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

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.

Diagnostics of the Prominence Plasma from Hα and Mg ii Spectral Observations

“…Such values indicate a relatively optically thick regime (Leenaarts et al 2013). In the case of prominences, the value of 1.39 fits well with the ratio already found in different prominences (Schmieder et al 2014b;Liu et al 2015;Levens et al 2017;Jejčič et al 2018). In an optically thin regime far above the limb, the ratio could increase and reach 1.96 (Kerr et al 2015;Alissandrakis et al 2018).…”

“…The double-peaked Mg II profiles can be caused also by a high optical thickness of the prominence plasma in these lines, as shown by Levens et al (2016a) and Jejčič et al (2018).…”

“…Using the 1D NLTE radiative transfer model, Heinzel et al (2014) showed that the nonreversed profiles can be reproduced by models with low gas pressure (p=0.01 dyne cm −2 ) and the reversed profiles by models with higher gas pressures (p=0.5 dyne cm −2 ). Jejčič et al (2018) statistically studied the reversal of the observed and synthetic Mg II h and k line profiles. Those authors did not find a complete agreement between the results of 1D isothermal and isobaric models and the IRIS observations.…”

“…In Section 3 we present spectroscopic results that combine the observations in Hα and in Mg II. We compute the ratio between integrated intensities of Mg II k and h lines for prominence compared with the solar disk and for parts of the prominence visible in Hα and not visible in Hα. The integrated intensity of Mg II k and h allows us to determine the difference of optical thickness of Mg II in both situations, as suggested by models of Jejčič et al (2018). After a careful coalignment of pixels in Mg II spectra and pixels in Hα spectra, we identify a list of 75 overlapping pixels and derive the ratio between the integrated intensities of Mg II and Hα in these pixels.…”

Some relationships between radiative and atmospheric quantities through 1D NLTE modeling of prominences in the Mg II lines

“…If so, it will offer a simple and supplementary tool for the diagnostic (density combined to thickness) of prominences. Let us note that a different approach has been taken by Jejčič et al (2018) who focused on the reversal of the h and k profiles. We simultaneously varied the temperature (between 8000 and 14 000 K) and the thickness (500, 1000 and 2500 km) while the whole range of pressures (and consequently electron density) was explored.…”