Analysis of HeI 10830 a line in a quiescent prominence

Abstract

In order to explain an enhanced emission in the wings of the photoelectrically observed HeI 10830 A line, Landman et al. (Astrophys. J. 218, 1977, 888) have introduced the concept of a multilayer prominence structure with different temperatures and/or turbulent velocities characterizing the prominence-corona transition region (PCTR). As previously reported (Heinzel et al., Contr.Astron.Obs. Skalnate Pleso 15, 1986, 171), our photographically measured profiles of this line also exhibit substantial surplus radiation (about i0 % ) in the line wings, which can hardly be explained in terms of simple isothermal models. However, isothermal models of cool structures can be used to fit the line cores of observed profiles(both peaks in our case), assuming that the line-core emission comes predominantly from cool parts of the prominence.Theoretical profiles corresponding to these models are typically lower in the wings, namely in the gap between both peaks. Intensity differences in the gap serve then us as an indicator of the amount of a hot plasma radiation, emergent from optically-thin PCTR structures. Since the amount of this surplus radiation was found to increase almost linearly with the total optical thickness of cool structures (which we relate to their number) , we deduce that cool prominence threads are surrounded by hot plasma sheets rather than that the prominence periphery as a whole is hot_ In the latter case, no simple relation between the surplus radiation and cool structures optical thickness was expected. Finally, we have also found an increase of PCTR emission with geometrical height in the prominence which may be connected with the coronal temperature rise.