Emission lines of Na-like ions in spectra obtained with the Solar EUV Research Telescope and Spectrograph (SERTS)

Theoretical emission-line ratios involving transitions in the 236–412  Å wavelength range are presented for the Na-like ions Ar viii, Cr xiv, Mn xv, Fe xvi, Co xvii, Ni xviii and Zn xx. A comparison of these with an extensive data set of the solar active region, quiet-Sun, subflare and off-limb observations, obtained during rocket flights by the Solar EUV Research Telescope and Spectrograph (SERTS), reveals generally very good agreement between theory and experiment. This indicates that most of the Na-like ion lines are reliably detected in the SERTS observations, and hence may be employed with confidence in solar spectral analyses. However, the features in the SERTS spectra at 236.34 and 300.25  Å, originally identified as the Ni xviii 3p ²P_3/2–3d ²D_3/2 and Cr xiv 3p ²P_3/2–3d ²D_5/2 transitions, respectively, are found to be due to emission lines of Ar xiii (236.34  Å) and possibly S v or Ni vi (300.25  Å). The Co xvii 3s ²S–3p ²P_3/2 line at 312.55  Å is always badly blended with an Fe xv feature at the same wavelength, but Mn xv 3s ²S–3p ²P_1/2 at 384.75  Å may not always be as affected by second-order emission from Fe xii 192.37  Å as previously thought. On the other hand, we find that the Zn xx 3s ²S–3p ²P_3/2 transition can sometimes make a significant contribution to the Zn xx/Fe xiii 256.43- Å blend, and hence care must be taken when using this feature as an Fe xiii electron density diagnostic. A line in the SERTS–89 active region spectrum at 265.00  Å has been re-assessed, and we confirm its identification as the Fe xvi 3p ²P_3/2–3d ²D_3/2 transition.