Renzo Ramelli, Goetz Stellmacher, Eberhard Wiehr, Michele Bianda
The only prominent line of singly ionized helium in the visible spectral range, He {\sc ii} 4686\AA, is observed together with the He {\sc i} 5015\AA{} singlet and the He {\sc i} 4471\AA {} triplet line in solar prominences. The Na D$_2$ emission is used as a tracer for He {\sc ii} emissions which are sufficiently bright to exceed the noise level near $10^{-6}$ of the disk-center intensity. The so selected prominences are characterized by small non-thermal line broadening and almost absent velocity shifts, yielding narrow line profiles without wiggles. The reduced widths [$\Delta\lambda_D/\lambda$] of He {\sc ii} 4686\AA{} are 1.5 times broader than those of He {\sc i} 4471\AA{} triplet and 1.65 times broader than those of He {\sc i} 5015\AA{} singlet. This indicates that the He lines originate in a prominence--corona transition region with outwards increasing temperature.
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http://arxiv.org/abs/1208.2153
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