Observations of light ions in the midlatitude and equatorial topside ionosphere

The terrestrial plasma located in altitude above the electron density peak is known as the topside ionosphere, and is coupled to the F region through flows parallel to the magnetic field. We present in this dissertation observations of both the mid-latitude and equatorial topside ionosphere using the technique of incoherent scatter.; After reviewing basic theories of topside dynamics, we introduce the incoherent scatter technique and describe its application to topside observations. In particular, we develop a novel application of constrained nonlinear least squares analysis techniques to solve a parameter ambiguity problem characteristic of incoherent scatter from the mixture of oxygen, hydrogen, and helium ions present at topside altitudes. The constraint method we employ encourages smooth behavior of electron and ion temperature variation, and yields physically reasonable and self-consistent measurements of density, temperature, and composition.; Using the results of the new technique, we then investigate three features of topside behavior seen in incoherent scatter measurements made at both the mid-latitude Arecibo Observatory and the equatorial Jicamarca Radio Observatory. The observations were taken at Arecibo during two periods characteristic of wintertime solar maximum and equinox solar minimum, with simultaneous Jicamarca topside measurements available during the latter period.; Large nighttime altitude variations in Arecibo horizontal density gradients are observed during wintertime but not at equinox, and are driven by conjugate heating from southern hemisphere photoelectron flux during early morning hours. We also present helium ion density measurements which peak during the daytime at the equator, but in contrast form a nighttime layer at mid-latitudes whose altitude is well predicted by simple ambipolar diffusion theory. Finally, we exploit the simultaneous Arecibo and Jicamarca topside data to derive the first complete field-aligned density profiles using ground-based radar observations, and we show that perpendicular density advection is a more significant supplier of oxygen ions to the nighttime F region than downwards flows from the protonosphere.; We close by summarizing the dissertation, and we offer suggestions for future studies.