Ionospheric Composition and Reactions: Our present knowledge of what ions are in the ionosphere, and why, is summarized

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.