8 Summary

The atomic calculations presented here are a significant advance over previous work. In particular, we have shown that resonances converging on the 3s3p3d electron configuration increase the thermally averaged collision strengths for several key diagnostic transitions.

Many of the discrepancies between observed and theoretical intensity ratios noted by Y98 for Fexiv now seem to have been resolved. In particular we find the following results.

• The theoretical 274/211 ratio is now in good agreement with the MH73 spectrum. There is still a discrepancy with the TN94 spectrum, but this may be due to an inaccurate first/second order SERTS-89 calibration;

• The 264/274 ratio now gives densities in both the MH73 and TN94 spectra that are consistent with other Fexiv ratios;

• The 270/274 ratio is found to be density sensitive, in contrast with previous models, and the range of variation is in good agreement with a variety of observations;

• The low density limit of the 353/334 ratio is in good agreement with theory;

• The 444/334 ratio is now consistent with ratios from other ions which suggested that the SERTS-89 calibration is in error above around 400Å. Previously this ratio was one of the few that suggested the calibration was not in error.

We recommend that our new atomic data be adopted for future analyses of solar and other astronomical spectra.

Acknowledgements

We acknowledge financial suport for the IRON Project from PPARC. SOHO is an NASA/ESA project. We thank the SOHO-CDS and SERTS teams for their assistance, in particular R.A. Harrison, C.D. Pike and R.J. Thomas.