The Formation of IRIS diagnostics. IV. The Mg II triplet lines as a new diagnostic for lower chromospheric heating

TL;DR

This study investigates the Mg II triplet lines as a new diagnostic tool for lower chromospheric heating, using simulations and observations to understand their formation and diagnostic potential.

Contribution

It introduces the Mg II triplet lines as a novel diagnostic for lower chromospheric heating, supported by simulation and observational analysis.

Findings

Emission lines are rare and caused by steep temperature increases.

Lines are sensitive to temperature changes at specific atmospheric depths.

Line profile shapes provide additional diagnostic information.

Abstract

A triplet of subordinate lines of Mg II exists in the region around the h&k lines. In solar spectra these lines are seen mostly in absorption, but in some cases can become emission lines. The aim of this work is to study the formation of this triplet, and investigate any diagnostic value they can bring. Using 3D radiative magnetohydrodynamic simulations of quiet Sun and flaring flux emergence, we synthesize spectra and investigate how spectral features respond to the underlying atmosphere. We find that emission in the lines is rare and is typically caused by a steep temperature increase in the lower chromosphere (above 1500 K, with electron densities above 10$^{18}$ m$^{-3}$). In both simulations the lines are sensitive to temperature increases taking place at column masses >= 5e-4 g cm$^{-2}$. Additional information can also be inferred from the peak-to-wing ratio and shape of the line profiles. Using observations from NASA's Interface Region Imaging Spectrograph we find both absorption and emission line profiles with similar shapes to the synthetic spectra, which suggests that these lines represent a useful diagnostic that complements the MgII h&k lines.