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The global atmospheric electrical circuit is the continuous movement of electrical current between the ionosphere and the earth's surface. This flow is powered by thunderstorms, which cause a build-up of positive charge in the ionosphere. In fair weather this positive charge slowly flows back to the surface.

History
Atmospheric electricity has been studied since 1750, when Franklin and D'Alibard began their famous thunderstorm experiments. In 1752, Lemonnier observed electrification in fair weather. Various others performed measurements thoughout the late 18th century, often finding consistent diurnal variations. During the 19th century, several long series of observations were made. Measurements near cities were heavily influenced by smoke pollution. In the early 20th century, balloon ascents provided information about the electric field in the upper atmosphere. Important work was done by the research vessel Carnegie, which produced standardised measurements around the world's oceans (where the air is relatively clean).

C.T.R. Wilson was the first to present a theory of a global circuit in 1920.

Mechanism
Thunderstorms generate an electrical potential difference between the earth's surface and the ionosphere, mainly by means of lightning. Because of this, the ionosphere is positively charged relative to the earth. Consequently, there is always a small current transporting charged particles between the ionosphere and the surface. Since air is a good insulator, this current is carried by a small number of ions present in the atmosphere (generated mainly by cosmic rays in the upper atmosphere, and by radioactivity near the surface).

The voltages involved are significant. At sea level, the typical potential gradient in fair weather is 120 V/m. Nonetheless, since the conductivity of air is limited, the associated currents are also limited. A typical value is 1800 Ampère over the entire planet.