Measuring the Solar He Abundance in the Outer Corona with SUMER.

Details Measuring the Solar He Abundance in the Outer Corona with SUMER. Measuring the Solar He Abundance in the Outer Corona with SUMER.

Oct 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.446..405l&link_type=abstract

8th SOHO Workshop: Plasma Dynamics and Diagnostics in the Solar Transition Region and Corona. Proceedings of the Conference hel

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We outline new spectroscopic methods for measuring relative element abundances in the solar corona, with special emphasis on the He abundance. At altitudes of about 1.05 solar radius or more, the solar corona becomes essentially isothermal. This is not unexpected in view of the quiescent nature of the plasma at these heights, and the long radiative cooling times at densities below log ne = 8.0. The isothermal nature of the plasma obviates many of the difficulties associated with the inverse problem of determining an emission measure distribution. This is particularly true in the cases of Li- and Na-like ions whose ionization balance fractions have long tails extending to high temperatures, making the inversion problem especially difficult. We present new calculations of intensity ratios as a function of temperature that can be used to determine element abundance ratios in the solar corona. The He abundance may be determined from He II lines relative to, for example, O VI, and we present an extensive analysis of the He II spectrum. In order to predict accurate line intensities, large close-coupling calculations are required. One special feature is that in order to get the excitation cross sections correct, impact ionization channels also need to be included in the calculation, for the temperatures relevant to the solar corona. The model ion also needs to include the many highly excited levels from which the Balmer series originates.

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