On the formation of Mg ii h and k lines in solar prominences

Abstract

Aims. With the recent launch of the IRIS mission, it has become urgent to develop the spectral diagnostics using the Mg ii resonance h and k lines. In this paper, we aim to demonstrate the behavior of these lines under various prominence conditions. Our results serve as a basis for analysis of new IRIS data and for more sophisticated prominence modeling. Methods. For this exploratory work, we use a canonical 1D prominence-slab model, which is isobaric and may have three different temperature structures: isothermal, PCTR-like (prominence-corona transition region), and consistent with the radiative equilibrium. The slabs are illuminated by a realistic incident solar radiation obtained from the UV observations. A five-level plus continuumMg ii model atom is used to solve the full NLTE problem of the radiative transfer. We use the numerical code based on the ALI techniques and apply the partial frequency redistribution for both Mg ii resonance lines. We also use the velocity-dependent boundary conditions to study the effect of Doppler dimming in the case of moving prominences. Finally, the relaxation technique is used to compute a grid of models in radiative equilibrium.Results. We computed the Mg ii h and k line profiles that are emergent from prominence-slab models and show their dependence on kinetic temperature, gas pressure, geometrical extension, and microturbulent velocity. By increasing the line opacity, significant departures from the complete frequency redistribution take place in the line wings. Models with a PCTR temperature structure show that Mg ii becomes ionized to Mg iii in the temperature range between roughly 15 000 and 30 000 K. Doppler dimming is significant for Mg ii resonance lines. At the velocity 300 km s −1 , the line intensity decreases to about 20% of the value for static prominences.Finally, we demonstrate the role of Mg ii h and k radiation losses on the prominence energy balance. Their dominant role is at lower pressures, while the losses due to hydrogen and Ca ii dominate at higher pressures.

Fast approximate radiative transfer method for visualizing the fine structure of prominences in the hydrogen Hαline

“…In the first test we used the series of 1D non-LTE isothermalisobaric models of Heinzel et al (2014). We compared the Hα line-center intensities obtained by the models with the intensities computed by the Hα synthesis when using the interpolation of parameters i and f from the tables provided in this paper and when using representative fixed values of parameters i and f for all models.…”

“…However, for this new Hα visualization method we evaluated these parameters for a broader range of temperatures using the most advanced 1D non-LTE radiative transfer models recently implemented by Heinzel et al (2014). The results are given in Table 1 for three representative altitudes of 10 000, 20 000, and 30 000 km and for a nominal slab thickness of 500 km.…”

“…The results are given in Table 1 for three representative altitudes of 10 000, 20 000, and 30 000 km and for a nominal slab thickness of 500 km. For the isothermal-isobaric slabs of Heinzel et al (2014) that are not optically thick in Hα, the atomic level populations and the electron densities are almost constant inside the slabs (Gouttebroze et al 1993), and we took the values at the slab center to tabulate i and f . For the same altitudes we also determined average values of the Hα line source function S as 3.22 × 10 −6 , 2.98 × 10 −6 , and 2.80 × 10 −6 erg s −1 cm −2 sr −1 Hz −1 , respectively.…”

Quiescent Prominences in the Era of Alma: Simulated Observations Using the 3d Whole-Prominence Fine Structure Model

“…This will help us to answer the question of energy balance of prominences, for which the temperature of the cool prominence core is a critical parameter (see e.g. Heinzel et al 2014). Moreover, the high-resolution information about the prominence plasma temperature distribution will be instrumental for studies of the PCTR -the transition region between the cool prominence plasma and the hot corona.…”

Mg ii Lines Observed During the X-class Flare on 29 March 2014 by the Interface Region Imaging Spectrograph

“…Hydrogen Ly α and Ly β lines were treated with the partial frequency redistribution (see Hubeny and Mihalas, 2014). Using the fixed electron density structure, the radiative transfer problem was solved for a five-level plus continuum atomic model of Mg ii with the two resonance lines k and h and the 3p -3d subordinate triplet around the k line, for details of the atomic model see Heinzel, Vial, and Anzer (2014). After modeling, we compare our synthetic spectra to the observed IRIS spectra.…”