Some spectral plasma diagnostics for prominences and structures in the middle chromosphere

The present investigation is concerned with the statistical equilibrium of the low-lying levels of Na, Sr(+), and Ba(+) under conditions appropriate to structures just above the solar surface. The solar and atomic data required for this objective are largely available with an adequate precision. However, the metastable (2)D term photoionization cross sections had to be calculated. In the calculations a variant of the scaled Thomas-Fermy method suggested by Kahler (1971) was used. The Na and Sr(+) resonance lines are readily measurable in ordinary prominences. The Ba(+) are weak. It is shown that the ratio of Na to Sr(+) resonance-line integrated intensities provides a useful and sensitive electron-density diagnostic for prominences, yielding values in the neighborhood of 10 to the 11th per cu cm. The statistical equilibrium solutions are applied to an analysis of data from the flash spectrum at eclipse.