The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets

TL;DR

This study uses non-LTE radiative transfer calculations to explore how coronal EUV irradiance influences the emission profiles of He I 10830 A and D3 multiplets, proposing a new diagnostic for coronal irradiance based on emission line ratios.

Contribution

It demonstrates that the intensity ratio of He I 10830 A multiplet components can serve as a diagnostic tool for coronal EUV irradiance, enhancing solar atmospheric diagnostics.

Findings

The blue/red emission ratio correlates with coronal EUV irradiance.

Modeling matches observed emission profiles at various heights.

The ratio can diagnose EUV radiation levels in the solar atmosphere.

Abstract

Two of the most attractive spectral windows for spectropolarimetric investigations of the physical properties of the plasma structures in the solar chromosphere and corona are the ones provided by the spectral lines of the He I 10830 A and 5876 A (or D3) multiplets, whose polarization signals are sensitive to the Hanle and Zeeman effects. However, in order to be able to carry out reliable diagnostics, it is crucial to have a good physical understanding of the sensitivity of the observed spectral line radiation to the various competing driving mechanisms. Here we report a series of off-the-limb non-LTE calculations of the He I D3 and 10830 A emission profiles, focusing our investigation on their sensitivity to the EUV coronal irradiation and the model atmosphere used in the calculations. We show in particular that the intensity ratio of the blue to the red components in the emission profiles of the He I 10830 A multiplet turns out to be a good candidate as a diagnostic tool for the coronal irradiance. Measurements of this observable as a function of the distance to the limb and its confrontation with radiative transfer modeling might give us valuable information on the physical properties of the solar atmosphere and on the amount of EUV radiation at relevant wavelengths penetrating the chromosphere from above.