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Astron. Astrophys. 329, 291-314 (1998)

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10. Carbon iso-electronic sequence

The main EUV lines emitted by the C-like ions in the SERTS-89 spectral range originate from 2s-2p excitations of the ground 2s configuration. There are three main groups of lines that represent excitations from the [FORMULA]P levels up to the [FORMULA]S, [FORMULA]P and [FORMULA]D levels. For ions higher than magnesium the ground 3 P levels come into Boltzmann equilibrium at coronal densities, so there exist useful density diagnostic ratios involving lines within these groups. For the lower ions, density diagnostics need to involve a line from one of these groups together with one of either the [FORMULA]D-[FORMULA]D or [FORMULA]D-[FORMULA]P transitions, the latter generally lying close in wavelength to the [FORMULA]P-[FORMULA]S transitions, while the former lie closer to the [FORMULA]P-[FORMULA]P transitions.

10.1. O III

The two transitions reported in the Thomas & Neupert catalogue are 2p-3s transitions and are found at 374.05 Å and 374.16 Å (the 2s-2p transitions lie above 500 Å). CHIANTI, however, predicts a total of six lines in the 373-375 Å region at 373.80 Å, 374.01 Å, 374.08 Å, 374.16 Å, 374.33 Å and 374.44 Å. There are further complications, since N III lines are predicted at 374.20 Å, 374.43 Å and 374.44 Å, as well as a second order line of Fe XII at 373.77 Å!

To resolve this mess of lines, we will match the CHIANTI 374.08 Å line to the observed 374.05 Å line, and assume that the observed 374.16 Å line is a blend of the CHIANTI O III 374.16 Å and N III 374.20 Å lines.

With these identifications we can then make estimates of the other O III line intensities from the branching ratios and density insensitive ratios given in Tables 9 and 10. We note that the 373.80/374.05 ratio then predicts an intensity for the 373.80 Å line of around 5 erg cm-2 s-1 sr-1. This is used later in Sect. 13.4 on Fe XII.


[TABLE]

Table 9. Branching ratios for the C-like ions.


[TABLE]

Table 10. Insensitive ratios for the C-like ions.


10.2. Ne V

Three Ne V lines are reported in the SERTS-89 catalogue, although CHIANTI predicts another two lines at 357.95 Å and 365.60 Å. The 357.95 Å line is most likely a blend of the feature measured at 357.89 Å and identified by Thomas & Neupert as Ne IV. With the branching ratio in Table 9 we estimate a Ne V contribution of 5.3 erg cm-2 s-1 sr-1 ; however, this then implies a rather small Ne IV contribution to the observed total of 7.8 erg cm-2 s-1 sr-1. The remaining branching ratio, 358.46/359.38, is in excellent agreement with observations.

The predicted 365.60 Å line is important as it shows strong density sensitivity relative to the 359.38 Å line. A Fe X line is reported in the catalogue at 365.57 Å and is stronger than theory predicts, suggesting a blend (see Sect. 13.2 on Fe X ). Assuming the extra component is due to Ne V, we can estimate an intensity of 11 erg cm-2 s-1 sr-1 from the Fe X branching ratio given in Table  18. As a check on this we note that the 365.60/416.21 ratio is density insensitive, with a theoretical value given in Table 10 predicting a Ne V component of around 13 erg cm-2 s-1 sr-1 in excellent agreement with the previous estimate.

Either of the 365.60/359.38 and 416.21/359.38 ratios can be used as a density diagnostic as the 359.38 Å line is principally excited from the [FORMULA]P levels, whereas the 365.60 Å and 416.21 Å lines are excited from the [FORMULA]D level, which comes into Boltzmann equilibrium at around 10[FORMULA] cm-3. We use the 416.21/359.38 ratio to derive a density of 10[FORMULA] cm-3, considerably lower than expected. We note that if the calibration scale is altered as suggested in Sect. 15.1, the predicted density becomes 10[FORMULA] cm-3.

10.3. Mg VII

The 3 P ground levels are in Boltzmann equilibrium for densities in the range 10[FORMULA]-10[FORMULA] cm-3 so all lines excited from these levels will be density insensitive relative to each other. The strongest lines emitted from the multiplets 2s2p 3 S, 3 P and 3 D are 278.41 Å, 367.68 Å (which is a blend of two Mg VII lines) and 434.92 Å, respectively. A comparison of the SERTS-89 observations of these lines with theory is reported in Table  10.

The 278.41 Å line is clearly seen to be discrepant with theory. This can be partly explained by a Si VII line not identified by Thomas & Neupert that is predicted to lie at 278.44 Å. From Sect. 12.3, we can estimate a contribution of around 34 erg cm-2 s-1 sr-1 for the Si VII component, but this still leaves approximately 46 erg cm-2 s-1 sr-1 of the 278.41 Å line unaccounted for.

Estimating the 278.41 Å intensity as 34 erg cm-2 s-1 sr-1 (deduced from the 367.68/278.41 insensitive ratio whose theoretical value is given in Table 10), we can estimate a Mg VII contribution to the 277.05 Å blend of 20 erg cm-2 s-1 sr-1 from the branching ratio presented in Table 9. This estimate of the Mg VII contribution to 277.05 Å is used later in Sect. 11.3 on Si VIII. We also give in Table  9the branching ratio involving the 276.14 Å line, which is not identified in the catalogue.

Dwivedi et al. (1997) report the identification of the CHIANTI 276.14 Å line in the SERTS-89 spectrum at 276.148 Å, and the full details of this fit are given in Table  24. We note that the intensity of this feature would then imply that the original 278.41 Å intensity of 114 erg cm-2 s-1 sr-1 is correct, in contradiction of the above discussion. We thus suggest that the 276.148 Å is a blend to which Mg VII provides a component.

The insensitive 434.92/367.68 ratio is inconsistent with theory, with the 434.92 Å line seeming too weak. Again, this may simply reflect a problem with the instrumental calibration, although the extent of the discrepancy is not as marked as it is for Ne VI or Mg VIII, say.

The theoretical branching/density-insensitive ratios among the 360-370 Å and 429-435 Å lines show reasonable agreement with observations; in particular, discrepancies among the 429-435 Å lines are small. The line at 434.69 Å was not reported in the Thomas & Neupert catalogue but has subsequently been fitted. The details of the transition and fitted profile are provided in Table  24.

The 1 D-1 D and 1 D-1 P transitions mentioned earlier occur at 319.03 Å and 280.74 Å in CHIANTI and ratios taken relative to lines principally excited from 3 P levels are sensitive to densities below around 10[FORMULA] cm-3. Of this pair, the SERTS-89 catalogue identified only the line at 319.02 Å which is blended with a Ni XV transition. Dwivedi et al. (1997) give a fit to a line at 280.749 Å (which we reproduce in Table  24) and suggest that it is the other Mg VII transition. We give in Table  11the predicted density from the 280.75/367.68 ratio. An estimate of the Mg VII contribution to the line at 319.02 Å is made in Sect. 14.2, and we use this value of 51 erg cm-2 s-1 sr-1 in the 319.02/367.68 ratio to give the density in Table 11.

[TABLE]

Table 11. Density sensitive line ratios for the C-like ions.

10.4. Si IX

The [FORMULA]P-[FORMULA]S transitions are found in the 220-230 Å region and so were not observed by SERTS-89. Only the one branching ratio (344.96/341.97) can be resolved in the spectrum and it gives excellent agreement with observations - see Table 9.

The line reported in the catalogue at 296.137 Å is identified as a blend of two Si IX transitions, which CHIANTI gives at 296.11 Å and 296.23 Å so making them potentially separable. Indeed, careful analysis of the SERTS-89 feature at 296.137 Å does reveal two components which we identify as the two Si IX lines. Details of these newly fitted profiles are listed in Table  24. The two components are density sensitive relative to each other, and we obtain an lower limit to the density, shown in Table 11.

The ground 3P levels do not approach Boltzmann equilibrium until around 10[FORMULA] cm-3 and so we find density sensitivity amongst lines within the three groups mentioned earlier. Such ratios are most useful for quiet Sun and coronal hole observations where we expect densities lower than 10[FORMULA] cm-3. This density sensitivity makes it difficult to identify density insensitive ratios appropriate for the SERTS-89 spectrum, but one solution is to sum together groups of lines, allowing the different degrees of sensitivity to cancel out. One such example is to sum the 292.80 Å, 296.13 Å and 296.23 Å lines and take the ratio relative to the sum of the 344.96 Å, 345.13 Å and 349.87 Å lines. We list this ratio in Table 10 and agreement is found within the error bars on the data.

For individual lines, we can check to see if the observed ratios lie close to their high density limit, as expected. Taking five ratios - 292.80/290.69, 296.13/292.80, 296.23/292.80, 345.13/341.97 and 349.87/345.13 - we display a comparison with theory in Table 10, the theoretical value being the average of the ratio over the 10[FORMULA]-10[FORMULA] cm-3 density regime - essentially the high density limit. Four of the ratios agree within the error bars on the data, while the 296.13/292.80 ratio lies marginally above the high density limit. In Table 11 we display the four ratios that agree with theory, but this time give the densities that they predict.

Only one of the two lines excited from the ground 1 D level is observed in the SERTS-89 spectrum at 258.095 Å (the [FORMULA]D-[FORMULA]D transition) and it provides a good density diagnostic for densities 10[FORMULA]-10[FORMULA] cm-3 when taken relative to any line excited from the ground [FORMULA]P levels. We choose to use the strong 296.13 Å line, leading to the estimated density shown in Table 11.

10.5. S XI

Lines from all of the three groups mentioned previously are seen in the SERTS-89 spectrum although the error bars on the lines are rather large due to low instrumental efficiency at these wavelengths: the [FORMULA]P-[FORMULA]S lines are seen in second order, while the [FORMULA]P-[FORMULA]P and [FORMULA]P-[FORMULA]D lines lie towards the short wavelength end of the bandpass. Two lines are reported as blended - 186.88 Å and 191.23 Å - although the Fe XIII line at 191.23 Å is expected to contribute very little (see Sect. 13.5 on Fe XIII ). S XI contributes only a weak component to the 186.88 Å line, as witnessed by the 186.88/191.23 branching ratio presented in Table 9. We estimate a S XI contribution of 55 erg cm-2 s-1 sr-1, just 4% of the total. The observed 285.58/281.44 ratio shows a discrepancy with theory and we suggest that the 285.58 Å line is blended with another (unknown) line. The 247.16/239.83 branching ratio indicates that we would expect a line at 247.16 Å which was not reported by Thomas & Neupert. Re-analysis of the SERTS-89 spectrum does show a line at this wavelength with an intensity which is just [FORMULA] from the predicted value. Details of this new fit are given in Table 24. The ground 3 P levels come into Boltzmann equilibrium at 10[FORMULA]-10[FORMULA] cm-3 which makes for many potentially good density diagnostics, but limits the number of density insensitive ratios that are available. CHIANTI does predict a pair of lines at 242.85 Å and 242.87 Å, not seen in the SERTS-89 spectrum, which are density insensitive relative to the 191.23 Å line. The latter is identified as a blend with Fe XIII, but even if it were due entirely to S XI, the calculated ratio shown in Table 10 then implies the 242.85/7 blend would have an expected intensity of no more than around 157 erg cm-2 s-1 sr-1. Thus the non-detection by SERTS-89 is quite reasonable, since its [FORMULA] -sensitivity limit is 225 erg cm-2 s-1 sr-1 at that wavelength. However, this predicted blend might be observable by future, more efficient instruments. No other useful density-insensitive ratios exist at these wavelengths for S XI. There are a number of potentially excellent density diagnostic ratios, including 247.16/246.89, 281.44/291.58 and 285.58/285.83. However, the 285.58 Å line needs to be used with care since it may be affected by blending as mentioned above. Although the 291.58 Å line was not seen by SERTS-89, we can use the [FORMULA] spectral sensitivity level to estimate an upper limit of 55 erg cm-2 s-1 sr-1 for its intensity, which implies a density value of [FORMULA]. The other measured ratios also give reasonable densities within their error bars, as shown in Table 11.

The [FORMULA]D-[FORMULA]D transition gives rise to a line at 215.97 Å and so would be seen in second order at around 431.94 Å but it is not reported in the catalogue. Since the line is predicted to gain significant intensity only above 10[FORMULA] cm-3 this is as expected.

10.6. Summary

The C-like ions provide many potentially excellent density diagnostics, and comparisons with the SERTS-89 spectrum show reasonably consistent results in general. However, a few problems were encountered that point out the need for some caution when using these diagnostics, as follows.

  • The Ne V 416.21/359.38 ratio gives a surpisingly low density of around 10[FORMULA] cm-3
  • The most useful Mg VII line for density work is 319.02 Å, but it is blended with a Ni XV line making interpretation difficult.
  • For Si IX many of the density diagnostics are useful only for quiet Sun or coronal hole studies. The 258.10 Å line can be used for active region work, however. In particular, the ratio 258.10/296.14 gives a density in good agreement with other ions.
  • There are several problems with the S XI lines which may be due to blending. Still, this ion has a number of ratios that are potentially very useful as density diagnostics, especially for solar active regions, if used with proper care.
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© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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