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Experimental data\ud acquired by the Ionospheric Digital Database of the National Geophysical Data\ud Center, Boulder, Colorado, from 1957 to 1990, are used to study the dependence\ud of the G condition, F1-layer, and NmF2 negative disturbance occurrence\ud probabilities on the solar zenith angle during summer, winter, spring, and\ud autumn months in latitude range 1 (between - 10° and + 10° of the geomagnetic\ud latitude, 8), in latitude range 2 (10° < |F| <\ud 30°), in latitude range 3 (30° < |j| <\ud 45°, 30° < |F| < 45°), in latitude\ud range 4 (45° < |j| < 60°, 45° < |F|\ud < 60°), and in latitude range 5 (60° < |F|\ud < 90°), where j is the geographic\ud latitude. Our calculations show that the G condition is more likely to occur\ud during the first half of a day than during the second half of a day, at all\ud latitudes during all seasons for the same value of the solar zenith angle. The\ud F1-layer occurrence probability is larger in the first half of a day in\ud comparison with that in the second half of a day for the same value of the\ud solar zenith angle in latitude range 1 for all seasons, while the F1-layer\ud occurrence probability is approximately the same for the same solar zenith\ud angle before and after noon in latitude ranges 4 and 5. The F1-layer and G\ud condition are more commonly formed near midday than close to post sunrise or\ud pre-sunset. The chance that the day-time F1-layer and G condition will be\ud formed is greater in summer than in winter at the given solar zenith angle in\ud latitude ranges 2–5, while the F1-layer occurrence probability is greater in\ud winter than in summer for any solar zenith angle in latitude range 1. The\ud calculated occurrence probability of the NmF2 weak negative disturbances\ud reaches its maximum and minimum values during daytime and night-time\ud conditions, respectively, and the average night-time value of this probability\ud is less than that by day for all seasons in all studied latitude regions. It is\ud shown that the NmF2 normal, strong, and very strong negative\ud disturbances are more frequent on average at night than by day in latitude\ud ranges 1 and 2 for all seasons, reaching their maximum and minimum occurrence\ud probability values at night and by day, respectively. This conclusion is also\ud correct for all other studied latitude regions during winter months, except for\ud the NmF2 normal and strong negative disturbances in latitude range 5. A\ud difference in the dependence of the strong and very strong NmF2 negative\ud disturbance percentage occurrences on the solar zenith angle is found between\ud latitude ranges 1 and 2. Our results provide evidence that the daytime\ud dependence of the G condition occurrence probability on the solar zenith angle\ud is determined mainly by the dependence of the F1-layer occurrence probability\ud on the solar zenith angle in the studied latitude regions for winter months, in\ud latitude range 2 for all seasons, and in latitude ranges 4 and 5 for spring,\ud summer, and autumn months. The solar zenith angle trend in the probability of\ud the G condition occurrence in latitude range 3 arises in the main from the\ud solar zenith angle trend in the F1-layer occurrence probability. The solar\ud zenith angle trend in the probabilities of strong and very strong NmF2\ud negative disturbances counteracts the identified solar zenith angle trend in\ud the probability of the G condition occurrence. Key words. Ionosphere (ionospheric disturbances,\ud ionosphere-atmosphere interactions, ion chemistry and composition

Solar zenith angle dependencies of F1-layer, &lt;i&gt;Nm&lt;/i&gt;F2 negative disturbance, and G-condition occurrence probabilities

Solar zenith angle dependencies of F1-layer, <i>Nm</i>F2 negative disturbance, and G-condition occurrence probabilities