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, using 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, �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 �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 minimu
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