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

9. Boron iso-electronic sequence

The dominant EUV transitions of the boron-like ions arise from 2s -2s2p doublet-doublet transitions. There are nine such transitions corresponding, in ascending wavelength order, to P-P (4 lines), P-S (2 lines), and P-D (3 lines) transitions. For ions with the ground P levels do not come into Boltzmann equilibrium until the electron density is at transition region/coronal densities and so yield useful density diagnostics. For ions density sensitivity comes from the 2s2p^{3 4} P levels coming into Boltzmann equilibrium with the ground levels, but this is not relevant for the SERTS-89 lines.

9.1. Ne VI

The P-D transitions lie outside the SERTS-89 bandpass at around 558-563 Å, while the remaining six transitions lie at around 400 Å and 430 Å. For typical transition region densities of 10 - 10 cm^-3 each of the six Ne VI observed lines are density insensitive relative to each other. There are three branching ratios and their values are in agreement with observation and reported in Table 6. The line at 403.30 Å is blended with a Mg VI line and the branching ratio 403.30/401.14 allows the Ne VI contribution to be estimated at around 17 erg cm^-2 s^-1 sr^-1 out of the total blended intensity of 45.6 erg cm^-2 s^-1 sr^-1.

Table 6. Branching ratios for the B-like ions.

There is a discrepancy between the theoretical intensity ratios involving the two lines at 433.16 Å and 435.63 Å and the other four observed transitions (see Table 7), with the former being around a factor of two too low. This is discussed later with regard to the calibration of the spectrometer.

Table 7. Insensitive ratios for the B-like ions.

9.2. Mg VIII

All of the nine transitions occur in the SERTS-89 wavelength range, although the line expected at 335.25 Å is hopelessly masked by the far stronger Fe XVI 335.40 Å line in the active region spectrum. From the branching ratio 335.25/339.00 (see Table 6) we can estimate that the expected intensity of the Mg VIII line is around 40 erg cm^-2 s^-1 sr^-1, as compared to the observed Fe XVI intensity of 10400 erg cm^-2 s^-1 sr^-1. However, Mg VIII may well be important and can even dominate the emission at this wavelength for cooler solar features, such as quiet Sun or coronal holes.

The 311.78 Å line is blended with a Ni XV line, and we use the 311.78/315.02 branching ratio to evaluate the Mg VIII contribution as around 48 erg cm^-2 s^-1 sr^-1 out of the observed total of 79 erg cm^-2 s^-1 sr^-1. The remaining branching ratio pair are well observed and are consistent with theory.

A weak transition is expected at 436.67 Å but is blended with the stronger 436.73 Å line, and only the latter is reported in the Thomas & Neupert catalogue. The contribution of the weaker line is expected to be 1/10 of the total intensity. Both lines have been considered for the intensity ratios here.

For the three density insensitive ratios given in Table 7 we see clearly that the lines at 430.45 Å and 436.73 Å are around a factor two lower than theory predicts. We feel that this may indicate a calibration problem, as discussed later in Sect. 15.1.

The ground ² P levels come into Boltzmann equilibrium for densities around 10⁹ cm^-3 and thus it is possible to use the Mg VIII lines to estimate the plasma density. Two ratios are suggested in Table 8, although both only give lower limits to the density. They are more useful for quiet Sun or coronal hole observations where the density is lower.

Table 8. Density sensitive line ratios for the B-like ions.

Bhatia & Thomas (1997) have recently reported new atomic data calculations for Mg VIII, as well as comparisons with SERTS-89 observations. Their conclusions are very similar to those presented here.

9.3. Al IX

The nine Al IX transitions all fall within the bandpass of the SERTS-89 spectrometer but only five are reported in the Thomas & Neupert catalogue. One surprise is the absence of the multiplet 2s² 2p ² P - 2s2p^{2 2} P with the exception of the 282.43 Å line. The strongest line of this multiplet (and of the entire Al IX spectrum) is expected to be observed at 284.04 Å, but lies close to the very strong Fe XV 284.16 Å line and so can not be reliably fitted. Branching ratios for the P-P multiplet are given in Table 6, while we give the density insensitive 305.09/284.04 ratio in Table 7 to give an indication of the expected intensity of the 284.04 Å line. The 300.56/305.09 branching ratio is not consistent with observations, and we suggest that the 300.56 Å line may be blended.

From Table 7 it is clear that the two lines observed at 385.02 Å and 392.41 Å are considerably weaker than expected as estimated from the 282.43 Å and 305.09 Å lines. We note that the same behavior is seen in the corresponding transitions of Mg VIII, although in that case the discrepancies were thought to indicate inaccuracies in the intensity calibration curve. Although both the 305.09/282.43 and 300.56/282.43 ratios are density sensitive below around 10 cm^-3, the error bars on the data are actually greater than the variation of the ratios with density. Despite this, we list both ratios in Table 8 and note that better agreement is found for the 305.09/282.43 ratio, strengthening the suggestion above that the 300.56 Å line is blended.

The 392.41/385.02 ratio has been recognised previously as a useful density diagnostic below around 10 cm^-3 (see, e.g., Keenan et al. 1994b), and with the SERTS-89 data we can derive a lower limit to the density, as shown in Table 8.

9.4. Si X

Again all nine boron-like transitions are found in the SERTS-89 spectral range, and all of them have been reported in the Thomas & Neupert catalogue. In addition there is a line identified at 292.25 Å as Si X, but we dismiss this identification as the Si X line predicted by CHIANTI at this wavelength is expected to be around two orders of magnitude weaker than the observed line. The line at 256.32 Å is blended with a stronger He II line; the Si X contribution can be estimated from the branching ratio presented in Table 6 as around 157 erg cm^-2 s^-1 sr^-1 out of the observed total of 1580 erg cm^-2 s^-1 sr^-1.

The 253.81/258.37 branching ratio shows a strong discrepancy with observations (Table 6), but we note that in the Malinovsky & Heroux (1973) quiet Sun spectrum this ratio was 0.19, suggesting that the SERTS-89 value may be in error. The remaining density insensitive ratios show good agreement.

The ground ² P levels do not come into Boltzmann equilibrium until around 10 cm^-3 and we have two useful ratios presented in Table 8 involving lines close in wavelength. Both ratios give consistent densities with the 356.03/347.41 ratio giving smaller error bars on account of greater sensitivity to the electron density.

9.5. S XII

Only the ² P - ² D transitions fall within the SERTS-89 wavelength range, and two of the three were detected. The ²P - ²D transitions are separated enough in wavelength to be resolved, but the component at 299.79 is not reported by Thomas & Neupert; it is related to the 288.40 Å line by the branching ratio given in Table 6 from which an intensity of 15 erg cm^-2 s^-1 sr^-1 can be estimated, significantly below the SERTS-89 sensitivity limit of 53 erg cm^-2 s^-1 sr^-1 there. 299.53/288.40 is an excellent density diagnostic showing strong variation over 10 - 10 cm^-3 and the SERTS-89 intensities give a value of 10 cm^-3 as shown in Table 8.

9.6. Summary

The B-like ions from magnesium to sulphur provide useful density diagnostics for typical coronal densities, and the agreement with theory for the density insensitive ratios is generally excellent.

• Some spectral lines show problems: Ne VI 433.16 Å and 435.63 Å, and Mg VIII 430.45 Å and 436.73 Å, which may be due to possible errors in the intensity calibration curve for the 430-450 Å spectral range.
• There are some problems with the Al IX transitions which may result from blending as these lines are all fairly weak.
• The Si X 356.03/347.41 and S XII 299.53/288.40 ratios are recommended for density diagnostic work as they involve lines close in wavelength, are highly sensitive, and give densities consistent with other ions formed at similar temperatures.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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