From AMS Glossary
(Also called Chapman region.) The idealized height distribution of ionization as a function of height produced solely by absorption of solar radiation.
Named for Sydney Chapman, who first derived the shape of such a layer mathematically. Some of the basic assumptions used to develop the equation were that the ionizing radiation from the sun is essentially monochromatic, that the ionized constituent is distributed exponentially (i.e., with a constant scale height), and that there is an equilibrium between the creation of free electrons and their loss by recombination.