Observed Variability of the Solar Mg II h Spectral Line

TL;DR

This study analyzes the variability of the Mg II h spectral line observed by IRIS across the Sun, revealing correlations with magnetic structures, center-to-limb effects, and discrepancies with existing models.

Contribution

It provides a comprehensive statistical analysis of Mg II h line profile variations related to solar magnetic features and compares observations with model predictions.

Findings

Average internetwork profile shows a reversed core and weak asymmetry.

Reversal depth inversely correlates with magnetic field strength.

Pronounced limb darkening observed, not predicted by models.

Abstract

The Mg II h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double gaussian model to fit the Mg II h profile for a full-Sun mosaic dataset taken 24-Aug-2014. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles which do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared with predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core which is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg II profiles observed by IRIS.