Excitation equilibrium for low lying levels in Cii,Niii, Oiv,Nevi, Mgviii,Six, and Siii

The populations of the excited state ² P _3/2 relative to the ground state ² P _1/2 have been investigated in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii by considering all the radiative and collisional processes including the collisional transitions to the higher states which cascade to the upper level. The relative populations are used for the calculation of the line emissivities. The intensities of 76 320, 30 258 and 14 302 lines of Nevi, Mgviii, and Six ions respectively in the chromosphere-corona transition region are also calculated.     

Forbidden line ratios from Si viii and S x coronal ions

The off-limb spectra, recorded by the NRL Skylab spectrograph, have been used to infer electron densities from theoretical curves of Si viii and S x density-sensitive line ratios. The derived densities are found to be in agreement with those reported by Feldman et al. (1978). Computed line intensities using the Kopp and Orrall (1976) model have also been used to estimate densities in the quite Sun and coronal holes from these curves.     

Theoretical ratios for the EUV lines of A1 VIII ions

Theoretical line ratios for the A1viii ion have been investigated using an atomic model including the first fifteen energy levels, and taking account of various physical processes. The line emissivities as a function of electron density have been computed. Density-dependent theoretical line ratios are presented and their diagnostic applications in the solar atmosphere are discussed with the help of theoretical line intensities computed using an atmospheric model for the quiet Sun.     

Density dependence of solar emission lines of oxygen-like ions

Assuming steady state conditions, the occupation of 9 levels of oxygen-like ions: Ne iii, Mg v, Si vii, S ix, and Ar xi have been computed as a function of electron density and temperature. The following physical processes have been considered: collisional excitations and spontaneous radiative de-excitations for permitted and intercombination transitions; collisional excitations and de-excitations, photo-excitations and spontaneous radiative transitions among the five levels of the ground term. This study indicates that line intensity ratios for oxygen-like ions can be used as a diagnostic in the determination of these two parameters of the solar plasma.Paper presented at the 4th Astronomical Society of India Meeting, held at Radio Astronomy Centre, Ootacamund, India, 7–10 March 1978.     

Solar XUV limb brightening observations

OSO-IV observations of the equatorial limb brightening of XUV resonance lines of Nv, O vi, Ne viii, Mg x and Si xii are interpreted with a modified version of a coronal model developed by Dupree and Goldberg (1967). Good agreement is obtained between the observed limb brightening and that predicted by the model. The sensitivity of the predicted limb-brightening curves to changes in parameters describing the model is discussed. Coronal abundances for N, O, Ne, Mg, and Si are obtained.     

On the Electron density in a coronal hole

Electron density in a coronal hole is rediscussed using the new calculation for the Mgviii 436.62/430.47 density-sensitive theoretical line ratio and with the help of available observations.     

The interpretation of XUV rocket measurements of intensity ratios of solar spectral lines of the lithiumlike ions O vi,Ne viii,and Mg x

A rocket borne spectrometer was flown to measure absolute intensities of extreme ultraviolet spectral lines from the three ions O vi, Ne viii, and Mg x present in the Sun. From these measurements, intensity ratios of lines from O vi, ratios of lines from Ne viii, and ratios of lines from Mg x were formed. These experimental ratios were compared with ratios calculated by using specific theoretical values of the ionization equilibrium in which dielectronic recombination was included in the processes establishing ionization balance. The effects of the electron density and temperature gradient on the temperature distribution of the flux of the spectral lines in the solar atmosphere have been taken into account in the calculations of the ratios. The agreement between the experimental and calculated ratios is good for the ions Ne viii and Mg x and satisfactory for the ion O vi for which the calculated ratio is subject to large uncertainties. A reliable measurement of the electron temperature in the lower corona was obtained from the experimental ratios for Mg x. This experimental temperature is in good agreement with the emission temperature of the spectral lines of Mg x predicted from the theoretical values of the ionization equilibrium. The design and photometric calibration of a new rocket spectrometer developed to measure the intensity ratios over the broad spectral region 50 to 1250 Å are also described.     

Intensity of lines from low-lying levels in C ii,N iii,O iv,Ne vi,Mg viii,Si x,and Si ii

The intensities of the lines for the transition ² P ₃ ^2/0 -² P ₁ ^2/0 in C ii, N iii, O iv, Ne vi, Mg viii, Si x, and Si ii in the chromosphere-corona transition region are investigated. The populations of the excited state ² P _f3/2 ^p0 relative to the ground state ² P _f1/2 ⁰ are obtained by considering all the radiative and collisional transition processes including the collisional transitions to the higher states which cascade to the upper level. It is found that in the transition region the intensity (which may be expressed as a function of temperature only) increases with the charge on the ion for a sequence.On leave from D.N. College, Gulaothi 245 408, India.     

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.     

NIV line ratios in the Sun

Theoretical Niv emission line ratios, which incorporate several improvements over previous estimates, are presented for R ₁ = I(923.2 Å)/I(765.1 Å) and R ₂ = I(1718.6 Å)/I(1486.5 Å), which are electron density and temperature sensitive, respectively. A comparison of R ₁ with observational data for several solar features obtained with the Harvard S-055 spectrometer on board Skylab reveals generally good agreement between theory and observation, except for the quiet Sun, which is probably due to the 923.2 Å line being blended with an Feiii transition in this instance. The observed value of R ₂, determined from a quiet-Sun spectrum obtained by the S082-B spectrograph on board Skylab, implies an electron temperature in excellent agreement with that of maximum Niv fractional abundance in ionisation equilibrium, which provides observational support for the accuracy of the diagnostic calculations.     

Some physical considerations on NGC 1313

The ionized gas in NGC 1313 was studied by spectrophotometric means. The radial behaviour of theI(H)/I(6584),I(6717)/I(6731),N(Nii)/N(Hii), andN(Nii)/N(Sii) ratios and the deduced electron densities are discussed. The abundance ratiosN(N)/N(H) andN(N)/N(S) for the nucleus and two emission regions were also derived and compared with previous data.     

Comparison of computed fluxes for Fex and Fexiv lines with observed values at 1980 eclipse

Fluxes have been computed for Fex (6374 Å) and Fe xiv (5303 Å) lines as a function of solar radii and at various coronal tempratures. The electron density derived from the white light corona during the total solar eclipse of 1980 were used in the computations. Fluxes in adjacent continua have also been computed. The computed ratios of line flux to the square of continuum flux at a coronal temperature of 1.6 × 10⁶ K show a good fit with the observed values for Fex line. Further, radiative excitation seems to dominate over collisional excitation beyond 1.3 solar radius.     

Mg vii and Si ix line ratios in the sun

Recent R-matrix calculations of electron excitation rates for Mg vii and Si ix are used to determine the theoretical density sensitive emission line ratios R ₁= I(2s2p ^{3 1} D ⁰ - 2s ² 2p ^{2 1} D ^e )/I(2s2p ^{3 3} S ⁰ - 2s ² 2p ^{2 3} P ₂ ^e ) and R ₂= I(2s2p ^{3 1} P ⁰ - 2s ² 2p ^{2 1} D ^e )/I(2s2p ^{3 3} S ⁰ - 2s ² 2p ^{2 3} P ₂ ^e ). These are found to be quite similar to the earlier results of Mason and Bhatia. Electron densities derived using observed R ₁ and R ₂ ratios from Skylab NRL XUV spectra of solar flares and active regions are in good agreement, and compare favourably with those deduced from ions formed at similar electron temperatures to Mg vii and Si ix.     

On the electron density in an active region observed by SERTS

Emission lines from an active region, observed by SERTS, have been used to determine electron densities from theoretical curves for Mgvii, Siviii, and Siix density-sensitive line ratios. Density diagnostics of Alviii 285.46/323.52 line emissivity ratio has also been investigated.     

A Comparison of Theoretical Si VIII Emission Line Ratios with Observations from Serts

Recent R-matrix calculations of electron impact excitation rates in N-like Si VIII are used to derive theoretical emission line intensity ratios involving 2s ²2p ³–2s2p ⁴ transitions in the 216–320 Å wavelength range. A comparison of these with an extensive dataset of solar active region, quiet-Sun, sub-flare and off-limb observations, obtained during rocket flights of the Solar EUV Research Telescope and Spectrograph (SERTS), indicates that the ratio R ₁= I(216.94 Å)/I(319.84 Å) may provide a usable electron density diagnostic for coronal plasmas. The ratio involves two lines of comparable intensity, and varies by a factor of about 5 over the useful density range of 10⁸–10¹¹ cm^?3. However R ₂= I(276.85 Å)/I(319.84 Å) and R ₃=I(277.05 Å)/I(319.84 Å) show very poor agreement between theory and observation, due to the severe blending of the 276.85 and 277.05 Å lines with Si VII and Mg VII transitions, respectively, making the ratios unsuitable as density diagnostics. The 314.35 Å feature of Si VIII also appears to be blended, with the other species contributing around 20% to the total line flux.

     

Mgix and Sixi line ratios in the sun

New theoretical emission line ratios for the Be-sequence ions Mgix and Sixi are presented. A comparison with observational data for two solar flares and an active region loop obtained with the Harvard EUV spectrometer and NRL XUV spectroheliograph aboard Skylab reveals that these plasmas are in ionization equilibrium at coronal temperatures. Unfortunately most of the density diagnostics are not particularly useful under solar plasma conditions, as they vary only slightly over the electron density range 10⁸–10¹³cm^–3. However the Sixi ratioI(³ P ^e ₂ -³ P ^o ₂)/I(³ P ^o ₁ –¹ S ^e ₀) is density sensitive in the range 10⁸ to 10¹⁰cm^–3, which is representative of electron densities found in solar active regions or small flares.     

Spatial distribution of XUV emission and density in a loop prominence

We have studied the spatial distribution of XUV emission in the 14 August, 1973 loop prominence observed with the NRL spectroheliograph on Skylab. The loop prominence consists of two large loops and is observed in lines from ions with temperatures ranging from 5 × 10⁴ K to 3 × 10⁶ K. The loops seen in low temperature (10⁶K) lines such as from He ii, Ne vii, Mg vii, Mg viii, and Si viii are systematically displaced from loops seen in higher temperature lines such as from Si xii, Fe xv, and Fe xvi. The cross section of the loop, particularly in cooler lines is nearly constant along the loop. For hotter loops in Si xii, Fe xv, and Fe xvi, however, emission at the top of the loop is more intense and extended than that near the footpoints, which makes the loops appear wider at the top.There is no evidence that the 14 August loop prominence consists of a cooler core surrounded by a hot sheath as in some active region and sunspot loops reported by Foukal (1975, 1976). Rather, the observed spatial displacement between cooler and hotter loops suggest that the 14 August loop prominence is composed of many magnetic flux tubes, each with its own temperature.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

Coronagraphic observations of an enhanced coronal region

Ratios of emission line intensities are used to calculate the variation of temperature and the variation of electron density as a function of ion class for differing paths through a coronal enhancement. The data indicate (a) a peak mean electron density of 2.3 × 10⁹ cm^–3, (b) a temperature maximum greater than 2.3 × 10⁶ K, and (c) the non-coincidence of the peak temperature and peak mean electron density. The latter demonstrates the invalidity of the assumption of symmetric models for coronal enhancements.The abundance of Ni was found to be equal to 0.045 that of Fe from the line ratio I( 6702)/ /I( 7059) and a density model based on the variation of the ratio I( 8024)/I( 6702).     

On the abundance of calcium in the solar corona

Measured values for the total intensity of the continuum and the ratio of integrated intensities I( 5694)/I/(5446) are used to estimate the fraction of electrons along the line of sight contributing to the excitation of Caxv. This estimate of electron density along with an estimate of the dimension of the emitting region are used to find a value of the abundance of Ca in the solar corona. The estimated abundance is logN _Ca/N _H = -4.35.     

Abundance ratios in quasi-stellar objects

Relative abundances of carbon and aluminium with respect to silicon have been calculated in the QSOs PHL957, PKS0237-23, 1331+170, 3C191 and M132. Relative abundance of Fe with respect to Mg has been also calculated in the QSOs 1331+170 and PHL938. The ratiosN(C)/N(Si),N(Al)/N(Si) andN(Fe)/N(Mg) in QSOs considered as a class are (5.5±1.6), (0.13±0.03) and (2.6±1.2), respectively.The ratioN(C)/N(Si) shows a mild trend of increase with decreasing emission redshift. This suggests the possibility that the QSOs might be evolving chemically.