Abstract
Much progress has been made in recent years in the development of theoretical models of minor neutral constituents, which include the effects of transport. Important sources of ion-pair production continue to be photoionization of O 2 and N 2 by X-rays, photoionization of NO by Lyman alpha, and ionization of O 2 and N 2 by galactic cosmic rays. Photoionization of O 2( 1 Δ) by solar UV is less important than thought originally. Ironically, the positive-ion chemistry in the disturbed high-latitude D-region appears to be understood better than that in the undisturbed D-region. Tentative positive-ion and negative-ion models of the undisturbed, mid-latitude D-region exist, but there are several problems. Also, attempts have been made to model D-region ionization for the sunrise transition. It is concluded that the true ambient composition and concentration of minor neutral constituents and positive and negative ions must be accurately measured before meaningful theoretical model studies can proceed.
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