Benchmarking atomic data for astrophysics: Fe VII and other cool lines observed by Hinode EIS

G. Del Zanna

doi:10.1051/0004-6361/200913082

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Volume 508 / No 1 (December II 2009)

A&A, 508 1 (2009) 501-511

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Issue		A&A Volume 508, Number 1, December II 2009


Page(s)		501 - 511
Section		Atomic, molecular, and nuclear data
DOI		https://doi.org/10.1051/0004-6361/200913082
Published online		24 September 2009

A&A 508, 501-511 (2009)

Benchmarking atomic data for astrophysics: Fe VII and other cool lines observed by Hinode EIS

G. Del Zanna

DAMTP, Centre for Mathematical Sciences, Wilberforce road Cambridge CB3 0WA, UK

Received 7 August 2009 / Accepted 11 September 2009

Abstract
The EUV spectrum of Fe VII is reviewed, using new rates for electron impact excitation, atomic structure calculations, and experimental data. In particular, solar observations of a sunspot loop spectrum obtained from the Hinode EUV Imaging Spectrometer (EIS) are used. Previous line identifications, mostly based on laboratory data, have been assessed. Large discrepancies between observed and predicted line intensities and wavelengths are found for the decays from the 3s² 3p⁵ 3d³ configuration, which are strong EUV lines. We ascribe these discrepancies to incorrect line identifications. A number of new identifications are proposed. With these, very good agreement between theory and experimental data is found. A few transitions, in particular from the 3s² 3p⁶ 3d 4s configuration, are observed for the first time, and are shown to provide a new important diagnostic for measuring the electron temperature in the solar transition region. The temperatures obtained at the base of solar coronal loops are found to be close to the temperature of maximum abundance in ionization equilibrium ( $\log T [$ K]= 5.4). The assessment of the Fe VII lines was done in conjunction with an assessment of all the strongest cool lines observed with EIS. This spectrum is rich in transition region lines. Some new identifications are presented, in particular for Fe IX. Most of the strongest transitions are identified, however a large number of lines still awaits firm identification.

Key words: atomic data - line: identification - Sun: transition region - techniques: spectroscopic

1 Introduction

This paper is one in a series where atomic data and line identifications are benchmarked against experimental data (Del Zanna et al. 2004, Paper I). A substantial amount of work has been devoted in the literature to the study of the visible and UV transitions of Fe VII, however little has been done on the EUV spectrum. Witthoeft & Badnell (2008, hereafter WB08) have recently performed a large electron scattering calculation for this ion as part of the Iron Project and the UK Rmax network (now superseded by the UK APAP network). For a description of previous work on electron impact excitation for this ion see WB08. The new rates, together with the new accurate Hinode EUV Imaging Spectrometer (EIS, see Culhane et al. 2007) observations, provide the opportunity to study in detail many of the strongest lines in the EUV spectrum of Fe VII. The aim of this paper is to reassess previous identifications, and suggest which lines are best for diagnostic purposes. This paper complements a similar paper (Del Zanna 2009) where the EUV spectrum of Fe VIII is discussed, using the same EIS observation.

In this paper, we focus on the n=3,4 lower configurations which produce the strong EUV transitions. Contrary to many other ions, very little experimental work has been done on Fe VII EUV lines. The identification of Fe VII EUV lines started with Fawcett & Cowan (1973, hereafter FC73), who identified five among the strongest transitions from the 3s² 3p⁵ 3d³configuration, using a laboratory vacuum spark spectrum and atomic structure calculations. Ekberg (1981, hereafter E81) also used a vacuum spark source and Hartree-Fock calculations to identify an impressively large (more than 400) number of lines, and many levels, in particular of the 3s² 3p⁵ 3d³ and 3s² 3p⁶ 3d 4p configurations. A number of UV lines were also identified. Later, levels of the 3s² 3p⁶ 3d 4d configuration were identified by Ekberg & Feldman (2003) using UV lines in a very nice piece of work.

To date, Ekberg's is the only work on the Fe VII EUV lines. Wavelengths were very accurate, to within a few m Å. The original spectra, which contained large numbers of lines, were not published, so for a number of cases it has not been possible to confirm identifications. The spectra contained lines from nearby ionization stages, so it is always possible that some of the lines identified by E81 were not due to Fe VII. Most level energies were obtained from various wavelength coincidences among decays to the levels of the ground configuration, which have been known with high accuracy, so at first it seemed that all of Ekberg's identifications must have been correct.

However, the benchmark iterative procedure has highlighted a number of problems with Ekberg's work. First, it turns out that a number of lines among those with largest gf values were not identified. This includes the line with by far the largest value. Second, for a number of levels, observed energies are very far from those predicted theoretically, based on level splittings. Third, for some levels, deviations from the ab-initio calculated energies are unreasonably large. A detailed assessment for each of the strongest spectral lines had to be done, and is described below. A number of new identifications are presented, while a number of uncertain ones are also suggested.

The assessment of the Fe VII lines was done in conjunction with an assessment of all the strongest transition region (TR) lines observed with EIS, which is also presented in this paper.

2 Atomic structure for Fe VII

As for Fe VIII, it is particularly difficult to obtain ab-initio level energies that match the observed ones for this ion. Configuration-interaction (CI) and mixing effects are also large, as described in WB08. For these reason, it has been particularly difficult to obtain firm identifications for Fe VII. Relativistic multi-reference many-body perturbation theory calculations such as those described in Ishikawa & Vilkas (2008) are needed, since they have been proved to provide very good level energies. Also, new experimental data would be useful to confirm the identifications proposed here.

Fortunately, mixing effects turn out to be not as important as it was the case for Fe VIII (Del Zanna 2009). This has been assessed by running various atomic structure calculations using the AUTOSTRUCTURE code (Badnell 1997). A basis which reproduced well all the levels from the main spectroscopically-important configurations could not be found. As a ``benchmark'' structure calculation, we chose the large 40-configuration basis described by WB08, with the same scaling parameters. To improve the level energies, term energy corrections (TEC) (see, e.g. Zeippen et al. 1977; Nussbaumer & Storey 1978) to the LS Hamiltonian matrix were applied, using the iterative procedure described in Paper I.

The corrections to the LS term energies were estimated from the weighted mean of the observed level energies, whenever available. Most TEC values for the important 3d³ configuration were only about 13 000 cm^-1, so this correction has been applied to all the LS terms from this configuration for which no experimental energies were available. The energies of this benchmark calculation ( $E_{\rm Bench.}$ ) are shown in Table 1. They are to be compared to the new experimental energies presented here ( $E_{\rm Exp.}$ ), together with those from the scattering calculation and those from the NIST database, which are derived from Ekberg's work for the 3s² 3p⁶ 3d 4s, 3s² 3p⁵ 3d³ configurations. The TEC iterative method has been essential to establish which spectral lines had correct identifications and which did not. Also, it has been used as a check for the validity of the scattering target. The target basis chosen by WB08 turns out to be quite accurate, given the complexity of this ion. In particular, the relative energies between strongly-mixed levels are close to the observed ones. This is reflected by the oscillator strengths. This is reassuring, and confirms the accuracy of the target adopted by WB08. Table 2 lists the weighted oscillator strengths (gf) for the strongest dipole-allowed transitions, compared to the WB08 values.

Line intensities were calculated with the WB08 rates and the transition probabilities from the benchmark (+TEC) calculation (adopting the WB08 probabilities changes the intensities by only up to 10%). We assumed plasma equilibrium conditions, and an electron density of 10⁹ cm^-3, typical for loop legs (Del Zanna & Mason 2003). The line intensities, listed in Table 2 in decreasing order, were calculated at the temperature $\log T [$ K] = 5.4. This is the temperature of peak ion abundance for Fe VII in ionization equilibrium, according to the latest ionization and recombination rates published within CHIANTI v.6 (Dere et al. 2009,1997).

All the identifications of the strongest lines have been checked, using laboratory and solar spectra, as described in the following Section. Line intensities, whenever available, were compared, in order to confirm identifications and assess the possible presence of blending. The results are also shown in Table 2.

Table 1: Level energies for Fe VII.

Table 2: List of the strongest Fe VII EUV lines in the 160-295 Å range.

3 Experimental data

One of the original plates from B.C.Fawcett was found to contain strong transition region lines, mostly from Fe VIII, Fe IX. The plate was scanned, and an averaged spectrum wavelength-calibrated. Lines from different ionization stages of Iron are present, as well as other C, O lines. This spectrum was used as an aid in the identification process, in particular for the wavelengths not observed by Hinode EIS. All the Fe VII lines with large gf values observed by EIS and by E81 were also observed in this spectrum. The E81 and Hinode EIS wavelengths are far more accurate than those of this spectrum, so they have been used. In a few cases, new tentative identifications based on this plate are proposed (see Table 2 for the spectroscopic identification). The firm identifications proposed here are based on the Hinode/EIS data, in particular on morphology, line intensity and wavelength.

$\begin{figure} \par\includegraphics[width=14.5cm,clip]{H13082_f1a.ps}\par\vspace*{2mm} \includegraphics[width=14.5cm,clip]{H13082_f1b.ps} \end{figure}$	Figure 1: Top: monochromatic images (negative) of the strongest Fe VII lines observed by EIS. Notice that all the Fe VII lines have a similar morphology. Also displayed are a few lines which have the same morphology as Fe VII but are considered as unidentified (u VII). A few lines formed over a range of temperatures are also displayed.
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$\begin{figure} \par\mbox{\includegraphics[width=5.5cm,clip]{H13082_f2a.ps}\hspace*{4mm} \includegraphics[width=6.1cm,clip]{H13082_f2b.ps} }\end{figure}$

Figure 2:

Left: doppler-gram of the Si VI 246.0 Å line showing strong (30 km s^-1) red-shifts in the legs of a fan of coronal loops anchored in a sunspot. The location of the area chosen to obtain averaged spectra from a sunspot loop leg is indicated by the crossing of the two sets of dashed lines. Right: doppler-shifts in a few lines formed at similar temperatures, along the N-S direction indicated by the dashed lines in the left figure. At the location of the loop leg, all lines are red-shifted by about 30 km s^-1.

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$\begin{figure} \par\includegraphics[width=15.5cm,clip]{H13082_f3.eps} \end{figure}$	Figure 3: Hinode EIS spectra (units are averaged counts per pixel) relative to two different areas. Thick lines refer to the spectrum over the sunspot leg, where transition region lines are much enhanced. The thick red line shows the foreground sunspot spectrum.
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Table 3: List of measured transition-region emission lines from the foreground-subtracted spectrum of the sunspot loop.

The Hinode/EIS instrument covers two wavelength bands (SW: 166-212 Å; LW: 245-291 Å). Here we consider a long-exposure (90 s) observation which started on 2007 Jan. 5 at 21:52 UT and observed a sunspot and various loops. A complex data processing, which included various geometrical corrections and a wavelength calibration procedure was applied to the data, as described in detail in Del Zanna (2009). More than 200 lines were fitted with Gaussian profiles using the cfit package (Haugan 1997), and their morphology examined in detail, one by one.

Figure 1 shows the resulting monochromatic images for a selection of Fe VII and other lines, to show how sensitive morphology is to the different ion stages. This allows to estimate the temperature of formation for the strongest unidentified lines, and to assess if/when Fe VII lines are blended.

A spectrum over an area in a sunspot loop leg was chosen for the benchmark. The area is indicated by the crossing of the two sets of dashed lines in Fig. 2. A ``foreground'' spectrum was subtracted, to remove the small contribution from coronal lines. The resultant spectrum has a wavelength uncertainty of about 5 m Å and very strong cool lines. The little coronal contamination inside the sunspot, and the foreground subtraction means that each feature in this spectrum can only be produced by a spectral line formed at transition-region temperatures. The only high-T residual emission is from Fe X, which is formed around 1 MK. Lines formed above 1 MK are not present. A sample of spectral windows from the sunspot loop and the foreground spectrum is provided in Fig. 3.

Table 3 provides the list of the strongest lines present in this spectrum, with their measured wavelengths $\lambda_{\rm o}$ and intensities. Notice that both the intensities in terms of total counts in the lines are given, as well as the calibrated ones. This was done to highlight the fact that many intrinsically-weak spectral lines which fall near the peak sensitivity of the channels do actually have large count rates.

As shown in Del Zanna (2007,2008), the legs of active regions loops present strong red-shifts, increasingly larger for lines formed at lower temperatures. The pattern is clearly shown in Fig. 2. The sunspot leg area selected presents a red-shift of about 20 km s^-1 in lines from Fe VIII, Si VII, Mg VII (Del Zanna 2009), while lines from Fe VII, Si VI, Mg VI are red-shifted by about 30 km s^-1, as shown in Fig. 2. Lines from higher-T such as those from Fe IX were red-shifted by only 10 km s^-1, while those at lower T by about 35 km s^-1. The corrections for these red-shifts have been applied to the measured $\lambda_{\rm o}$ to obtain the ``rest'' wavelengths $\lambda_{\rm c}$ (Å), also shown in Table 3. This was done in all cases when a line had an established formation temperature. The overall cumulative uncertainty on the $\lambda_{\rm c}$ values is estimated to be about 5 m Å. Many values are within a few m Å from the literature values, also shown in Table 3. This agreement is remarkable. The table also clearly indicates that a considerable number of rest wavelengths, in particular for Si VII, need to be revised.

4 Fe VII line identifications

For the line identification, we make use of the ``emissivity ratio'' technique, whereby the observed intensity of a line is divided by its emissivity (as a function of electron temperature or density) and by a normalization factor. This allows, in one single plot, to assess at once for a group of lines how good observed vs. theoretical intensities are (see Del Zanna et al. 2004 for details).

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f4a.ps}\pa... ...*{2mm} \includegraphics[angle=90,width=7.5cm,clip]{H13082_f4b.ps} \end{figure}$

Figure 4:

The emissivity ratio curves relative to some of the strongest Fe VII EUV transitions observed in the ``foreground-subtracted'' sunspot loop leg by Hinode EIS. The curves were calculated at $\log N_{\rm e}$ [cm^-3] =9. $I_{\rm ob}$ : observed intensity; bl: blend; sbl: self-blend; N: new identification proposed here. Top: using the previous identifications from E81. Bottom: using the present identifications.

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4.1 3s ² 3p ⁶ 3d ²-3s ² 3p ⁵ 3d ³ transitions

The emissivity ratio curves for the stronger lines are given in Fig. 4, while those for the weaker ones in Fig. 5. The same normalization factor was used.

The strongest line (3-135) is the main decay from a highly-mixed level, with a dominant component originating from the ³F term. It was correctly identified by FC73, together with the two other main decays from levels originating from the same term, the 2-133 and 1-131 transitions, among the top brightest lines. There is excellent agreement between predicted and observed intensities of these three transitions, once blending is taken into account. The 2-133 176.927 Å line is blended with a strong Fe IX line, while the 1-131 177.171 Å with a strong well-known Fe X (Del Zanna et al. 2004).

The second strongest line (3-148) was also correctly identified by FC73, together with the other main decay from a level originating from the same ³D term (2-147).

The third brightest transition (8-150) was identified by E81 with a line observed at 173.442 Å, and 2 coincidences. The identification of this decay from the ¹G₄ is inconsistent with the atomic data and the observations. Hinode observed a TR line which could be this transition, but with a lower intensity than predicted, and a wavelength of 173.434 Å. The predicted intensity is on firm grounds. The benchmark calculation confirms the gf value of WB08, indeed the level is not highly mixed. FC73 did suggest an alternative identification, a line observed at 167.6 Å (not observed by E81). However, the energy of the ¹G₄ would be very far from the predicted one. It is possible that the identification of this strong line has been hindered by blending with another strong line. We give a tentative suggestion that it is the strong Fe IX 171.077 Å.

The identification from E81 of the fourth strongest transition (3-110) with the 195.388 Å line must also be incorrect, on various grounds. Firstly, the 195.388 Å line has a morphology close to Fe VIII, and not Fe VII, as Fig. 1 shows. Secondly, the predicted intensity does not match the observed one. Thirdly, the predicted energy splitting between level 110 and those mainly originating from the ³G term is inconsistent with the energies of levels 108 and 106. Level 110 is very mixed, however the benchmark calculation gives gf=2.9, close to 2.99, the value pertinent to the scattering calculation, hence the predicted intensity of the 3-110 line should be accurate. The main decay from level 108 (2-108) was identified by E81 with the 196.243 Å line, while the main decay of the (relatively pure) 106 level (³G₃) was identified by E81 with the 196.045 Å line. The benchmark calculation gives a gf value for the 1-106 transition in good agreement with that from WB08, while a lower value is found for the 2-108 transition. The same calculation also suggests, based on the predicted splittings between these levels, that the three transitions 3-110, 2-108, 1-106 should be identified with the lines observed by Hinode EIS at 196.043, 196.209, 197.364 Å respectively. Notice that E81 identified the 196.045 Å line with the 1-106 transition instead. The predicted intensities for these three lines are in excellent agreement with the observed ones, as shown in Fig. 4. Notice that the 196.209 Å is a self-blend, and the 197.364 Å is blended with a strong Fe VIII line (see Del Zanna 2009). The previous identifications, on the other hand, are inconsistent with the atomic data, as Fig. 4 shows.

E81 did not identify the 8-153 transition, predicted to be the fifth strongest, and that one which has the largest gf value of all the EUV lines. Level 153 is highly mixed, however the benchmark calculation suggests that a value of gf=10 is correct. The only plausible explanation for E81 not having identified the strongest line is that it was blended in the spectrum. The only reasonable candidate from E81's list is the strong 165.087 Å line, blended with the 2-147 transition, possibly still blended with the weak 9-177 (gf=1.7) transition, which was the original identification.

The next strongest line was not identified by E81. It is the main decay from the ¹H₅ (8-116). The level is not highly mixed and the predicted intensity should be accurate. This line must be strong in the EIS spectrum. It is identified here with the 196.209 Å self-blend, on wavelength and intensity grounds.

Levels 19 and 20 were not identified by E81, despite producing transitions of equal strength (3-19, 3-20). Given their strength, these lines ought to be well observable by EIS. A search through the reasonable spectral region suggest that these two lines are observed at 260.676 and 254.051 Å. No other possibilities were found. The 254.051 Å line is too strong to be blended with either of the Fe VIII 253.95, 255.101 Å lines (Del Zanna 2009).

The following levels have a slightly more uncertain identification. Level 118 (³D₂) was identified by E81 with 4 coincidences, the strongest transition being the 6-118, identified with a line observed at 189.450 Å. The predicted intensity is weaker than the observed one from Hinode EIS, however this could be due to blending with another TR line. If this identification is correct, this means that the positioning of the ³D term is known. The main other level, very mixed, originating from this term is level 119. The main transition is the 4-119, identified by E81 at 185.547 Å. Morphology and intensity measured from Hinode EIS are in good agreement with the predicted one, however the splitting of the ³D term predicts a wavelength 2.5 Å away. An alternative suggestion, which gives TEC in agreement with the others, the correct splitting and the correct intensity is given instead: the 4-119 is blended with the Fe XII self-blend (first identified by Del Zanna & Mason 2005). This self-blend is of particular importance because is one of the main density diagnostics for EIS. The 6-118 is identified with a self-blend observed at 188.18 Å, and normally blended with various other transitions, the main one being from Fe XI (Del Zanna et al. 2009).

The 7-134 line, identified by E81 at 183.825 Å using four coincidences, is twice as strong as predicted, in the EIS spectra. An alternative, which gives good TEC agreement and good match in intensity is the 182.133 Å line, normally blended with an Fe XI transition.

The 4-121 line was identified by E81 at 186.656 Å. This line would be blended with a much stronger Fe VIII transition. Again, TEC and intensity arguments suggest that this transition is the 183.823 Å line instead.

The mixed levels 97, 98, 101, originating from a ³G term, produce the 1-101, 3-97, 2-98 transitions, all of similar strengths and observable by EIS. The identifications proposed by E81 (211.931, 212.663, 207.712 Å) cannot be reconciled with the predicted splittings. A good match in both wavelengths and intensities is found for the three lines observed at 206.754, 209.731, 208.823 Å. The 212.664 Å line was observed by EIS, but is assigned to the 3-93 transition, again on wavelength and intensity grounds.

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f5.ps} \end{figure}$	Figure 5: The emissivity ratio curves relative to some of the weaker Fe VII EUV transitions observed by Hinode EIS. The curves were calculated at log $N_{\rm e}$ [cm^-3] =9.
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4.2 3s ² 3p ⁶ 3d ²-3s ² 3p ⁶ 3d 4p transitions

These transitions fall around 240 Å and are not observable by Hinode EIS, however all the identifications proposed by E81 appear correct. All levels have similar TEC, and all observed energy splittings are in good agreement with theory.

4.3 3s ² 3p ⁶ 3d ²-3s ² 3p ⁶ 3d 4s transitions

The few decays from the 3s² 3p⁶ 3d 4s configuration are of particular importance because they offer a good temperature diagnostic, when observed with the decays from the much higher 3d or 4p levels. The energies of the 3s² 3p⁶ 3d 4s levels were obtained by E81 indirectly, from various UV lines of the transition array 3d 4s-3d 4p, and knowing the energies of the 3d 4p levels. For the first time, EIS has observed these decays. The lines are not very strong because they are near the edge of the detector, where the sensitivity is low, however are well observed. Their wavelengths are in very good agreement (within uncertainty) with the values predicted by E81. The 1-10 is observed at 290.303, rather than 290.307 Å. The 1-11 at 289.838 Å, instead of 289.831 Å. The 3-12 is blended with the 2-11, which should be at 290.724 Å. E81's energies predict a wavelength of 290.756 Å for the 3-12 line, in good agreement with the observed blend at 290.748 Å.

5 Benchmarking other ions

For the following benchmark the atomic data in CHIANTI v.5.2 (Landi et al. 2006) were used.

5.1 Fe IX

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f6.ps} \end{figure}$	Figure 6: The emissivity ratio curves relative to the Fe IX EUV transitions observed by Hinode EIS calculated at log $N_{\rm e}$ [cm^-3] =9. N are new identifications, while T N are tentative new ones.
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The emissivity ratio curves relative to the Fe IX EUV transitions observed by Hinode EIS are shown in Fig. 6. The strongest is the resonance 1-13 171.07 Å line, which is the strongest EUV line in quiet Sun conditions. For EIS, this line is close to the edge, where the sensitivity is low, so long exposures are required to obtain a good measurement of the line. Young (2009) used the atomic data produced by Storey et al. (2002) (present in CHIANTI v.5) to identify four new lines from the 3s² 3p⁴ 3d² configuration. The three main decays from the ³G_4,5,3 were identified with the lines observed at 189.941, 188.497, 191.216 Å. These identifications are confirmed on intensity and wavelength grounds. As shown by Young (2009), the combination of one of these lines with the resonance line provides an electron temperature diagnostic. A value of $\log T [$ K] =5.65 is obtained, which is significantly lower than the temperature of maximum abundance in ionization equilibrium. The sensitivity, however, is not very high, and a broad range of temperatures are consistent with the data. Further, the accuracy of the EIS ground radiometric calibration (used here) toward the edge at 171.07 Å is difficult to assess.

The main decay from the ³D₃ (4-86) is predicted to be a line well-visible by EIS. The energy difference between ab-initio energies and those of the ³D₃ predict that the 4-86 line should fall around 197.2 Å. The only line with the appropriate morphology is a line observed at 197.854 Å. There is excellent agreement between the predicted and observed intensity. The same line was instead identified by Young (2009) with the main decay from the 3s² 3p⁵ 4p (13-140), observed at 197.862 Å. There are no other observed levels from the 3s² 3p⁵ 4p configuration, so it is not easy to identify any lines originating from this configuration. If we assume for the 3s² 3p⁵ 4p the same correction as for the 3s² 3p⁴ 3d² to the ab-initio energies, we can estimate that the 13-140 transition should fall around 187 Å. The only viable candidate on intensity grounds is the line observed at 194.784 Å, which would be blended.

Using the same energy corrections, three further weaker transitions from the 3s² 3p⁴ 3d² configuration have been identified. These identifications should be treated as tentative, however excellent agreement between observed and predicted intensities is present as Fig. 6 shows.

5.2 Fe X

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f7.ps} \end{figure}$	Figure 7: The emissivity ratio curves relative to the Fe X EUV transitions observed by Hinode EIS calculated at log $N_{\rm e}$ [cm^-3] =9.
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The emissivity ratio curves relative to the Fe X EUV transitions observed by Hinode EIS are shown in Fig. 7. The identifications and the atomic data for these lines is presented in Del Zanna et al. (2004). It is interesting to notice that the three strongest transitions (1-30 at 174.53 Å, 1-28 at 177.24 Å blended with Fe VII, and the self-blend 257.26 Å line) have observed intensities in very good agreement with theory, and consistent with an isothermal plasma at $\log T [$ K] =5.4. However, this result is very uncertain, considering that temperature sensitivity is not very high, as it was the case for Fe IX. Also, that there is a density dependence in the 257.26 Å line above $\log N_{\rm e} =9$ . The other weaker lines appear to be blended in this spectrum.

5.3 Si VII and Mg VII

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f8a.ps}\pa... ...*{2mm} \includegraphics[angle=90,width=7.5cm,clip]{H13082_f8b.ps} \end{figure}$	Figure 8: The emissivity ratio curves relative to the Si VII and Mg VII EUV transitions observed by Hinode EIS, calculated at $\log T [$ K]= 5.8.
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The emissivity ratio curves relative to the Si VII and Mg VII EUV transitions observed by Hinode EIS are shown in Fig. 8. They were calculated at log T [K]= 5.8, however there is no temperature dependence for the lines considered here. The 2-8 line from Si VII is normally blended with a strong Fe XIV transition. The foreground subtraction leaves an intensity for the line which provides a measurement of the electron density of $\log\rm Ne = 8.8$ . Via a branching ratio, it is possible to estimate quite accurately, using the strong 1-6 transition, the intensity of the 2-6 line, which blends the Mg VII 3-14 line, which is an important density diagnostic for the EIS spectral range. This line gives a density of $\log \rm Ne = 9.5$ , when used in conjunction with the strong 4-15 line. There is disagreement with the Si VII measurement. The Mg VII branching ratio suggests that the 2-14 transitions should be 80% the intensity of the blend with Si VIII observed at 277. Å. There is another transition from Si VIII, observed at 276.85 Å, this time blended with the Si VII 276.84 Å, which intensity can be estimated accurately via another branching ratio with the strong 272.64 Å. The model for Si VIII however provides a disagreement of a factor of 2 between the two lines. This could be ascribed to a further TR line blending at 276.84 Å. In summary, more work needs to be done to properly assess Si VII and Mg VII lines before they can be used reliably.

5.4 O VI and O V

$\begin{figure} \par\includegraphics[angle=90,width=7.5cm,clip]{H13082_f9a.ps}\pa... ...degraphics[angle=90,width=7.5cm,clip]{H13082_f9b.ps} \vspace*{3mm} \end{figure}$	Figure 9: The emissivity ratio curves relative to the O VI and O V EUV transitions observed by Hinode EIS.
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The emissivity ratio curves relative to the O VI and O V EUV transitions observed by Hinode EIS are shown in Fig. 9. The density obtained from the O V lines is very close to what expected. Excellent agreement is found between observed and predicted intensities. This rules out the possibility of blending with other TR lines. This is an important issue for the 192.8 Å blend, where the Ca XVII resonance line and at least two other Fe XI lines are present. This is because the Ca XVII is the strongest line in an important temperature range, and efforts are on-going to estimate the Ca XVII from the observed blend (see, e.g. O'Dwyer et al. 2009).

5.5 Other ions

The strongest transitions from Cr VI, Cr VII, Cr VIII, Mn VII, Mn VIII, Mn IX, were identified by Gabriel et al. (1965) and are also observed in the EIS spectra.

6 Summary and conclusions

Spatially-resolved high-resolution solar spectroscopy is very valuable for line identification purposes. After very careful data analysis, Hinode EIS spectra can provide very accurate wavelengths (down to a few m Å) and line intensities. The overall benchmark of atomic data using a spectrum emitted at transition-region temperatures is very satisfactory. Most of the strong lines in the spectrum have been identified, and have good agreement (within 20%) between expected and observed intensities. Some wavelengths have been revised. However, a number of strong transitions still await firm identifications. Work is in progress to improve some of the atomic models for some ions. The fact that many lines are blended has been highlighted. Notice, however, that only blending with cool lines was considered here. Blending in other conditions is described in a follow-up paper.

A series of inconsistencies in the (otherwise excellent) work of Ekberg (1981) on Fe VII were found. The large-scale electron scattering calculation of Witthoeft & Badnell (2008) appears to be very accurate, so the inconsistencies in Ekberg's work could only be ascribed to mis-identifications. Further experimental data will be needed to confirm many of the identifications of the weaker lines that have been proposed here. Identifications along the sequence are being revisited.

A few important temperature diagnostics for the solar transition region and independent from the assumption of ionization equilibrium have been highlighted here. All the temperature diagnostics are consistent with loop legs close to being isothermal but at temperatures well below the peak ion abundance in ionization equilibrium for ions formed at upper transition region temperatures such as Fe VIII (Del Zanna 2009), Fe IX, Fe X. This is not surprising, considering that loops are radiatively-cooling structures of down-flowing plasma (Del Zanna 2008; Bradshaw 2008). A full study of this issue is the subject of a follow-up paper.

Acknowledgements

Support from STFC (Advanced Fellowship and APAP network) is acknowledged.
I warmly thank B.C. Fawcett for helping in identify some of the best plates of his archive.
The analysis of this Hinode/EIS observation was stimulated by the participation to the Coronal Loop Workshop held in Santorini in June 2007 and by various communications with M. Witthoeft.
The excellent Hinode Science Data Centre Europe was used to search the EIS database.
Hinode is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as domestic partner and NASA and STFC (UK) as international partners. It is operated by these agencies in co-operation with ESA and NSC (Norway).

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Footnotes

... database: http://physics.nist.gov/PhysRefData/ASD/index.html
... CHIANTI: www.chianti.rl.ac.uk

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