The solar cycle as seen by low-ℓ p-mode frequencies: comparison with global and decomposed activity proxies

Abstract

We present a detailed study of variations observed in low-angular degree solar p-mode frequencies during solar cycles 22 and 23, and their relation to global and spatially decomposed proxies of the surface activity. To do so, we have analysed 11 yr of unresolved (Sun-as-a-star) Doppler velocity observations of the solar disc made by the Birmingham Solar-Oscillations Network (BiSON). The sensitivity of these observations to different azimuthal orders, m, is such that for the degree range studied (0 ≤ℓ≤ 2) extracted frequencies can be regarded as providing a measure of the response of the sectoral modes (with |m| =ℓ). After allowing for the dependence of the frequency shifts on mode frequency and inertia, we find the average ℓ= 0 shift is significantly weaker than that at ℓ= 2; the magnitude of the average ℓ= 1 shift lies in between the two. The comparative sizes of the shifts are observed to match those of the corresponding spherical harmonic (Legendre) components of both the Kitt Peak magnetogram (KPMI) and He I equivalent width activity indices, and reflect, therefore, the sensitivity of the observed mode components to the distribution of activity over the solar surface (i.e. a ‘spatial’ contribution to the shifts). When the falling and rising parts of the cycles are analysed independently, we uncover a significant difference in behaviour at ℓ= 1. Possible reasons for this are discussed.