No swan song for Sun-as-a-star helioseismology: performances of the Solar-SONG prototype for individual mode characterisation

TL;DR

This study evaluates the Solar-SONG spectrograph's ability to perform helioseismic measurements, demonstrating its capability to characterize low-frequency p modes and comparing its performance with established instruments like GOLF, BiSON, and HMI.

Contribution

The paper introduces a new Python data processing pipeline and assesses Solar-SONG's performance in helioseismology, highlighting its effectiveness in low-frequency mode detection.

Findings

Solar-SONG can characterize p modes at lower frequencies than BiSON and GOLF.

The new Python pipeline improves data reduction and analysis.

Solar-SONG's performance is comparable to or better than existing instruments in certain frequency ranges.

Abstract

The GOLF instrument on board SoHO has been in operation for almost 25 years but aging of the instrument has now strongly affected its performance, especially in the low-frequency p-mode region. At the end of the SoHO mission, the ground-based network BiSON will remain the only facility able to perform Sun-integrated helioseismic observations. Therefore, we want to assess the helioseismic performances of an \'echelle spectrograph like SONG. Indeed, the high precision of such an instrument and the quality of the data acquired for asteroseismic purpose calls for an evaluation of the instrument ability to perform global radial-velocity measurements of the solar disk. Data acquired during the Solar-SONG 2018 observation campaign at the Teide Observatory are used to study mid- and low-frequency p modes. A Solar-SONG time series of 30-day duration is reduced with a combination of the traditional IDL iSONG pipeline and a new Python pipeline described in this paper. A mode fitting method built around a Bayesian approach is then performed on the Solar-SONG and contemporaneous GOLF, BiSON, and HMI data. For this contemporaneous time series, Solar-SONG is able to characterise p modes at a lower frequency than BiSON and GOLF (1750{\mu}Hz against 1946 and 2157 {\mu}Hz respectively), while for HMI it is possible to characterise a mode at 1686 {\mu}Hz. The decrease of GOLF sensitivity is then evaluated through the evolution of its low-frequency p-mode characterisation abilities over the years. [abridged]