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J.T. Schmelz (Univ. of Memphis), A. Fludra (Rutherford Appleton Laboratory)
Knowledge of the abundances of trace elements relative to hydrogen -- absolute abundances -- in the solar corona is essential for the understanding of plasma conditions. Both spectroscopic and solar energetic particle data agree that the coronal-to-photospheric abundance ratios of elements with low First Ionization Potential (FIP <10 eV) seem to be enhanced by about a factor of four relative to those with high FIP (>11 eV). The observations, however, do not agree on the normalization of the trace elements with respect to hydrogen, a result which is problematic in both the spectroscopic and particle data analysis. Two different empirical models have been suggested in the literature: (1) low-FIP elements may be enhanced by about a factor of four with respect to their photospheric values while high-FIP elements are the same in the corona and the photosphere; or (2) low-FIP elements may be the same in the corona and the photosphere while high-FIP elements are depleted by about a factor of four with respect to their photospheric values. Unfortunately, however, neither of these two empirical models accurately represents the data.
We have used the absolute coronal abundance results from several groups using both spectroscopic and energetic particle data to show that a much better representation is achieved with a 'hybrid' set of abundances. In this empirical model, there is both low-FIP enhancement as well as high-FIP depletion, each by about a factor of two. The data clearly show that it is impossible for one model to explain all observations. It is also vital to account for the possibility of abundance variability when analyzing any data set. However, it is often useful to begin the analysis with an assumed set of coronal abundances. The hybrid abundances represent the best average values for all available data.
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