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

Abstract

The goal of this paper is to derive the physical conditions of the prominence observed on 2017 March 30. To do so, we use a unique set of data in Mg II lines obtained with the space-borne Interface Region Imaging Spectrograph (IRIS) and in Hα line with the ground-based Multi-Channel Subtractive Double Pass spectrograph operating at the Meudon solar tower. Here, we analyze the prominence spectra of Mg II h and k lines, and the Hα line in the part of the prominence which is visible in both sets of lines. We compute a grid of 1D NLTE (i.e., departures from the local thermodynamical equilibrium) models providing synthetic spectra of Mg II k and h, and Hα lines in a large space of model input parameters (temperature, density, pressure, and microturbulent velocity). We compare Mg II and Hα line profiles observed in 75 positions of the prominence with the synthetic profiles from the grid of models. These models allow us to compute the relationships between the integrated intensities and between the optical thickness in Hα and Mg II k lines. The optical thickness τ Hα is between 0.05 and 2, and t Mg IIk is between 3 and 200. We show that the relationship of the observed integrated intensities agrees well with the synthetic integrated intensities for models with a higher microturbulence (16 km s −1 ) and T around 8000 K, ne=1.5×10 10 cm −3 , p=0.05 dyne. In this case, large microturbulence values could be a way to take into account the large mixed velocities existing in the observed prominence.

Solar prominence diagnostics from non-LTE modelling of Mg II h&k line profiles

“…This issue is seen when attempting direct one-dimensional forward modelling of Hα prominences. For example, Ruan et al (2019) demonstrated that areas with wide Hα line profiles can be better fit by models with high microturbulent velocities. Whether this is also true for Mg ii line profiles is currently unclear.…”

Spectro-imagery of an active tornado-like prominence: formation and evolution

“…integrated intensity and FWHM of Mg ii and Hα lines in each pixel, the second method is based on the fitting of Mg ii h and k line profiles with synthesised profiles using the radiative transfer code. The first method has already been used in previous studies of prominences (Wiik et al 1992;Heinzel et al 2015;Jejčič et al 2018;Ruan et al 2019).…”

“…The broad profiles could also be due to high microturbulence. In Ruan et al (2019) paper, high micro turbulence was used as a proxy of multi structure velocities. A 2D NLTE radiative model would be suitable to fit more profiles with wide FWHM.…”

“…The differences in apparent morphology of prominences observed in different chromospheric lines have been discussed in the past. The optical thickness of the Mg ii h or k line is ∼100 times greater than that of Hα (Ruan et al 2019). This explains why all the low density structures can be better seen in Mg ii (Heinzel et al 2015;Levens et al 2016).…”

“…In each pixel of the MSDP prominence field of view a Hα profile is retrieved and Dopplershifts were computed with a bisector method. Hα profiles in prominences are relatively narrow compared to profiles on the disk, but they are broader than synthetic profiles obtained by radiative transfer codes Ruan et al (2018Ruan et al ( , 2019. The Dopplershifts were determined as following: we consider the blue and the red points in a Hα profile distant of 0.6 Å and having the same intensity.…”