FFT spectrum of amplitude envelope of the semidiurnal variation of TEC (bandpass 1.8–2.2 cpd) in Southern China (blue line). The envelope is modulated by periods of the solar cycle, seasonal variations (annual oscillation AO, semiannual oscillation SAO and further harmonics). The beat of the lunar and solar semidiurnal variation generates a spectral peak at the beat frequency 2 cpd − M₂ (equal to a period of 14.7653 days or half of a lunar synodic month). The magenta line denotes the 95% confidence level.