CHIANTI—AN ATOMIC DATABASE FOR EMISSION LINES. XIII. SOFT X-RAY IMPROVEMENTS AND OTHER CHANGES

The previous model for O v included data for the entire n = 2 and n = 3 complexes for a total of 46 fine-structure levels. CHIANTI version 7.1 extends the model to include all the n = 4 and n = 5 configurations, for a total of 166 fine-structure levels.

Observed energy levels have been taken from version 5 of the National Institute of Standards and Technology (NIST; Kramida et al. 2012) for levels belonging to the n = 2, 3 complexes, and from the compilation of Moore (1993) for the levels in the n = 4, 5 configurations.

Theoretical energy levels, radiative transition rates, and collision strengths have been taken from Bhatia & Landi (2012), who used the Flexible Atomic Code (FAC; Gu 2003) to calculate the full set of data included in CHIANTI. Bhatia & Landi (2012) included all configurations of complexes up to n = 9 in the target model but the 2snl (n = 6–9) configurations have been omitted here. Levels from these configurations had lower energies than those in the 2p5l configurations, but their omission has no consequence either for the total radiative decay from the 2p5l levels, or on the overall O v population calculations under solar coronal conditions. Collision strengths were calculated at six values of the incident electron energy using the distorted wave (DW) approximation ranging from threshold to 41 Ry. However, resonant excitation has important effects on the O v Maxwellian-averaged collision strengths, so the R-matrix calculations included in the previous CHIANTI version (described in Landi et al. 2006), along with the energy separation in the SPLUPS file, have been retained for the n = 2, 3 levels.

The extended O v model allows the prediction of lines in the soft X-ray range between 110 Å and 170 Å which can be observed by high-resolution spectrometers and also fall within the passband of the SDO/AIA 131 Å channel.

The previous model for C ii was based on the R-matrix collision strengths of Blum & Pradhan (1992) that consisted of 18 fine-structure levels. The current model is based on the calculations of Tayal (2008) that provide oscillator strengths, A-values, and electron effective collision strengths for an atom with 37 fine-structure levels. Observed energies and wavelengths are taken from the compilation of NIST (version 5; Kramida et al. 2012). In addition, five observed wavelengths between the 2s²2p²P and the 2s2p^{2 4}P levels are taken from the analysis of Young et al. (2011).

The electron collision data for O iv have been largely unchanged since CHIANTI 1 (Dere et al. 1997), for which R-matrix data of Zhang & Sampson (1994) were used for transitions among the n = 2 levels and unpublished data of D.H. Sampson for transitions involving n = 3 levels. These data are completely replaced here with the data from Aggarwal & Keenan (2008), who provided effective collision strengths for all transitions between the 75 levels of the configurations 2s²2p, 2s2p², 2p³, 2s²3l (l = s, p, d), 2s2p3l (l = s, p, d), and 2s²4l (l = s, p, d, f). The previous CHIANTI model contained 57 levels from the 2p²3l (l = s, p, d) configurations, which are dropped from the new model. These levels have high energies (many are above the O iv ionization limit) and do not produce significant emission. Coupled with the fact that the data for these levels were never published, it has been judged better to remove them.

The Aggarwal & Keenan (2008) effective collision strengths were computed with the DARC code and are tabulated for 12 temperatures between 3 × 10³ and 1 × 10⁶ K. As the tabulated temperatures were not evenly distributed when projected onto a logarithmic scale, they were interpolated to provide additional values at 1.7 × 10⁴ and 1.7 × 10⁵ K. Nine-point spline fits were then performed to the effective collision strengths tabulated at 14 temperature points. For allowed transitions, high-temperature limit points (Burgess & Tully 1992) were derived using theoretical energies and oscillator strengths provided by Aggarwal & Keenan (2008). For a number of allowed transitions, the scaled effective collision strengths did not tend toward the high-temperature limit and in some cases it was necessary to remove one or two of the highest temperature points in order to get a good fit. For other transitions it was necessary to remove between one and three of the lowest temperature points to get a good fit. Care was taken to ensure that low-temperature points were not removed for transitions that are important for low temperature, photoionized plasmas, however. The fits reproduce the original data to better than 1.85% in the 10⁴–5 × 10⁵ K temperature range.

The only O iv levels with significant population for astrophysical applications are the energetically lowest five levels, and so only electron collision data for transitions involving these levels are included.

Three sources for radiative decay rates are used. For all transitions among levels of the 2s²2p, 2s2p², 2p³, 2s²3l (l = s, p, d) configurations (levels 1–20 in the CHIANTI indexing), except the ground transition, data from Corrégé & Hibbert (2004) are used. The ground transition decay rate is from Tachiev & Froese Fischer (2000), although we note that this value is in very good agreement with the earlier calculations of Flower & Nussbaumer (1975) and Galavís et al. (1998). Rates for decays from the 2s2p3s⁴P_J levels (numbers 21–23 in the CHIANTI indexing) are also from Tachiev & Froese Fischer (2000). Decay rates for all other transitions are from Aggarwal & Keenan (2008); however, these values were computed using the authors' theoretical energies and so they have been scaled using experimental energies to yield more accurate values.

Experimental energies are available for all of the 75 levels of the CHIANTI model. The 2s²2p²P_3/2 level (number 2 in the CHIANTI model) energy is from Feuchtgruber et al. (1997), and the 2s2p^{2 4}P_J (numbers 3–5 in the CHIANTI model) energies are from Young et al. (2011). All other energies are from version 4.1 of the NIST database (Ralchenko et al. 2011).

Giunta et al. (2012) identified a problem with the previous CHIANTI model whereby transitions from the 2s²3s²S_1/2 level were underpredicted. They demonstrated that using the Aggarwal & Keenan (2008) atomic data fixes this problem, and so the new CHIANTI model resolves this issue. This has consequences for studies of the λλ279.63, 279.93 emission lines in Hinode/EIS spectra (Culhane et al. 2007; Muglach et al. 2010) which are emitted from this level.

The N ii lines observed in the visible provide a useful density diagnostic at densities above 10⁴ cm⁻³. The previous model for N ii included 23 fine-structure levels and was based on the effective collision strength calculations of Stafford et al. (1994). The new model includes 58 fine-structure levels. Observed energies and wavelengths are taken from the compilation of NIST (version 5; Kramida et al. 2012). In addition, two observed wavelengths for transitions within the 2s²2p² configuration and one transition between the 2s²2p² and the 2s2p³ configurations are taken from the analysis of Young et al. (2011). The A-values are from Tayal (2011) with the exception of the forbidden transitions within the ground configuration, which are from Tachiev & Froese Fischer (2001). In addition, wavelengths for three transitions within the ground configuration are taken from Young et al. (2011). The effective collision strengths were calculated by Tayal (2011) over a temperature range of 5.0 × 10² K to 10⁵ K.

The O ii lines at 3729 Å and 3726 Å provide an important density diagnostic ratio that is often used in the analysis of astrophysical plasmas such a planetary nebulae (Wang et al. 2004). The previous model for O ii consisted of 15 fine-structure levels. The current model has been expanded to 35 fine-structure levels with observed energy levels and wavelengths taken from the NIST compilation (version 4; Ralchenko et al. 2011). The A-values for the ground configuration were calculated by Zeippen (1987) and those for allowed transitions were calculated by Tayal (2007). Effective collision strengths were calculated by Tayal (2007) over a temperature range from 2 × 10³ K to 10⁵ K. Following the suggestion of Kisielius et al. (2009), the collision strength indices for levels 4 and 5 were reversed.

The radiative rates included in the previous version of CHIANTI for Ca xiv were taken from Bhatia & Landi (2005). These have been retained, but an error was corrected in the numbering of the levels in the WGFA file that caused the rates of levels 48 and 52 to be exchanged, and those of level 49 to be omitted from the file itself. All the rest of the data are unchanged.

Ca ix data were left unchanged since the first release of CHIANTI in 1996, and consisted of a 16-level model where all data were taken from the DW calculation of Christensen et al. (1986). The entire data set has been superseded in CHIANTI version 7.1 by the FAC calculations of Landi & Bhatia (2012), who provided the energy levels, A-values, and collision strengths between the 283 fine-structure levels of the n = 3, n = 4, and n = 5 configurations.

Energy levels were taken from a number of sources (Parker & Phillips 1940; Fawcett 1970, 1976; Ekberg 1971; Edlen & Boden 1976; Litzen & Redfors 1987; Redfors 1988; Churilov et al. 1989; Curdt et al. 2004). Collision strengths were calculated at six values of the incident electron energy ranging from threshold to up to 57.9 Ry. Only the collision data for transitions originating in the lowest four levels in the atomic model (corresponding to the ground level and the 3s3p³P triplet) have been retained, as they were the only ones relevant for the level population calculation for astrophysical applications.

The version 7.0 data for levels in the 3s²3p⁶nl (n = 5, 6, l ⩽ n − 1) and 3s²3p⁶7l' (l' = s, p, d, f) configurations have been replaced with the calculations from O'Dwyer et al. (2012).

Experimental energy levels come from a combination of measurements from NIST (version 2; Martin et al. 1999), Ekberg & Feldman (2003), Young & Landi (2009), Del Zanna (2009), and Landi & Young (2010). These have been used to determine a correction to predicted energies which has been used to calculate improved wavelengths for transitions where experimental energy levels were not available.

The R-matrix calculation by Griffin et al. (2000) was retained for n = 3, 4 levels. As shown in Del Zanna (2009), significant limitations in the target adopted by Griffin et al. (2000) were present. To improve them, the collision strengths of the dipole-allowed transitions were scaled by Del Zanna (2009) according to the ratio of the gf values. These scaled values were also adopted in the previous version 7.0 of CHIANTI. Radiative data for the n = 3, 4 levels are taken from Del Zanna (2009).

Collisional and radiative data were calculated by O'Dwyer et al. (2012) for levels up to n = 7 using autostructure. These replace old calculations from Czyzak & Krueger (1966).

The CHIANTI model for Fe ix was expanded to include many more configurations and levels. The original model included the 3s²3p⁶, 3s²3p⁵3d, 3s3p⁶3d, 3s²3p⁴3d², and 3s²3p⁵4s and 4p configurations for a total of 140 fine-structure levels, whose data were taken from the calculations of Storey et al. (2002). The present model adds the 3s3p⁵3d², 3p⁶3d², 3s²3p⁵nl (n = 4–6, l ⩽ n − 1), and 3s3p⁶4l' (l' = s, p, d, f) configurations, reaching 379 fine-structure levels.

The data from Storey et al. (2002), already used in the previous version of CHIANTI, were retained for all levels, with the only exception of radiative data for the 3s²3p⁵4s and 4p configurations which are replaced by the Landi & Young (2009) calculation.

The Fe ix energy level and radiative data files were also updated with revised energies and wavelengths for transitions involving levels 3s²3p⁴(¹D)3d^{2 3}D_{1, 2, 3} (with indices 110–112 in the CHIANTI model). The energies are from Young & Landi (2009) who gave new experimental energies for levels 110 and 111, and an updated theoretical energy for level 112. All other experimental energies for the new levels come from version 3 of NIST (Ralchenko et al. 2005). In the benchmark study of Del Zanna (2012a) some discrepancies between predictions and observations (from Manson 1972) in several Fe ix lines were noted, and alternative identifications have been proposed. New laboratory measurements of these lines will be needed to assess which identifications are correct. Observed energies have been used to calculate corrections to the energies of levels where no experimental entry was available; these corrected energies have been used to calculate improved values of unobserved wavelengths.

Radiative data and DW collision strengths have been taken from the autostructure calculations of O'Dwyer et al. (2012). Recently, also Foster & Testa (2011) carried out Fe ix DW calculations using the FAC code. The actual collision strengths obtained by O'Dwyer et al. (2012) with autostructure and those from FAC were very similar (within 10% in most cases), although some differences in the line intensities were present, due to different modeling.

The new CHIANTI model for Fe x includes 825 fine-structure levels from 27 configurations. Energy levels, A-values, and collision data for the lower 54 levels come from version 7.0 of CHIANTI, and consist of the same R-matrix calculation from Del Zanna et al. (2004) described in Landi et al. (2006).

Data for the additional levels 55–825 were calculated by Landi & Dere (2013) and are from the 3s²3p³3d², 3s²3p²3d³, 3s3p⁵3d, 3s3p⁴3d², 3s3p³3d³, 3p⁶3d, 3p⁵3d², 3s²3p⁴4l, 3s²3p³3d4l, 3s3p⁵4l, and 3s²3p⁴5l' (with l = s, p, d, f and l' = s, p, d, f, g). A much larger set of configurations was adopted by Landi & Dere (2013) for the Fe x target, which included 31,598 levels from the complete n = 3 complex, as well as from the n = 4, 5 and n = 6 complexes.

Observed energy levels are taken from Fawcett et al. (1972), with additional levels from Del Zanna et al. (2012a) and Del Zanna (2012a). Differences between observed and calculated levels have been used to determine a correction to the predicted energy of levels for which an experimental energy was not available; the resulting corrected energies were used to calculate improved wavelengths. A-values were calculated for all transitions within the 825 levels. Collision strengths were calculated for six values of the incident electron energy ranging from threshold to up to 202 Ry; we retain in CHIANTI the collisional data for transitions originating in the ground 3s²3p^{5 2}P doublet as well as from the metastable levels in the 3s²3p⁴3d configuration, whose population is large enough to influence the overall ion population in astrophysical plasmas.

The Fe xi model has been extended from 365 to 999 fine-structure levels with the addition of 3s²3p3d³, 3s3p²3d³, 3p⁴3d², 3s²3p³4l (l = p, d, f), 3s³3p²3d4s, and 3s3p⁴4l' (l' = s, p, d, f). The lowest levels of some of these configurations had lower energies than the highest levels in the version 7.0 model, so that some renumbering was necessary. However, all the version 7.0 data available for the 365 levels in the version 7.0 model have been retained.

New data for the additional levels have been taken from the FAC calculations of Landi & Dere (2013). The Fe xi atomic target model adopted by Landi & Dere (2013) included the complete set of configurations in the n = 3 and n = 4 complexes, as well as the 3s²3p³5l configuration, for a total of 38,850 levels. The observed energies for these configurations were taken from Fawcett et al. (1972); additional energy levels have been taken from Del Zanna (2012a). Differences between observed and calculated level energies have been used to determine a correction to the predicted energy of levels for which an experimental value was not available; the resulting corrected energies were used to calculate improved wavelengths. A-values have been computed for all transitions within the additional levels, and between the latter and those in the configurations already included in version 7.0 of the database.

Collision strengths have been calculated at six values of the incident electron energy, ranging from threshold to up to 90 Ry using the DW approximation. Effective collision strengths have been calculated and retained only for transitions originating from the ground configuration and from the some metastable levels with significant population at log N_e = 12.0: 3s²3p³3d⁵D₄, ³G_{4, 5}, ¹G₄, (²D)³F₄, and (²P)³F₄.

The version 7.0 model of Fe xii included 143 levels from the lowest six configurations in the n = 3 complex. Version 7.1 extends it to 898 fine-structure levels by including additional levels from 28 additional configurations: 3s²3d³, 3s3p²3d², 3s3p3d³, 3p⁴3d,3p³3d², 3s²3p²4l, 3s²3p3d4l, 3s3p³4l (l = s, p, d, f), and 3s²3p²5l' (l' = s, p, d, f, g). The new Fe xii model retains all the data from version 7.0 for the lowest 143 levels, and uses the Landi & Dere (2013) calculations for the additional levels.

Landi & Dere (2013) used FAC on an Fe xii model which included the complete n = 3–5 complexes, for a total of 33,680 fine-structure levels. Experimental energies for the lowest 41 levels are the same as in version 7.0, coming from Del Zanna & Mason (2005); those for the higher-energy configurations are taken from Fawcett et al. (1972), with additional values from Del Zanna (2012a) and Del Zanna et al. (2012b). The differences between experimental and calculated energies were used to determine a correction to the energy of all other levels; corrected energies were used to calculate transition wavelengths. A-values were provided by Landi & Dere (2013) for all transitions included in the present version of CHIANTI.

Collision strengths were calculated at six values of the incident electron energy ranging from threshold to 141 Ry using the DW approximation. Effective collision strengths were calculated and retained for all levels within the ground configurations.

The 7-configuration, 114-level model available in version 7.0 has been expanded to include data from 34 additional configurations: 3s3p3d², 3s3d³, 3p²3d², 3p3d³, 3s²3p4l, 3s²3d4l, 3s3p²4l, 3s3p3d4l, 3p³4l, 3s²3p5l', and 3s3p²5l' with l = s, p, d, f and l' = s, p, d, f, g. The new CHIANTI model includes 950 fine-structure levels. The data in version 7.0 for the lowest 114 levels have been retained.

Experimental values for the new energy levels were taken from Fawcett et al. (1972); additional values were taken from Del Zanna (2012a) and Del Zanna & Storey (2012). A correction to the calculated energies was determined using the difference between the available experimental values and their theoretical counterpart; the corrected energies were used to provide more accurate wavelengths for the transitions involving the unobserved levels. The target ion model adopted by Landi & Dere (2013) included the entire n = 3–5 complexes, as well as the additional configurations 3s²3pnl, 3s²3dnl, and 3s3p²nl, with n = 6, 7 and l ⩽ n − 1. A-values were provided for all transitions involving the 950 levels in the ELVLC file.

Collision strengths were calculated by Landi & Dere (2013) using the FAC code and the DW approximation. Six values of the electron energy ranging from transition threshold to 119 Ry. Effective collision strengths were calculated and retained for transitions originating in the five levels of the ground configuration, as well as from metastable level 18 (3s²3p3d³F₄).

The version 7.0 model of Fe xiv, including data from 16 configurations and 197 fine-structure levels from Liang et al. (2010), has been expanded to include additional 542 levels from 39 additional configurations: 3s3p4f, 3s3d4l, 3p²4l, 3s3d4l, 3s3p5l', 3s3p6s, and 3s²nl'', where l = s, p, d, f, l' = s, p, d, f, g, n = 4–7, and l'' ⩽ n − 1. The total number of levels in CHIANTI is now 739. All the data in the previous version of CHIANTI have been retained.

Experimental energy levels for the additional configurations were not available, while we added the value for levels 122, 125, and 136, where the new identifications proposed by Del Zanna (2012a) have been included. The lack of observed energies in the additional configurations has prevented the calculation of corrected wavelengths.

All data for the additional configurations come from Landi & Dere (2013), who used FAC with an Fe xiv atomic model that included the entire n = 3 to n = 7 complexes, for a total of 5079 levels. A-values were calculated for all transitions involving the new configurations. Collision strengths for the new levels were calculated by Landi & Dere (2013) using the DW approximation at six incident electron energies ranging from transition threshold to 256 Ry. Effective collision strengths were calculated and retained for transitions originating in the ground configuration.