Imprint of the magnetic activity cycle on solar asteroseismic characterisation based on 26 years of GOLF and BiSON data

Asteroseismic modelling will play a key role in future space-based missions, such as PLATO, CubeSpec, and Roman. Despite remarkable achievements, asteroseismology has revealed significant discrepancies between observations and theoretical predictions of the physics used in stellar models, which have the potential to bias stellar characterisation at the precision level demanded by PLATO. The current modelling strategies largely overlook magnetic activity, assuming that its effects are masked within the parametrisation of the so-called `surface effects'. Given the presence of activity cycles in multiple solar-like oscillators and activity variations in a significant fraction of Kepler observations of main-sequence stars, it is therefore relevant to investigate systematic errors in asteroseismic characterisations caused by our incomplete understanding of magnetic activity. Based on 26.5 years of GOLF and BiSON observations, we measured the impact of magnetic activity on the asteroseismic characterisation of the Sun as a star, a reference target for assessing the PLATO mission requirements. The GOLF and BiSON observations, which fully cover solar cycles 23 and 24, were divided into yearly overlapping snapshots, each delayed by a quarter of a year. For each snapshot, an advanced asteroseismic characterisation, similar to that to be adopted by the PLATO pipeline, was performed with standard prescriptions for the parametrisation of the surface effects. This allowed us to monitor the apparent temporal evolution of fundamental solar parameters such as mass, radius, and age. The correlation of these parameters with the 10.7 cm radio emission flux, a proxy of the solar activity cycle, was then measured. The effects of magnetic activity are partially absorbed into the parametrisation of the surface effects when suitable prescriptions are used, and they do not significantly affect the measured solar mass or radius. However, contrary to literature expectations, we find a significant imprint on the age determination, with variations of up to 6.5 between solar minima and maxima. This imprint persists across both BiSON and GOLF datasets. Considering that the Sun exhibits low levels of activity, our study highlights the looming challenge posed by magnetic activity for future photometry missions, and it prompts a potential reevaluation of the asteroseismic characterisation of the most active Kepler targets.