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Context. The paper analyzes the structure and dynamics of a quiescent prominence that occurred on October 22, 2013 and was observed by several instruments including the Interface Region Imaging Spectrograph (IRIS). Aims. We aim to determine the physical characteristics of the observed prominence using Mg ii k and h (2796 and 2803 Å) , C ii (1334 and 1336 Å), and Si iv (1394 Å) lines observed by IRIS. In addition we study the dynamical behavior of the prominence. Methods. We employed the one-dimensional non-LTE (departures from the local thermodynamic equilibrium -LTE) modeling of Mg ii lines assuming static isothermal-isobaric slabs. We selected a large grid of models with realistic input parameters expected for quiescent prominences (temperature, gas pressure, effective thickness, microturbulent velocity, height above the solar surface) and computed synthetic Mg ii lines. The method of Scargle periodograms was used to detect possible prominence oscillations. Results. We analyzed 2160 points of the observed prominence in five different sections along the slit averaged over ten pixels due to low signal to noise ratio in the C ii and Si iv lines. We computed the integrated intensity for all studied lines, while the central intensity and reversal ratio was determined only for both Mg ii and C ii 1334 lines. We plotted several correlations: time evolution of the integrated intensities and central intensities, scatter plots between all combinations of line integrated intensities, and reversal ratio as a function of integrated intensity. We also compared Mg ii observations with the models. Results show that more than two-thirds of Mg ii profiles and about one-half of C ii 1334 profiles are reversed. Profiles of Si iv are generally unreversed. The Mg ii and C ii lines are optically thick, while the Si iv line is optically thin.Conclusions. The studied prominence shows no global oscillations in the Mg ii and C ii lines. Therefore, the observed time variations are caused by random motions of fine structures with velocities up to 10 km s −1 . The observed average ratio of Mg ii k to Mg ii h line intensities can be used to determine the prominence's characteristic temperature. Certain disagreements between observed and synthetic line intensities of Mg ii lines point to the necessity of using more complex two-dimensional multi-thread modeling in the future.

Context. Movies of prominences obtained by space instruments e.g. the Solar Optical Telescope (SOT) aboard the Hinode satellite and the Interface Region Imaging Spectrograph (IRIS) with high temporal and spatial resolution revealed the tremendous dynamical nature of prominences. Knots of plasma belonging to prominences appear to travel along both vertical and horizontal thread-like loops, with highly dynamical nature. Aims. The aim of the paper is to reconstruct the 3D shape of a helical prominence observed over two and a half hours by IRIS.Methods. From the IRIS Mg ii k spectra we compute Doppler shifts of the plasma inside the prominence and from the slit-jaw images (SJI) we derive the transverse field in the plane of the sky. Finally we obtain the velocity vector field of the knots in 3D. Results. We reconstruct the real trajectories of nine knots travelling along ellipses. Conclusions. The spiral-like structure of the prominence observed in the plane of the sky is mainly due to the projection effect of long arches of threads (up to 8 × 10 4 km). Knots run along more or less horizontal threads with velocities reaching 65 km s −1 . The dominant driving force is the gas pressure.

Context. Prominence oscillations have been mostly detected using Doppler velocity, although there are also claimed detections by means of periodic variations in half-width or line intensity. However, scarce observational evidence exists about simultaneous detection of oscillations in several spectral indicators. Aims. Our main aim here is to explore the relationship between spectral indicators, such as Doppler shift, line intensity, and line half-width, and the linear perturbations excited in a simple prominence model. Methods. Our equilibrium background model consists of a bounded, homogeneous slab, which is permeated by a transverse magnetic field, having prominence-like physical properties. Assuming linear perturbations, the dispersion relation for fast and slow modes has been derived, as well as the perturbations for the different physical quantities. These perturbations have been used as the input variables in a one-dimensional radiative transfer code, which calculates the full spectral profile of the hydrogen Hα and Hβ lines. Results. We have found that different oscillatory modes produce spectral indicator variations in different magnitudes. Detectable variations in the Doppler velocity were found for the fundamental slow mode only. Substantial variations in the Hβ line intensity were found for specific modes. Other modes lead to lower and even undetectable parameter variations. Conclusions. To perform prominence seismology, analysis of the Hα and Hβ spectral line parameters could be a good tool to detect and identify oscillatory modes.