Modeling Mg ii h, k and Triplet Lines at Solar Flare Ribbons

Abstract

Observations from the Interface Region Imaging Spectrograph (IRIS) often reveal significantly broadened and non-reversed profiles of the Mg II h, k and triplet lines at flare ribbons. To understand the formation of these optically thick Mg II lines, we perform plane parallel radiative hydrodynamics modeling with the RADYN code, and then recalculate the Mg II line profiles from RADYN atmosphere snapshots using the radiative transfer code RH. We find that the current RH code significantly underestimates the Mg II h & k Stark widths. By implementing semi-classical perturbation approximation results of quadratic Stark broadening from the STARK-B database in the RH code, the Stark broadenings are found to be one order of magnitude larger than those calculated from the current RH code. However, the improved Stark widths are still too small, and another factor of 30 has to be multiplied to reproduce the significantly broadened lines and adjacent continuum seen in observations. Non-thermal electrons, magnetic fields, three-dimensional effects or electron density effect may account for this factor. Without modifying the RADYN atmosphere, we have also reproduced non-reversed Mg II h & k profiles, which appear when the electron beam energy flux is decreasing. These profiles are formed at an electron density of ∼ 8 × 10 14 cm −3 and a temperature of ∼ 1.4 × 10 4 K, where the source function slightly deviates from the Planck function. Our investigation also demonstrates that at flare ribbons the triplet lines are formed in the upper chromosphere, close to the formation heights of the h & k lines.

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…In addition to previous studies (Rubio da Costa and , recent work by Zhu et al (2019) has succeeded in reproducing some of these profiles assuming very large electron energy fluxes (typical of a X-class flare) and increased Stark widths. Nevertheless, it is still unclear how to reproduce such single-peaked profiles in the case of smaller flares, and how to fully explain their increased broadening.…”

“…Puzzling blue-wing enhancements in the Mg II lines (Tei et al, 2018;Huang et al, 2019a) have also been observed and could be indicative of "cool upflows", caused by non-thermal electrons penetrating much deeper into the lower atmosphere than expected and causing an upward motion in the upper chromosphere (Huang et al, 2019a;Zhu et al, 2019;Hong et al, 2020). A recent study has also shown that the location where the cool upflows occur varies in different flare models (Hong et al, 2020).…”

“…Polito, Testa, and De Pontieu, 2019;Kerr, Allred, and Polito, 2020;Mandage and Bradshaw, 2020), improvements in the modeling and synthesis of the chromospheric diagnostics (e.g. Zhu et al, 2019;Graham et al, 2020), as well as new approaches to the analysis of the observations based on statistical studies and machine-learning techniques (e.g. Panos et al, 2018;Panos and Kleint, 2020;Sadykov et al, 2019).…”

Modeling the IRIS Lines During a Flare. I. The Blue-wing Enhancement in the Mg II k Line

“…If we increase the micro-turbulent velocities in our models, the blue peak in the line would be smoothed out and shown as a blue asymmetry instead. In addition, an increased Stark broadening would also help remove the discrepancy significantly (Zhu et al 2019).…”

Modeling Mg ii during Solar Flares. II. Nonequilibrium Effects

“…The importance of non-equilibrium (NEQ) ionisation and recombination in determining the atomic level populations (and therefore the radiative response to the disturbance) will likely vary from species to species, and the charge states of those species. However, when modelling the radiative response of the atmosphere to flares it is often the case that the statistical equilibrium (SE) solution is sought, given the computational demands of performing time dependent simulations that include physical processes, such as partial frequency redistribution, that are required for certain transitions (recent examples include Rubio da Costa & Kleint 2017;Zhu et al 2019;. This is partly mitigated by using the non-equilibrium electron density from dynamic simulations.…”