The Temperature Diagnostic Properties of the MG I 457.1 NM Line

TL;DR

This paper investigates the formation and diagnostic potential of the Mg I 457.1 nm line for measuring temperature in the lower solar chromosphere, emphasizing non-LTE effects and model simplifications.

Contribution

It provides a detailed analysis of non-LTE effects on the Mg I 457.1 nm line and develops a simplified model atom for efficient temperature diagnostics in solar atmospheres.

Findings

The line is strongly coupled to local temperature and LTE conditions.

Non-LTE ionization is dominated by photo-ionization and collisional recombination.

The line is effective for diagnosing temperature in the lower chromosphere, especially in cooler regions.

Abstract

We analyze the important formation processes for the Mg I 457.1 nm line. This line is an intercom- bination line and the source function is close to the local thermodynamic equilibrium (LTE) value. The strong coupling to the local temperature and the relatively high population of the lower level (the ground state of Mg I) makes this line an ideal candidate for temperature diagnostics in the lower chromosphere/temperature minimum region. Linking the temperature probed to an absolute phys- ical height is non trivial because of Non-LTE ionization. We analyze the Non-LTE effects and find that photo-ionization from the lower energy levels together with cascading collisional recombination dominate the ionization balance. Taking properly into account the line-blanketing in the UV is essen- tial for obtaining the right photoionization rates. The identification of the main Non-LTE effects in the line allows us to construct a quintessential model atom, ideal for computationally demanding tasks, e.g. full 3D and/or time-dependent radiative transfer. Furthermore we analyze the diagnostic potential to temperature of this line in solar-like atmospheres, by synthesizing the line from a series of parametrized atmospheric models. These models have been constructed with fixed effective temper- ature, but with a variable heat term in the energy equation to obtain a chromospheric temperature rise at different heights. We conclude that the line has a significant potential in the diagnostics of the lower chromosphere temperature structure, especially for cooler atmospheres, such as sunspots.