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The He I D3 Line as a Proxy for Magnetic Activity using EXPRES Solar Observations

Momo Ellwarth, Joe Llama, Lily L. Zhao, John M. Brewer, Andrew Szymkowiak, Rachael M. Roettenbacher, James T. Sikora, Alex S. Polanski, Steven H. Saar

Abstract

Stellar activity remains one of the primary challenges in the detection and characterization of low-mass exoplanets, as it can induce radial velocity (RV) variations that mask or mimic planetary signals. Identifying reliable activity proxies is essential in order to distinguish stellar variability from genuine planetary signatures. In this study, we examine the variability of the chromospheric He I D3 line in high-resolution solar spectra and assess its potential as an activity indicator. We find a strong correlation between the He I D3 line intensity variation and the Sun's unsigned magnetic flux derived from SDO HMI data as well as with the solar RVs. Our results suggest that the He I D3 line offers a promising and straightforward proxy for magnetic activity, which may complement existing stellar activity indicators. Its inclusion could help disentangle stellar signals in RV measurements and ultimately improve the detection of Earth-like exoplanets.

The He I D3 Line as a Proxy for Magnetic Activity using EXPRES Solar Observations

Abstract

Stellar activity remains one of the primary challenges in the detection and characterization of low-mass exoplanets, as it can induce radial velocity (RV) variations that mask or mimic planetary signals. Identifying reliable activity proxies is essential in order to distinguish stellar variability from genuine planetary signatures. In this study, we examine the variability of the chromospheric He I D3 line in high-resolution solar spectra and assess its potential as an activity indicator. We find a strong correlation between the He I D3 line intensity variation and the Sun's unsigned magnetic flux derived from SDO HMI data as well as with the solar RVs. Our results suggest that the He I D3 line offers a promising and straightforward proxy for magnetic activity, which may complement existing stellar activity indicators. Its inclusion could help disentangle stellar signals in RV measurements and ultimately improve the detection of Earth-like exoplanets.

Paper Structure

This paper contains 9 sections, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Data
  3. EXPRES Solar Data
  4. Unsigned Magnetic Flux
  5. Results
  6. He D3 as Proxy for Magnetic Activity
  7. Comparison with established Activity Indices
  8. Differential Rotation
  9. Summary and Conclusion

Figures (8)

  • Figure 1: Averaged EXPRES spectrum over the year 2024. Purple: Observed Spectrum with telluric lines. Black: Telluric corrected spectrum. The gray area shows the wavelength range of interest.
  • Figure 2: EXPRES spectra of the He I D3 line, averaged by year from 2021 to 2025. The red lines mark the laboratory wavelengths of the He D3 line (Table \ref{['tab:table0']}). The gray area mark the wavelength range used to calculate the line index.
  • Figure 3: Top: EXPRES master spectrum (pink), and one single observation (black) of the He I D3 line. Bottom: Ratio of this wavelength range obtained by dividing the master spectrum by the single observation. The He D3 index is obtained by integrating the gray-shaded area under these ratios.
  • Figure 4: Magnetic activity indicators as a function of time. Cyan: Unsigned magnetic flux calculated with SolAster. Magenta: Daily He D3 index derived from EXPRES solar observations. The long-term gap starting in 2022 was caused by a broken fiber in the setup, while the large observation gap in 2023 was due to smoke near the telescope site.
  • Figure 5: Same as Fig. \ref{['fig:sdo_he0']}, but zoomed in for the time range from April 2024 till end of March 2025 (during the solar maximum).
  • ...and 3 more figures