Average p₁ power spectrum obtained from SDO/HMI dopplergrams. The sampling is given by h_kR_⊙ = 24.5 and h_ω/ 2π = 34.7μHz. Left panel: cut at frequency ω/ 2π = 3.4 mHz. Right panel: cut at k_y = 0. The dark parts correspond to large values of the power spectrum and the white ones to small values.