Influence of the Moon phase on ionospheric total electron content observed by GNSS remote sensing

The average influence of the Moon phase on ionospheric total electron content (TEC) is derived from TEC maps of the International Global Navigation Satellite System (GNSS) Service (IGS) from 1998 to 2024. At low latitudes, we find three spectral lines in the Fast Fourier Transform (FFT) amplitude spectra of TEC having frequencies of lunar variations at M 2 − 1 cpd (diurnal), M 2 (semidiurnal), and M 2 + 1 cpd (terdiurnal) where cpd stands for cycles per day and M 2 is the lunar semidiurnal variation with a period of 12.4206 hours. These spectral lines of the lunar variations are close to the spectral lines of the solar diurnal, semidiurnal and terdiurnal variations, so that a beat of the solar and lunar oscillations occurs with a frequency of 2 − M 2 which corresponds to a beat period of 14.7653 days (half of a lunar synodic month). In the modulation spectrum of the semidiurnal amplitude envelope, a significant spectral peak is at 1/14.7653 cpd and is about 0.8 TECU (TEC Units) in Southern China which is comparable to the strength of the seasonal modulation of the semidiurnal amplitude. A composite analysis of the amplitude envelope of the semidiurnal TEC variation is performed with respect to the Moon phase. Generally, the maximal semidiurnal TEC amplitude is reached 2–3 days after New Moon (or Full Moon). The influence of the Moon phase on the diurnal and terdiurnal amplitudes is similar but less clear as for the semidiurnal amplitude. The world map of the semidiurnal beat amplitude is presented. It shows increased beat amplitudes at the equatorial ionization anomaly (EIA). The maximal semidiurnal amplitude at low latitudes is mostly reached a few days after New Moon. In some regions, the maximal semidiurnal amplitude appears at First Quarter Moon or at New Moon.