title: Ionospheric Composition and Reactions: Our present knowledge of what ions are in the ionosphere, and why, is summarized.
abstract: Recent measurements of ion densities in the ionosphere seem to be more or less compatible with current data for ion-molecule reaction rates in some parts of the ionosphere under conditions in which a steady state prevails. There is no such agreement for the upper ionosphere, where the densities of He(+) and H(+) are difficult to understand in terms of present concepts concerning the relevant production and loss processes. Sources of ionizing radiation are needed to explain night- time observations in the E region and perhaps also in the F region. The D region remains a poorly understood laboratory of negative-ion and positiveion chemistry, where neither observation nor theory is as yet adequate.
authors.forename: T M
authors.initials: TM
authors.lastname: Donahue