Visibility of HL1 mode in consecutive three-year power spectra, using MWO (1969–2010) and HMI (2011–2022) data. Panel a: example MWO time series (1984–1986) of $V_{\mathrm{zonal}}^{(-)}$ averaged over latitudes 60° ≤λ < 75° (i.e., $V_{\mathrm{zonal}}^{(\mathrm{band})}$). Panel b: power spectra of $V_{\mathrm{zonal}}^{(-)}$ for the same time period in various latitudinal bands of 15° (labeled at the top right corner of each panel). The vertical red lines denote the reference HL1 mode frequency ${\nu }_{HL1}^{\mathrm{synodic}}=338$ nHz and the excess power around this frequency is highlighted in blue. The shaded pink areas indicate the differential rotation rate in the outer 5% of the Sun from global helioseismology as viewed from the Earth. Panels c and d: same as above but for an example HMI three-year time series during 2017–2019. Panel e: all available three-year power spectra of $V_{\mathrm{zonal}}^{(\mathrm{band})}$ stacked on top of each other. The crosses highlight the reference frequency of 338 nHz.