The global atmospheric electric circuit and the Carnegie curve

Lightning and the SR are part of a broader system — the global atmospheric electric circuit. In this circuit, storms act as a generator, a kind of planetary "battery": through their discharges and the separation of charge they maintain a positive potential of the ionosphere relative to the Earth's surface. In fair weather (away from storms), a weak return current then flows through the atmosphere back to the ground, closing the circuit.

Because the total potential is governed primarily by the instantaneous global lightning activity, the circuit voltage follows the diurnal rhythm of the three continental centers (see #28). This diurnal variation is called the Carnegie curve after measurements from the research voyages of the ship Carnegie. The key point is that the curve repeats at the same Universal Time regardless of where on Earth we measure — which proves that it is a truly global, not local, phenomenon.

The Carnegie curve and the diurnal course of the SR therefore reflect the same thing: how much lightning is striking worldwide right now.