SOLARNET Conference: The Many Scales of the Magnetic Sun

A quiescent prominence was observed on October 22, 2013 quasi-simultaneously and nearly co-spatially in the Lyman series of hydrogen by SoHO/SUMER and in the Mg II h&k lines by IRIS. Such spectroscopic observations comprising multiple lines offer an excellent opportunity for diagnostics of prominence plasma and the dynamics of its fine structures. However, it also invokes significant challenges when compared with synthetic spectra provided by radiative transfer modelling. Only a few similar coordinated datasets of Lyman and Mg II h&k observations were ever obtained in prominences. We present here the first spectroscopic analysis of this unique dataset. Moreover, for the first time, we assess the influence of noise on the statistics of profile characteristics. We focus on the following characteristics – the number of distinct peaks, integrated line intensity, center-to-peak ratio describing the depth of the reversal of two-peaked profiles, and the asymmetry of peaks. There occur many profiles of both Lyman and Mg II lines which except of two major peaks, contain also lower peaks – hereafter called as minor peaks. Nature of these, possibly spurious minor peaks is uncertain. They may be either due to noise or vigorous dynamics of the prominence. We show that noise has a negligible effect on the integrated intensities of all lines but it significantly affects the classification of spectral profiles using the number of distinct peaks, the reversal depth and also the peak asymmetry. We also demonstrate that by taking into account an influence of noise, we can assess which characteristics of particular lines are suitable for diagnostics of different properties of prominence. For example, we show that the minor peaks of the Lyman line profiles are mostly caused by noise, which means that only the dominant peaks should be used for statistical analyses or comparisons with synthetic spectra. On the other hand, for Mg II k&h, distinction between multi-peaked profiles with low peaks and profiles with deep reversals lies mainly in the dynamics of multiple fine structures located along a line of sight. The complex, multi-peaked profiles are observed in places where multiple fine structures with different line-of-sight velocities are crossing the line of sight while the profiles with deep reversals likely correspond to instances when we observe single fine structure or multiple fine structures but with similar line-of-sight velocities. Such results lead to a conclusion that if we are interested in the diagnostics of dynamics of prominence fine structures, the best approach is to use a combination of profile asymmetry in the Lyman lines together with the complex profiles of Mg II h&k lines. On the other hand, if we want to diagnose the temperature and pressure of individual prominence fine structures, we need to focus on deeply reversed Mg II h&k line profiles in combination with the Lyman lines and to analyse the depth of central reversal and the integrated intensity.