On the center-to-limb variations of the He I 10 830 \AA\ triplet

TL;DR

This study investigates the center-to-limb variations of the He I 10830 Å triplet in the quiet Sun, revealing increased optical depth and reversed convective blueshift near the limb through high-resolution spectroscopic observations and modeling.

Contribution

It provides detailed observations and analysis of the He I triplet CLV, including inversion results, highlighting new insights into the line's behavior across the solar disk.

Findings

He I triplet shows increased optical depth towards the limb.

Reversed convective blueshift observed in the red component.

He I CLV is steeper compared to nearby continuum.

Abstract

We present high-resolution spectroscopic observations of the quiet-Sun center-to-limb variations (CLV) of the He I triplet at 10 830 {\AA} and the nearby Si I 10 827 {\AA} line, observed with GREGOR Infrared Spectrograph (GRIS) and the improved High-resolution Fast Imager (HiFI+). The observations cover the interval $\mu = [0.1,\, 1.0]$, where $\mu$ is the cosine of the heliocentric angle. At each $\mu$-position, the spectra are spatially averaged over 0.02 $\mu$, and the resulting CLVs are given both as these averaged data points and as smooth polynomial curves fitted across each wavelength point. The He I spectra were inverted using the HAnle and ZEeman Light (HAZEL) code, showing an increase in optical depth towards the limb and a reversed convective blueshift for the red component, while the blue component was entirely absent. In addition, we find a strong increase in the steepness of the He I CLV compared to that of the nearby continuum. The Si I showed a behavior more typical of photospheric lines, namely shallower CLV, a reduction in width and depth, and a more typical convective blueshift.