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

TL;DR

This paper introduces a novel non-LTE radiative transfer modeling method to diagnose plasma parameters in solar prominences from Mgii h&k line profiles observed by IRIS, effectively capturing dynamic plasma conditions and line core shifts.

Contribution

The study develops a new approach that compares observed Mgii line profiles with a large grid of non-LTE models to accurately infer prominence plasma parameters and line core shifts.

Findings

Models recover plasma temperatures from 6000 to 50,000K.

Mean electron densities range from 7.3×10^8 to 1.8×10^11 cm^-3.

The method identifies regions with a prominence-corona transition region (PCTR).

Abstract

Aims: We investigate a new method to for obtaining the plasma parameters of solar prominences observed in the Mgii h&k spectral lines by comparing line profiles from the IRIS satellite to a bank of profiles computed with a one-dimensional non-local thermodynamic equilibrium (non-LTE) radiative transfer code. Methods: Using a grid of 1007 one-dimensional non-LTE radiative transfer models we carry out this new method to match computed spectra to observed line profiles while accounting for line core shifts not present in the models. The prominence observations were carried out by the IRIS satellite on 19 April 2018. Results: The prominence is very dynamic with many flows. The models are able to recover satisfactory matches in areas of the prominence where single line profiles are observed. We recover: mean temperatures of 6000 to 50,000K; mean pressures of 0.01 to 0.5 dyne cm$^{-2}$; column masses of 3.7$\times10^{-8}$ to 5$\times10^{-4}$ g cm$^{-2}$; a mean electron density of 7.3$\times10^{8}$ to 1.8$\times10^{11}$ cm$^{-3}$; and an ionisation degree ${n_\text{HII}}/{n_\text{HI}}=0.03 - 4500$. The highest values for the ionisation degree are found in areas where the line of sight crosses mostly plasma from the PCTR, correlating with high mean temperatures and correspondingly no H$\alpha$ emission. Conclusions: This new method naturally returns information on how closely the observed and computed profiles match, allowing the user to identify areas where no satisfactory match between models and observations can be obtained. Regions where satisfactory fits were found were more likely to contain a model encompassing a PCTR. The line core shift can also be recovered from this new method, and it shows a good qualitative match with that of the line core shift found by the quantile method. This demonstrates the effectiveness of the approach to line core shifts in the new method.