Moon phase of maximal semidiurnal amplitude of TEC. Maximum at New Moon (or Full Moon) is at phase 0°, maximum at First Quarter Moon (or Third Quarter Moon) is at 180°. Light brownish colors (e.g., at location of red cross) means that the maximum occurs a few lunar days after New Moon or Full Moon. The white areas are regions where the beat amplitude is too small (<0.17 TECU). The magenta line denotes the geomagnetic equator. The red cross denotes the location in Southern China (25°N latitude, 110°E longitude) which we analysed in the Figures 1–5.