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Aims. Prominences observed by Hinode show very dynamical and intriguing structures. To understand the mechanisms that are responsible for these moving structures, it is important to know the physical conditions that prevail in fine-structure threads. In the present work we analyse a quiescent prominence with fine structures, which exhibits dynamic behaviour, which was observed in the hydrogen Hα line with Hinode/SOT, Meudon/MSDP and Ondˇ/HSFA2, and simultaneously in hydrogen Lyman lines with SoHO/SUMER during a coordinated campaign. We derive the fine-structure physical parameters of this prominence and also address the questions of the role of the magnetic dips and of the interpretation of the flows. Methods. We calibrate the SoHO/SUMER and Meudon/MSDP data and obtain the line profiles of the hydrogen Lyman series (Lβ to L6), the Ciii (977.03 A) and Svi (933.40 A), and Hα along the slit of SoHO/SUMER that crosses the Hinode/SOT prominence. We employ a complex 2D radiation-magnetohydrostatic (RMHS) modelling technique to properly interpret the observed spectral lines and derive the physical parameters of interest. The model was constrained not only with integrated intensities of the lines, but also with the hydrogen line profiles. Results. The slit of SoHO/SUMER is crossing different prominence structures: threads and dark bubbles. Comparing the observed integrated intensities, the depressions of Hα bubbles are clearly identified in the Lyman, Ciii ,a nd Svi lines. To fit the observations, we propose a new 2D model with the following parameters: T = 8000 K, pcen = 0.035 dyn cm −2 , B = 5 Gauss, ne = 10 10 cm −3 , 40 threads each 1000 km wide, plasma β is 3.5 × 10 −2 . Conclusions. The analysis of Ciii and Svi emission in dark Hα bubbles allows us to conclude that there is no excess of a hotter plasma in these bubbles. The new 2D model allows us to diagnose the orientation of the magnetic field versus the LOS. The 40 threads are integrated along the LOS. We demonstrate that integrated intensities alone are not sufficient to derive the realistic physical parameters of the prominence. The profiles of the Lyman lines and also those of the Hα line are necessary to constrain 2D RMHS models. The magnetic field in threads is horizontal, perpendicular to the LOS, and in the form of shallow dips. With this geometry the dynamics of fine structures in prominences could be interpreted by a shrinkage of the quasi-horizontal magnetic field lines and apparently is not caused by the quasi-vertical bulk flows of the plasma, as Hinode/SOT movies seemingly suggest.

2D radiative-magnetohydrostatic model of a prominence observed by Hinode, SoHO/SUMER and Meudon/MSDP