Statistical analysis of UV spectra of a quiescent prominence observed by IRIS^⋆

¹ Astronomical Institute, The Czech Academy of Sciences, 25165 Ondřejov, Czech Republic
e-mail: e-mail: sonja.jejcic@guest.arnes.si
² Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia
³ Astronomical Institute of Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovak Republic

Received: 21 May 2018
Accepted: 27 June 2018

Abstract

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.