The Balmer Lines of He II in the Blue Wing of the Hydrogen Lyman alpha Line Observed in a Quiescent Prominence

International audienceWe revisit the prominence observations in the Lyman α line of Curdt et al. (Astron. Astrophys. 511, L4, 2010) and focus on the bump in the blue wing of the line, which we identify with He ii Balmer lines. We determine the transition candidates, derive an upper limit for the width of the profile and an associated non-thermal velocity close to 0 km s−1, with the assumption that the kinetic temperature is equal to the formation temperature. We compare the total intensity with the corresponding H Lyman α intensity and find a ratio much lower than that measured by Ebadi, Vial, and Ajabshirizadeh (Solar Phys. 257, 91, 2009) in other Lyman lines. We confirm this result with observations performed by Schwartz et al. (private communication, 2014), we discuss a possible interpretation, and suggest that this issue needs to be addressed closely in future observations