What do iris observations of Mg II k tell us about the solar plage chromosphere?

TL;DR

This study uses multi-instrument solar observations and numerical models to analyze Mg II k line profiles, revealing a hot, dense chromosphere and high transition region in solar plage, with implications for understanding chromospheric structure.

Contribution

It provides new insights into the solar plage chromosphere by linking Mg II line profiles with chromospheric temperature, density, and velocity structure through combined observations and modeling.

Findings

Mg II k profiles indicate a high transition region and hot chromosphere.

Large wing widths imply steep temperature rise in the chromosphere.

Absence of subordinate line emission constrains temperature and height of temperature rise.

Abstract

We analyze observations from the Interface Region Imaging Spectrograph of the Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to better understand the solar plage chromosphere. We also make comparisons with observations from the Swedish 1 m Solar Telescope of the H{\alpha} line, the Ca II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the coronal 19.3 nm line. To understand the observed Mg II profiles, we compare these observations to the results of numerical experiments. The single-peaked or flat-topped Mg II k profiles found in plage imply a transition region at a high column mass and a hot and dense chromosphere of about 6500 K. This scenario is supported by the observed large-scale correlation between moss brightness and filled-in profiles with very little or absent self-reversal. The large wing width found in plage also implies a hot and dense chromosphere with a steep chromospheric temperature rise. The absence of emission in the Mg II subordinate lines constrain the chromospheric temperature and the height of the temperature rise while the width of the O I 135.6 nm line sets a limit to the non-thermal velocities to around 7 km/s.