Two-horn quiescent prominence observed in Hα and Mg II h&k lines with THEMIS and IRIS

Context. Prominences are large magnetic structures in the corona filled by cool plasma with fast evolving fine structure.Aims. We aim to better understand the plasma conditions in the fine structure of a quiescent prominence including two transient horns observed at the bottom of the cavity using the high resolution Interface Region Imaging Spectrograph (IRIS) and the MulTi-Raies (MTR) spectrograph of the Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires (THEMIS) in the Canary Islands.Methods. We analysed the spectra obtained in H alpha by THEMIS and Mg II by IRIS and compare them with a grid of 23 940 1D radiative transfer models which include a prominence-to-corona transition region (PCTR). The full observed profiles of Mg II in each pixel are fitted completely by synthesised profiles with xRMS (Cross RMS; an improved version of the rolling root mean square (rRMS) method). When the RMS is below a certain threshold value, we recover the plasma conditions from the parameters of the model best fitting the observed line profile. This criterion is met in two regions (the horns and edge of the prominence) where the line profiles can generally be described as single peaked.Results. The 1D models suggest that two different kinds of model atmospheres correspond to these two regions. The region at the edge is found to be fitted mainly with isothermal and isobaric models, while the other area (the horns) is seen to be fitted with models with a PCTR that have optical thicknesses of less than 5. In the prominence edge, the theoretical relationship between the integrated intensities in H alpha and Mg II is verified and corresponds to low emission measure values. In these regions the electron density is around 10(10 )cm(-3), while it is one order of magnitude less in the horn regions around 10(9) cm(-3).Conclusions. In the horns, we find some profiles are best fitted with models with high mean temperatures. This suggests that the hot PCTR found in the horns could be interpreted as prominence plasma in condensation phase at the bottom of the coronal cavity.