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

TL;DR

The paper addresses center-to-limb variations of the He I 10 830 Å triplet in the quiet Sun and its relation to coronal coupling, using high-resolution GRIS+HiFI+ observations across $μ \in [0.1,1.0]$. It employs the HAZEL inversion framework to extract optical depth and Doppler parameters, revealing an increasing opacity toward the limb and a reversed convective signature for the red component, with the blue component effectively absent. The He I CLV is found to be steeper than the nearby continuum, contrasting with the Si I reference line which shows more typical photospheric behavior. The resulting CLV dataset provides a benchmark for radiative-transfer modeling of chromospheric lines and informs interpretations of stellar activity and exoplanet-atmosphere signatures, with the data made publicly available via CDS.

Abstract

We present high-resolution spectroscopic observations of the quiet-Sun center-to-limb variations (CLV) of the He I triplet at 10 830 Å and the nearby Si I 10 827 Å line, observed with GREGOR Infrared Spectrograph (GRIS) and the improved High-resolution Fast Imager (HiFI+). The observations cover the interval $μ= [0.1,\, 1.0]$, where $μ$ is the cosine of the heliocentric angle. At each $μ$-position, the spectra are spatially averaged over 0.02 $μ$, 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.

Figures (6)

• Figure 1: Solar disk with the seven pointings used in this work. Ten concentric rings from $\mu=0.9$ to $\mu=0.0$ indicate steps in heliocentric angle. The background shows an AIA 1600 Å image Lemen2012 taken at 08:10:38 UT on the same day as the observations.
• Figure 2: HAZEL fit (orange) of HeI triplet observations at $\mu=1$ (black dashed). The rest velocities of the red and blue components are marked with vertical lines in the respective colors. Basic information and the best-fit parameters are given in the box at the upper-left corner. These include the heliocentric angle $\theta$, the Doppler velocity $v$, the Doppler width $\Delta v$, the optical depth $\tau$, and the quality of fit $\chi^2$.
• Figure 3: CLV maps of the GRIS spectral window. Mean CLV profiles (left) across the full $\mu$-range. Oscillations are visible across the entire range. Cleaned profiles (middle) after polynomial fitting. Normalized profiles (right) of the middle panel, showing the relative CLV behavior across the spectral window.
• Figure 4: A one dimensional version of the right panel of Fig. \ref{['fig:Fig1']} showing the changes in the spectrum as a function of $\mu$. A zoomed-in window focuses on the HeI line at 10 830 Å. The black vertical line marks the wavelength of the intensity calibration.
• Figure 5: Inferred spectral line behavior as a function of $\mu$. Left: The optical depth $\tau$ of the HeI line. Right: Doppler velocity for the median HeI (black) and SiI (gray) line profiles, together with their polynomial fits shown in red and blue dashed lines, respectively.