B. A. Miller

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.

We present low-degree solar p-mode linewidths from the analysis of several high-resolution frequency spectra generated from high-quality Doppler velocity data collected — in integrated sunlight — by the Birmingham Solar-Oscillations Network (BiSON) between 1991 July and 1995 July. We have used linewidths, Δv, measured in a 32-month frequency spectrum, to test the power-law dependence of the widths at low frequencies. Over the range 1473 ≤ v ≤ 1822 μHz, we find Δv α v^7.0±1.5. If an effort is made to correct for finite observing time systematics, the fitted index increases to 7.3 (and even higher, to 7.8, if a naive, single-bin- width correction is applied). This is somewhat steeper than reported in previous observations of intermediate-degree modes, and is more in line with theoretical calculations which predict Δv α v⁸ over this frequency range. If one assumes that the steep dependence reported here persists down to lower frequencies, low-degree-mode coherence (e-folding) times at 800 μHz of > 14 yr, and at 300μHz (i.e. near the frequency expected for the fundamental) of > 13 000 yr are implied.