Explanation of the activity sensitivity of Mn I 5394.7 \AA

TL;DR

This study reveals that the unusual activity sensitivity of the Mn I 5394.7 Å line is due to its hyperfine structure, not chromospheric effects, making it a useful diagnostic for strong magnetic fields in the solar photosphere.

Contribution

The paper demonstrates that hyperfine structure causes the Mn I 5394.7 Å line's activity sensitivity, challenging previous chromospheric-based explanations and clarifying its diagnostic potential.

Findings

Hyperfine structure explains Mn I line's activity sensitivity.

Chromospheric effects like Mg II h & k are not responsible.

Mn I lines are promising for weak field diagnostics.

Abstract

There is a long-standing controversy concerning the reason why the Mn I 5394.7 A line in the solar irradiance spectrum brightens more at larger activity than most other photospheric lines. The claim that this activity sensitivity is caused by spectral interlocking to chromospheric emission in Mg II h & k is disputed. Classical one-dimensional modeling is used for demonstration; modern three-dimensional MHD simulation for verification and analysis. The Mn I 5394.7 A line thanks its unusual sensitivity to solar activity to its hyperfine structure. This overrides the thermal and granular Doppler smearing through which the other, narrower, photospheric lines lose such sensitivity. We take the nearby Fe I 5395.2 A line as example of the latter and analyze the formation of both lines in detail to demonstrate and explain granular Doppler brightening. We show that this affects all narrow lines. Neither the chromosphere nor Mg II h & k play a role, nor is it correct to describe the activity sensitivity of Mn I 5394.7 A through plage models with outward increasing temperature contrast. The Mn I 5394.7 A line represents a proxy diagnostic of strong-field magnetic concentrations in the deep solar photosphere comparable to the G band and the blue wing of H-alpha, but not a better one than these. The Mn I lines are more promising as diagnostic of weak fields in high-resolution Stokes polarimetry.