The sun controls the earth's global electrical circuit
Data collected from electrosondes (balloons measuring atmospheric electrical currents) over the Antarctic ice caps infer that solar flares stimulate large surges in the flow of electrical charge from the upper atmosphere to the earth's surface. Because this unidirectional flow of fair-weather electricity must ultimately be balanced by thunderstorms somewhere on the planet, it follows that the frequency and severity of terrestrial thunderstorms are dictated, at least on the average, by solar activity. Formerly, global circuit theory had it that the thunderstorms themselves were the driving force behind the fairweather current flow. Now it seems that the sun calls the tune and that thunderstorms do not arise at random.
(Anonymous; "Solar Activity and Terrestrial Thunderstorms," New Scientist, 81:256, 1979.)