Impact of the solar activity cycle on frequency separation ratios in helioseismology

Roxburgh & Vorontsov have recently proposed the use of ratios of small to large frequency separations of low angular degree p-modes as a means of eliminating from asteroseismic data the unwanted influence of the structure of the near-surface layers of stars. Here we have studied the impact of the solar activity cycle on the magnitude of these so-called frequency separation ratios using data collected by Sun-as-a-star observations. The ratios are observed to change with the shifting level of global solar activity. The effect, which we detect in BiSON Doppler velocity data at a marginal level of significance, is shown to be a consequence of the influence of acoustic asphericity from the surface activity on the azimuthally dependent Sun-as-a-star frequencies. The results suggest that any analysis that makes use of ratios formed from long helioseismic data sets may therefore show effects of bias. While the effect is less significant in shorter data sets, of length similar to what will soon be available from asteroseismic campaigns, an approximate doubling of the effects from the solar asphericity may be sufficient to cause complications for stellar analyses.