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THE GLOBAL ATMOSPHERIC-ELECTRICAL CIRCUIT 206 15 The Global Atmospheric-Electrical Circuit Raymond G. Roble and Israel Tzur National Center for Atmospheric Research Lightning was recognized as a grand manifestation of static electricity within thunderstorm clouds in the eighteenth century. It was also recognized that electrical phenomena are not confined to thunderclouds and that a weak electrification exists as a permanent property of the atmosphere even during fair weather. Further research established that the Earth's surface is charged negatively and the air is charged positively, with a vertical electric field of about 100 V/m existing in the atmosphere near the Earth's surface. An electrostatic explanation for the phenomena was sought at first, and one theory suggested that the electric field of the atmosphere was the result of an intrinsic negative charge on the Earth, probably collected during the Earth's formation. With the discovery of cosmic-ray ionization in the early twentieth century, it was realized that air possesses an electrical conductivity due to its ion content. As a result of the finite electrical conductivity, vertical conduction currents flow from the atmosphere to the Earth, tending to neutralize the charge on the Earth. On the basis of actual conductivity values it was calculated that charge neutralization would take place in less than an hour, and the continued existence of an electric field suggested some generation mechanism to oppose the leakage currents flowing to the Earth. The search for this generation mechanism soon became the main object of research on global atmospheric electricity. In the early twentieth century the concept of a global circuit of atmospheric electricity slowly began to evolve (IsraÃ«l, 1973; Pierce, 1977). The net positive space charge in the air between the ground and a height of about 10 km is nearly equal to the negative charge on the surface of the Earth. The electrical conductivity of the air increases rapidly with altitude, and the product of the local vertical electric field and local conductivity at any altitude within an atmospheric column gives a constant air-earth current flowing downward. This constant air-earth current with respect to altitude implies that the current flow is mainly driven by a constant difference in potential between the surface of the Earth and some higher altitude in the atmosphere. The discovery of the highly conducting ionosphere in the 1920s explained the long-range propagation of radio waves and was important for the evolution of the concept of the global electric circuit. The ionosphere, with its large electrical conductivity, provided a means of closing the global circuit. It, however, is not a perfect conductor parallel to the Earth's surface, but it possesses a finite conductivity, and the electric currents and fields within it are driven by the combined action of the ionospheric and magnetospheric dynamo systems as well as by current generation from the lower atmosphere. Wilson (1920) first demonstrated that a thunderstorm supplies a negative charge to the Earth. In the 1920s, it was also known that over the oceans and in polar areas