Abstract
Quiescent prominences
It is found that Heii 4686 is emitted in the same cold region of ≲ 10000 K as hydrogen, metal and neutral helium emission lines. This conclusion is based on the finding that the observed width of λ4686 is the same as the calculated width of λ4686. The calculated width is derived from the observed widths of hydrogen and metallic lines. The large intensity of Heii 4686 in ≲ 10000 K can be explained by the ionization of Heii due to the UV radiation below 228 Å that comes from the corona and the transition region.
Loop prominences
The very broad width (30 to 50 km s−1) of λ4686 for two post-flare loop prominences shows that the Heii line is emitted in hot regions different from regions of hydrogen and metal emission. From the widths of the Balmer lines and many metallic lines the kinetic temperature for one loop is found to be 16000 K in one part and 7600 K in another part. The electron densities are 1012.0 cm−3 and less than 1011.0 cm−3 respectively.
Chromosphere
The intensity of λ4686 in the chromosphere can be interpreted in terms of a temperature of ≲ 10000 K with the ionization due to UV radiation. But, since observations of the width of λ4686 are not available, a definitive conclusion for the chromosphere cannot be reached.
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Hirayama, T. Ionized helium in prominences and in the chromosphere. Sol Phys 24, 310–323 (1972). https://doi.org/10.1007/BF00153371
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DOI: https://doi.org/10.1007/BF00153371