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Astron. Astrophys. 360, 1148-1156 (2000)

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6. Conclusions

In the present work, a SOHO-CDS observation of an active region has been used to determine the element abundances and the DEM of the emitting plasma by means of EUV line intensities. During the study, three different datasets for the ion fractions have been adopted in turn, under the assumption of ionization equilibrium. Since these ion fraction datasets (Shull & Van Steenberg 1982; Arnaud & Rothenflug 1985 with the latest revisions for iron from Arnaud & Raymond 1992; and Mazzotta et al. 1998) show significant differences for several ions relevant to the present analysis, the present work allows us to check the effects of uncertainties in the ionization and recombination rates on plasma diagnostics.

The present work shows that the differences found in the ion fraction datasets adopted in the analysis cause large effects in the measurements of plasma physical quantities when diagnostic techniques involving EUV line intensities are used. Both the DEM curves and the element abundances determined in the present analysis show significant variations according to the different ion fraction dataset adopted in the analysis.

From these results, two main points can be stressed.

The first point concerns differences in the DEM curves. These can be significant, and can be found for any value of the electron temperature, according to the ion fraction dataset used. This may have great effects on theoretical models in the case the DEM curves are used for plasma modeling. This is expecially true at transition region temperatures, where the differences in the curves are largest.

The second point concerns the large differences found in the relative low-FIP/high-FIP abundance corrections. These can significantly alter any quantitative conclusion drawn on the FIP effect, as the factor around two between the results found adopting in turn each ion fraction dataset is comparable in size to the factor between 3 and 4 difference between solar photospheric and coronal low-FIP/high-FIP relative abundance.

These results show that the choice of ion fractions for plasma diagnostics is an important source of uncertainties for plasma diagnostics.

Allen et al. 2000 report that using the same three ion fraction datasets considered in the present work no difference was found in electron temperature diagnostics at the solar limb. Allen et al. 2000 measured the electron temperature by means of three different plasma diagnostic techniques: line intensity ratios, DEM analysis and a variant Emission Measure analysis. The agreement they find in temperature values when ion fractions are changed is probably due to the fact that the plasma analyzed by Allen et al. 2000 is isothermal, while the disk spectrum analyzed in the present study is formed by plasma whose temperature ranges from chromospheric to coronal values, so that the temperature dependence of ion fractions must be fully taken into account.

As a consequence of this work, we stress the need of new and accurate ionization and recombination rates in order to obtain more reliable calculations of ion fraction datasets, so that the uncertainty that this fundamental parameter provides to the diagnostics results can be minimized.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000