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Table 2.

Summary of the nature, wavelength, and formation height of the absorption line used by the instruments considered in this paper.

Instrument Line λ (Å) Height (km)
MDI/GONG Ni I 6768 170
GOLF Na I D1/D2 5896/5890 340
HMI Fe I 6173 100

Notes. The formation heights are given with respect to the photosphere. References: Fleck et al. (2011; MDI/GONG, HMI) and Baudin et al. (2005; GOLF). Note that the MDI instrument and the GONG network use the same spectral line and therefore observe the modes at the same altitude.

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