpopulation of the singlet level 2¹ P may be explained by combination of weak excitation mechanism (recombinations and formation of the diffuse field 584 Å) and strong deexcitation mechanism (spontaneous transitions into the level 1¹ S).     

The Violet Opacity in the Red Peculiar Stars – III. Spectral analysis of the SC star GP Ori

Medium-resolution (1.5-2.5 Å) spectrum has been obtained for SC star GP Ori in a wide range of wavelengths from 3730 to 6250 Å. It is characterised mainly by strong atomic lines, as well as moderately strong bands of the CN violet system and CH (0,0) and (0,1) bands at 4315 and 4890 Å. Weak bands of C₂, C₃, and ZrO molecules are observed, however, no evidence is found for the presence of either SiC₂ (Merrill-Sanford bands) or YO. The most prominent atomic lines along with the NaID12 are those of CaII (K,H), CaI at 4227 Å, SrI at 4607 Å, SrII at 4077 and 4215 Å, and BaII at 4554 Å. H seems to be in emission.     

Further X-ray spectra of solar active regions

Measurements of the solar X-ray spectrum between 3 Å and 15 Å are reported. They were made with two slitless Bragg crystal spectrometers flown on a Sun-pointed Skylark rocket on 8 August, 1967.The use of a beryl crystal has provided higher spectral resolution than hitherto in the spectral range 12 Å and 15 Å leading, in particular, to a revised identification of the strong line at 13.71 Å. Separate components of the stronger emission lines are clearly seen from each of three coronal active regions which may be identified on radio and X-ray spectroheliograms.The absolute line fluxes are used to determine a model for each active region in terms of the differential emission measure as a function of electron temperature. Emission lines due to the transition 1s ^{2 1} S ₀ – 1s2s ³ S ₁ in several helium-like ions are identified and values of the local electron density derived from measurement of the line flux in these ions.     

The structure of the monochromatic corona in the surroundings of prominences

It is well known that in the immediate surroundings of stationary filaments the white light of the corona is strongly reduced. The same effect is observed in the monochromatic emissions of the lines 5303 and 6374 Å, for which in Figures 1–3 some examples are given. This diminuation of light can only be explained by a corresponding diminuation of the density. As the density in the vicinity of the prominence may be as small as one tenth of that of the undisturbed corona, or even smaller in some cases, these features are called cavities. The intensity distribution of the line 5303 Å in the region of a stable prominence observed on October 6, 1969 has been analysed (Figure 4). The cavity turns out to be of the shape of a half-ellipsoid. Its length (in heliographic longitude) was found to be 0.68, its width (in heliographic latitude) 0.20 and its height 0.16 R (Figure 5). The corresponding dimensions of the enclosed filament are 0.56 (length), 0.10 (width) and 0.08 (height) R .     

The chromosphere in continuum emission observed at the total solar eclipse on 7 March 1970

Direct images of the Sun were photographed in continuum emission centered at 6900 Å by the jumping film method near the second contact of the Mexico eclipse on 7 March 1970. The band width was 150 Å defined by a combination of a sharp cut filter and KODAK IV F film. The intensity distribution of the solar outer layers obtained shows a steep decrease by a factor of 0.9 in logarithmic units around 2500 km. This is interpreted as the boundary of the chromosphere and corona. Spicules observed at 3500 km are explained by log n _e = 11.25 and T _e 6000 K. Discussions are made in relation to the other observations and some chromosphere models.     

A comparison of theoretical line strengths for the 2s 22p 2 − 2s2p 3 transitions in Ne v with solar data

Results are presented for several theoretical line ratios in Nev involving transitions between multiplets in the 2s ²2p ² and 2s2p ³ configurations. A comparison of these with solar data from the S082A and S-055 instruments on board Skylab reveals generally good agreement between theory and experiment, especially in the case of the high-resolution (S082A) observations. However the 2s ²2p ^{2 1} D – 2s2p ^{3 1} P (365.6 Å) and 2s ²2p ^{2 3}P – 2s2p ^{3 3} S (359 Å) lines appear to be blended, possibly with transitions in Fex and Fexi/Fexiii, respectively. We note that the intensity ratio I(365.6 Å)/I(416.2 Å) should be a valuable calibration check for a high-resolution extreme ultraviolet instrument in the spectral range 360–420 Å.