The morphology and dynamics of the tropical ionosphere are distinguished by the strong influence of the geometry of the magnetic field and the various sources of ionization. The emphasis in this symposium was on those ionization sources which are characteristic of the ionosphere at low latitudes.
An overview of the characteristic properties of the tropical ionosphere was given by J. C. G. Walker. It was followed by a review of nighttime photoionization sources by D. F. Strobel, C. B. Opal, and R. R. Meier. At midnight the principal source of photons in the E region is found to be stellar continuum in the 911–1026-Å wavelength range, producing as much as 2.5 ions cm−3 s−1 at 110 km over the equator. The sidereal time dependence of starlight results in a seasonal modulation of the ionization rate of a factor of 4. Since the majority of hot stars are in the southern hemisphere, the ionization rate at southern mid-latitudes can be a factor of 2 higher than at the equator. Resonantly scattered H and He airglow are also important sources in theE region. The radiative recombination continuum of O+ at 910 Å is a major F region photoionization source in the Appleton Anomaly.