Eleven-year periodic components and power spectra of HL1 mode parameters. Left column, from top to bottom: sunspot number, HL1 mode amplitude, eigenfunction tilt, and frequency. The 55-yr long time series combine the MWO (1968 – 2010) and HMI (2011 – 2022) measurements taken from Figs. 7 and 8. The gray curves are fitted parabolas representing the long-term trend. The pink curves show the 11-yr periodicity in the data added to the long-term trend. Right column: power spectra of the time series from the left column with the temporal mean subtracted. The vertical pink lines indicate the frequency of (11yr)⁻¹ = 2.88 nHz that corresponds to the average sunspot cycle period. The horizontal dashed and dotted lines represent the 95% and 90% confidence levels, respectively. The 11-year periodicity is highly significant for the mode amplitude, and only marginally significant for the eigenfunction tilt. No evidence is found for an 11-year periodicity in the mode frequency.