As a container of electrical charges, the Earth-ionosphere system is a leaky capacitor: continually charged by thunderstorms, currents "leak off" of this system in areas of fair weather in the form of lightning. In the 1930s, the research ship Carnegie measured this fair weather current. These researchers found that the current exhibits a diurnal (daily) variation that is periodic, and speculated that this variation was the result of an uneven distribution of land around the Earth (only about 10% of all thunderstorms occur over the oceans). Peak thunderstorms typically occur at 4:00PM local, so the thunderstorm contributions move around the globe with the Sun.
Recently, researchers from The National Space Science and Technology Center (NSSTC, consisting of NASA, USRA's Science and Technology Institute, University of Alabama in Huntsville and others) found that the sole use of lightning data did not accurately reproduce the Carnegie curve. Rather, when all of the data was totaled, a periodic variation did exist, but not quite like the one the Carnegie researchers estimated. In order to discover the cause of the variation, for the past 15 years, Earth scientists have made high-altitude aircraft flights over thunderstorms, measuring currents above all types of clouds, not just clouds that produce the lightning seen by satellites. The aircraft measurements revealed that electrified, non-lightning-producing clouds in the tropics were needed to reproduce what was measured from Carnegie.
This improved agreement explains 90% of the daily variation of the Carnegie curve, thereby solving an 80-year-old puzzle about how the Earth system works. These research findings have just been published as a three-paper series in the Journal of Geophysical Research.