In 1749 Benjamin Franklin made a fundamental discovery—that lightning is an electrical discharge between a thundercloud and Earth. Such a discharge can only occur if the atmosphere, which is normally an insulator, undergoes electrical breakdown. Therein lies our first mystery.
The conventional breakdown taught in textbooks originates with free electrons heated in an electric field. Fast electrons in the tail of the thermal distribution function have enough energy—about 10–20 eV—to ionize matter and therefore to generate new free electrons. Electrons with lower energies disappear when they recombine with the ionized molecules in the air. When the electric field E exceeds a threshold, E E thr, the generation rate of new electrons from ionization exceeds their recombination rate, and the number of free electrons begins to exponentially increase: Electrical breakdown occurs. Because the electrons responsible for ionization are out in the high-energy tail of the distribution, the mean electron energy...
