Few, Arthur, A Department of Space Physics and Astronomy, Rice University, Houston, Texas.
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The acoustic radiation produced by thermal lightning channel processes. For an electric discharge to propagate through air, the leading tip of the channel must be very hot. This is necessary for photoionization to occur and thereby produce the electrons needed to continue developing the channel. The hot channels produce light and shock waves; hence, all discharges create sound. Lightning flashes to the ground are special because the ground is a conductor in comparison to air and the ground can supply very large currents, called return strokes, to the lightning channel. The sound from this process is what is most commonly called thunder. The lightning return stroke is a high surge of electric current (about 20,000 A) that occurs when the lightning flash makes contact with the Earth. The current surge has a very short duration, depositing approximately 95% of its electrical energy during the first 20 μs with its peak power occurring at 2 μs. Spectroscopic studies have shown that the lightning channel is heated to temperatures in the 20,000–30,000 K (36,000–54,000°F) range by this process. The lightning channel at this time has a diameter of approximately 1 cm (0.4 in.), and the pressure of the hot channel exceeds 10 atm (106 pascals). The hot, high-pressure channel expands supersonically and reaches a radius of 5 cm (2 in.) within the 20-μs period during which it is being heated. The channel continues expanding and forms a shock wave as it pushes against the surrounding air. Because of the momentum gained in expanding, the shock wave overshoots, causing the pressure in the core of the channel to fall below atmospheric pressure temporarily. The outward-propagating wave separates from the core of the channel, forming an N-shaped wave that eventually decays into an acoustic wavelet. The core remains hot at atmospheric pressure and cools by mixing with the surrounding air. The length of the N-shaped wave and the resulting acoustic wave increase with the energy deposited in the channel, and also increase as the ambient atmospheric pressure decreases. See also: Photoionization; Shock wave; Storm electricity
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