(a) Power in the k_h − ω diagnostic diagram of the line core intensity of the Fe I λ 5434 Å line. The phase (b) and coherence (c) spectra between the line-core intensity of the Fe I λ 5576 Å and Fe I λ 5434 Å lines are also shown. The white line marks the dispersion relation of the surface gravity waves (f-mode). The coloured contour lines in (a)–(c) mark the coherence levels with the reference colours shown in the colourmap of (c). (d) Power as a function of k_h integrated over the IGW region for the continuum intensity and line-core intensities of the Fe I λλ 5576 Å and 5434 Å lines. Also shown in black is the mean coherence in the IGW region.