Solar cycle variation of $\nu_{\rm max}$ in helioseismic data and its implications for asteroseismology

The frequency, $\nu_{\rm max}$, at which the envelope of pulsation power peaks for solar-like oscillators is an important quantity in asteroseismology. We measure $\nu_{\rm max}$ for the Sun using 25 years of Sun-as-a-Star Doppler velocity observations with the Birmingham Solar-Oscillations Network (BiSON), by fitting a simple model to binned power spectra of the data. We also apply the fit to Sun-as-a-Star Doppler velocity data from GONG and GOLF, and photometry data from VIRGO/SPM on the ESA/NASA SOHO spacecraft. We discover a weak but nevertheless significant positive correlation of the solar $\nu_{\rm max}$ with solar activity. The uncovered shift between low and high activity, of $\simeq 25\,\rm \mu Hz$, translates to an uncertainty of 0.8 per cent in radius and 2.4 per cent in mass, based on direct use of asteroseismic scaling relations calibrated to the Sun. The mean $\nu_{\rm max}$ in the different datasets is also clearly offset in frequency. Our results flag the need for caution when using $\nu_{\rm max}$ in asteroseismology.