Abstract. We present a study of statistical relationships between the G condition, F1-layer and NmF2 negative disturbance occurrence probabilities and geomagnetic and solar activity indices Kp and F10.7, season, and geomagnetic latitude, busing experimental data acquired by the Ionospheric Digital Database of the National Geophysical Data Center, Boulder, Colorado from 1957 to 1990. It is shown that the dependence of the G condition occurrence probability, y_G, on Kp is mainly determined by processes that control the behaviour of the F2 layer with Kp changes. We found that the relationship for log y_G versus Kp is very close to the linear one. The G condition occurrence probability decreases from 0.55% to 0.17% as the value of  F10.7 increases from low to middle values, reaches its minimum at the middle solar activity level of F10.7 = 144 – 170, increasing from the minimum value of 0.17% to 0.49% when the F10.7 index increases from the middle solar activity level to F10.7 = 248 – 274. Interhemispheric asymmetry is found for the G condition occurrence probability in the ionosphere, with a stronger enhancement seen in the magnetic latitude range close to the northern magnetic pole and a deep minimum of the G condition occurrence probability in the low magnetic latitude range from – 30° to 30°. The measured magnetic latitude variation of the F1-layer occurrence probability is also asymmetrical relative to the geomagnetic equator. Our results provide additional evidence the F1-layer is more likely to be formed in summer than in winter. The Northern Hemisphere peak F1-layer occurrence probability is found to exceed that in the Southern Hemisphere. The G condition occurrence probability has maximum values of 0.91 and 0.75% in summer, and minimum values of 0.01 and 0.05% in winter for the Northern and Southern Hemisphere, respectively.

Key words. Ionosphere; ion chemistry and composition; ionosphere-atmosphere interactions; ionospheric disturbances