Line-of-sight Doppler velocity associated with the HL1 mode as viewed from the Earth on 09 September 2018 when the solar tilt angle B₀ is 7.24°, extracted from time series of daily HMI images by applying a band-pass filter around ${\nu }_{HL1}^{\mathrm{synodic}}$ (see Eq. (8)). The LOS velocity pattern in the polar regions is associated with the HL1 mode. The image was smoothed using a 2D Gaussian kernel with a FWHM of 5°. A comparison with the reconstructed $V_{\mathrm{los}}^{HL1}$ using 5°-tile ring-diagram flow maps is shown in Fig. A.3 and in the online movie.