Statistical analysis and model of spread F occurrence in China

The spread F data obtained over Lanzhou (36.1A degrees N, 103.9A degrees E), Chongqing (29.5A degrees N, 106.4A degrees E) and Haikou (20.0A degrees N, 110.3A degrees E) of China during the period from 1978 to 1997 are used to analyze the occurrence characteristics. The statistical results show that the post midnight spread F occurrence is maximum during the summer solstice months of the lower solar activity period, while post sunset spread F is dominant in equinoxes of higher solar activity period over Haikou station. Over Chongqing and Lanzhou stations, spread F mostly occurs at post midnight and relates negatively with solar activity. Using regression method and Fourier expansion, the preliminary single-station model of spread F occurrence is established and the accuracy of the model is evaluated.