Power spectrum of south-north travel-time shifts for m = 2 (black solid line). The blue vertical line indicates the frequency of the classical m = 2 sectoral r mode. The red solid line is the background B estimated by a fit to the power in the frequency range between −350 and −150 nHz. The red dashed line is the threshold for 95% confidence level; that is, the noise in the background only has a 5% chance of being higher than this threshold for at least one frequency bin. We note that the spike around −295 nHz (on the right side of the blue line) is above the background but much lower than the 95% confidence level. The three green lines indicate the power that would correspond to a m = 2 sectoral mode with rms velocity of 0.4, 0.5, or 0.6 m s⁻¹.