On understanding the meaning of ℓ = 2 and 3 p-mode frequencies as measured by various helioseismic instruments

Aims. Frequencies of the low-degree p-mode oscillations of the Sun may be extracted either from data collected by instruments that make full-disc observations of the Sun as a star, or from data collected by instruments that resolve, or image, the Sun's surface onto many detector elements. The two methods can show marked differences in their sensitivity to modes having certain combinations of degree and azimuthal order. These different sensitivities lead to differences in measurements of the central frequencies of the modes, which must be properly accounted for if data from two different instruments are to be compared, or combined. Methods. We perform an analytical derivation of the p-mode frequency offsets expected between contemporaneous Sun-as-a-star and resolved-Sun data. Results. Here, we demonstrate that the empirical factors derived by Chaplin et al. ( 2004, A& A, 424, 713) are reproduced by our analysis, but with a more marked dependence upon the mode linewidth