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Aims. In this paper we report on calculations of energy levels, radiative rates, oscillator strengths, line strengths, and effective collision strengths for transitions among the lowest 362 levels of the (1s22s22p6) 3s23p5, 3s3p6, 3s23p43d, 3s3p53d, 3s23p33d2, 3s3p43d2, 3p63d, and 3s23p44$\ell$configurations of Cr viii.

Aims. We generate theoretical ultraviolet and extreme-ultraviolet emission line ratios for O IVand show their strong versatility as electron temperature and density diagnostics for astrophysical plasmas.

Aims. In this paper we report electron impact collision strengths and excitation rates for transitions among the lowest 89 levels of Ni xix.

Aims. In this paper we report calculations for energy levels, radiative rates, and excitation rates for transitions in O IV.

Aims. In this paper we report on calculations for energy levels, radiative rates, collision strengths, and effective collision strengths for transitions among the lowest 25 levels of the n≤5 configurations of H-like Ar xviii.

Aims.In this paper we report calculations for energy levels, radiative rates and excitation rates for transitions in Ni xi.

Aims.In this paper we report calculations for energy levels and radiative rates for transitions in Co xi.

Aims.In this paper we report calculations for energy levels and radiative rates for transitions in Fe xvi.

Aims.In this paper we report calculations for energy levels and radiative rates for transitions in Ni XIX.

Aims.In this paper we report calculations for energy levels and radiative rates for transitions in Fe IX.

Previously, large discrepancies have been found between theory and observation for $\ion{Fe}{xv}$emission line ratios in solar flare spectra covering the 224-327 Å  wavelength range, obtained by the Naval Research Laboratory's S082A instrument on board Skylab. These discrepancies have been attributed to either errors in the adopted atomic data or the presence of additional atomic processes not included in the modelling, such as fluorescence. However our analysis of these plus other S082A flare observations (the latter containing $\ion{Fe}{xv}$transitions between 321-482 Å), performed using the most recent $\ion{Fe}{xv}$atomic physics calculations in conjunction with a chiantisynthetic flare spectrum, indicate that blending of the lines is primarily responsible for the discrepancies. As a result, most $\ion{Fe}{xv}$lines cannot be employed as electron density diagnostics for solar flares, at least at the spectral resolution of S082A and similar instruments (i.e. ~0.1 Å). An exception is the intensity ratio I(3s3p 3P2–3p23P1)/I(3s3p 3P2–3p21D$_{2}) = I$(321.8 Å)/I(327.0 Å), which appears to provide good estimates of the electron density at this spectral resolution.

Energies for 524 levels of Ar XIII, 460 levels of Ar XIV and 156 levels of Ar XV have been calculated using the graspcode of Dyall et al. (1989). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with the limited results available in the literature, and the accuracy of the data is assessed. Our energy levels are estimated to be accurate to better than 1%, whereas results for other parameters are probably accurate to better than 20%. Additionally, the level lifetimes derived from our radiative rates are in excellent agreement with measured values.

Energy levels, radiative rates, collision strengths, and effective collision strengths for all transitions up to and including the $n = 5$levels of Al XIII have been computed in the jjcoupling scheme including relativistic effects. All partial waves with angular momentum $J \le 60$have been included, and resonances have been resolved in a fine energy grid in the threshold region. Collision strengths are tabulated at energies above thresholds in the range $170.0 \le E \le 300.0$Ryd, and results for effective collision strengths, obtained after integrating the collision strengths over a Maxwellian distribution of electron velocities, are tabulated over a wide temperature range of $4.4 \le \log \,T_{\rm e} \le 6.8$K. The importance of including relativistic effects in a calculation is discussed in comparison with the earlier available non-relativistic results.

Energies of the 54 levels belonging to the (1s22s22p6) 3s23p5, 3s3p6, 3s23p43d and 3s3p53d configurations of Fe X have been calculated using the GRASP code of Dyall et al. ([CITE]). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with results available in the literature, and the accuracy of the data is assessed. Our energy levels are estimated to be accurate to better than 3%, whereas results for other parameters are probably accurate to better than 20%. Additionally, the agreement between measured and calculated lifetimes is better than 10%.

Energies of the lowest 157 levels belonging to the (1s2) 2s22p6, 2s22p53${\ell}$, 2s22p54${\ell}$, 2s22p55${\ell}$, 2s2p63$\ell$, 2s2p64$\ell$and 2s2p65$\ell$configurations of Fe XVII have been calculated using the GRASP code of Dyall et al. ([CITE]). Additionally, radiative rates, oscillator strengths, and line strengths are calculated for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among these levels. Comparisons are made with the results already available in the literature, and the accuracy of the data is assessed. Our energy levels are expected to be accurate to better than 1%, whereas results for other parameters are probably accurate to better than 20%.

Energy levels and radiative rates for transitions among the lowest 97 fine-structure levels belonging to the (1s22s22p6) 3s23p2, 3s3p3, 3s23p3d, 3p4, 3s3p23d and 3s23d2configurations of Fe XIII have been calculated using the fully relativistic GRASP code. Additionally, collision strengths for transitions among these levels have been computed using the Dirac Atomic R-matrix Code (DARC) of Norrington & Grant ([CITE]). Radiative rates and oscillator strengths are tabulated for all allowed transitions among the 97 fine-structure levels, while collision strengths are reported for some transitions at a few energies above thresholds. Comparisons are made with the available results, and the accuracy of the data is assessed.

Energy levels and the corresponding transition probabilities for allowed and forbidden transitions among the levels of the ground configuration and first 23 excited configurations of fluorine-like $\ion{Fe}{xviii}$have been calculated using the multiconfigurational Dirac-Fock graspcode. A total of 379 lowest bound levels of $\ion{Fe}{xviii}$is presented, and the energy levels are identified in spectroscopic notations. Transition probabilities, oscillator strengths and line strengths for electric dipole (E1), electric quadrupole (E2) and magnetic dipole (M1) transitions among these 379 levels are also presented. The calculated energy levels and transition probabilities are compared with experimental data.

Collision strengths for transitions among the energetically lowest 53 fine-structure levels belonging to the (1s22s22p6) 3${\ell}^2$, 3${\ell}$3${\ell}^{'}$, 3s4${\ell}$and 3p4s configurations of Fe XV are computed, over an electron energy range below 160 Ryd, using the Dirac Atomic R-matrix Code (DARC) of Norrington & Grant ([CITE]). Effective collision strengths, obtained after integrating the collision strengths over a Maxwellian distribution of electron energies, have also been calculated. These results of effective collision strengths are tabulated for all 1378 inelastic transitions over a wide temperature range of 105to 107K. Comparisons are also made with other R-matrix calculations and the accuracy of the results is assessed.?

Collision strengths for transitions among the energetically lowest 46 fine-structure levels belonging to the (1s2) 2s22p2, 2s2p3, 2p4, and 2s22p3$\ell$configurations of Ca XV are computed, over a wide electron energy range below 300 Ryd, using the Dirac Atomic R-matrix Code (DARC) of Norrington & Grant ([CITE]). Resonances in the threshold region have been resolved in a fine energy mesh, and excitation rates are determined over a wide electron temperature range below 107K. The results are compared with those available in the literature, and the accuracy of the data is assessed.

Energy levels and oscillator strengths (transition probabilities) have been calculated for transitions among 46 fine-structure levels of the (1s2) 2s22p2, 2s2p3, 2p4, 2s22p3s, 2s22p3p and 2s22p3d configurations of C-like K XIV, Sc XVI, Ti XVII, V XVIII, Cr XIX and Mn XX using the GRASP code. Configuration interaction and relativistic effects have been included while generating the wavefunctions. Calculated values of energy levels agree within 3% with the experimentally compiled results, and the length and velocity forms of oscillator strengths agree within 20% for a majority of allowed transitions.

Collision strengths for transitions among the lowest 48 fine-structure levels belonging to the (1s22s22p6) 3s23p4, 3s3p5, 3s23p33d and 3p6configurations of Fe XI have been calculated using the Dirac Atomic R-matrix Code (DARC) of Norrington & Grant (2003). Results are tabulated at energies above thresholds in the range $10 \le E \le 100$Ry, although resonances have been resolved in a fine energy mesh in the thresholds region. Effective collision strengths, obtained after integrating the collision strengths over a Maxwellian distribution of electron velocities, are also tabulated over a wide electron temperature range below $5\times 10^6$K. Comparisons with other available results are made, and the accuracy of the present data is assessed.

Collision strengths for transitions among the n = 5 levels of Al XIII have been computed in the LS coupling scheme using the R-matrix code. All partial waves with angular momentum [MATH] have been included, and resonances have been resolved in a fine energy grid in the threshold region. Collision strengths have been computed at energies below 220.0 Ryd, and effective collision strengths, obtained after integrating the collision strengths over a Maxwellian distribution of electron velocities, are calculated over a wide temperature range of [MATH].

Energy levels and radiative rates for transitions among the 107 fine-structure levels belonging to the (1s22s22p6) 3s23p63d10, 3s23p63d94l, 3s23p53d104l, and 3s3p63d104l configurations of Ni-like ions with [MATH] have been calculated using the GRASP code. The collision strengths ([MATH]) have also been computed for transitions in Gd XXXVII at energies below 800 Ryd, using the DARC code. Resonances have been resolved in a fine energy mesh in the threshold region, and excitation rate coefficients have been calculated for transitions from the ground level to excited levels at temperatures below 2500 eV. These have been compared with those available in the literature, and enhancement in the values of rates, due to resonances, has been observed up to an order of magnitude for some of the transitions.

Using theR-matrix approach new calculations have been made for the electron impact excitation of the fine structure transitions within the 1s22s22p2 ground configuration of Mgvii. The computations have been made at a large number of energies in order to account for the contribution of resonances. All partial waves withL = 9 are included in the calculations which are considered to be sufficient for the convergence of collision strengths in the energy range below 65 Ry. From this collision strength data, excitation rate coefficients have been calculated at a series of electron temperatures which are employed in the computation of population of the five lowest levels of Mgvii. The line intensity ratios for the transitions3P1 ?1D2 and3P2 ?1D2 to3P1 ?1S0 are then calculated in the temperature range of 105 to 107 K at electron densities in the range 106 to 1010 cm-3. The calculated values are in good agreement with the earlier available results.

Using configuration interaction wavefunctions, the excitation thresholds for the twelve lowest terms arising from the configurations 1s22s22p2, 1s22s2p3, and 1s22p4 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.

Electron impact excitation rates for Fexxi, calculated with theR-matrix code, are used to determine theoretical electron density sensitive emission line ratios involving transitions in the 121–146 Å wavelength range. The observed ratios for a solar flare, obtained with a grazing spectrometer on board the OSO-5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.3 dex. In addition, the derived values ofNe are similar to those estimated for the high temperature regions of other solar flares. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations.

Results are presented for several theoretical line ratios in Nev involving transitions between multiplets in the 2s22p2 and 2s2p3 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 2s22p21D - 2s2p31P (365.6 Å) and 2s22p23P - 2s2p33S (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.

Relative level populations in Oiii, determined using R-matrix calculations of electron impact excitation rates, are used to derive the theoretical emission line ratios R1 = I(525.80 Å)/I(599.62 Å), R2 = I(507.41 Å)/I(599.62 Å), R3 = I(507.71 Å)/I(599.62 Å), and R4 = 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, logTmax = 4.96. These results provide experimental support for the accuracy of the atomic data adopted in the line ratio calculations.

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 R1= I(2s2p31D0 - 2s22p21De)/I(2s2p33S0 - 2s22p23P2e) and R2= I(2s2p31P0 - 2s22p21De)/I(2s2p33S0 - 2s22p23P2e). These are found to be quite similar to the earlier results of Mason and Bhatia. Electron densities derived using observed R1 and R2 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.

Relative level populations for O III, derived using electron impact excitation rates calculated with the R-matrix code, are used to deduce the electron-temperature-sensitive emission-line ratioR=I(2s22p21D–2s22p21S)/I(2s22p23P1,2–2s22p21D) =I(4363 Å)/I(4959 + 5007 Å) for a range ofTe= (7500–20000 K) applicable to planetary nebulae. Electron temperatures deduced from the observed values ofRin several planetary nebulae are in excellent agreement with those determined fromTe-sensitive line ratios in other species, including CIII]/C [II], [NII] and [ArIII], which provides support for the accuracy of the atomic data adopted in the level population calculations.

Theoretical ArXIII electron-density-sensitive emission line ratios, derived using electron impact excitation rates interpolated from accurateR-matrix calculations, are presented forR1 =I(242.22 Å)/I(236.27 Å),R2 =I(210.46 Å)/I(236.27 Å), andR3 =I(248.68 Å)/I(236.27 Å). Electron densities deduced from the observed values ofR1,R2, andR3 for solar flares obtained with the NRL S082A slitless spectrograph on boardSkylab are in excellent agreement, and furthermore compare favorably with those determined from line ratios in CaXV, which is formed at a similar electron temperature to that of ArXIII. These results provide experimental support for the accuracy of the atomic data adopted in the analysis, as well as for the techniques used to calculate the line ratios.

The recent level population calculations for Ne v by Aggarwal are used to determine the theoretical emission line ratios R1 = I(2s2p31Do - 2s22p21De)/I(2s2p33D20- 2s22p23P1e) and R2 = I(2s2p31Do-2s22p21De)/I(2s2p33D30-2s22p23P2e). A comparison of these with observational data for a solar flare and erupting prominence obtained with the NRL XUV spectrograph on board Skylab reveals that R1 and R2 are in their predicted high density limits. Although the ratios cannot be used as density diagnostics for values of ne typical of the solar transition region, it is shown that they are temperature sensitive and hence may be employed to determine the electron temperatures of Ne v line emitting regions.

Context. The $\ion{Na}{x}$X-ray lines between 10.9 and 11.2 Å have attracted little attention but are of interest since they enable an estimate of the coronal abundance of Na to be made. This is of great interest in the continuing debate on the nature of the FIP (first ionization potential) effect. Aims. Observations of the $\ion{Na}{x}$lines with the Solar Maximum Mission Flat Crystal Spectrometer and a rocket-borne X-ray spectrometer are used to measure the Na/Ne abundance ratio, i.e. the ratio of an element with very low FIP to one with high FIP.Methods. New atomic data are used to generate synthetic spectra which are compared with the observations, with temperature and the Na/Ne abundance ratio as free parameters.Results. Temperature estimates from the observations indicate that the line emission is principally from non-flaring active regions, and that the Na/Ne abundance ratio is 0.07 ±50%.Conclusions. The Na/Ne abundance ratio is close to a coronal value for which the abundances of low-FIP elements (FIP <10 eV) are enhanced by a factor of 3 to 4 over those found in the photosphere. For low-temperature (Te$\leqslant$1.5 MK) spectra, the presence of $\ion{Fe}{xvii}$lines requires that either a higher-temperature component is present or a revision of ionization or recombination rates is needed.

Context. The $\ion{Na}{x}$X-ray lines between 10.9 and 11.2 Å have attracted little attention but are of interest since they enable an estimate of the coronal abundance of Na to be made. This is of great interest in the continuing debate on the nature of the FIP (first ionization potential) effect.

Mean electron tempertures for 106 planetary nebulae are presented, which have been derived using calculations of the values of electron temperature-sensitive line ratios involving forbidden transitions among the 2s22p23P, 1D, and 1S levels of N+and O++, based on new electron impact rates and transition probabilities. Comparison of these results with values of Te[N II] and Te[O III] determined previously by Kaler reveal that the present electron temperatures are systematically lower for both ions, and that this discrepancy is correlated with the electron density in the nebula. It is also shown that the average difference tween Te[N II] and Te[O III] in a planetary nebula is somewhat smaller than that derived by Kaler, with the present results implying that the N II and O III temperatures disagree on average by 2070 K as opposed to the 2210 K average found by Kaler.