Ion Layers, Tides, Gravity Waves, and Electric Fields in the Upper Atmosphere, Inferred from Arecibo Incoherent Scatter Radar Measurements.

This thesis uses data accumulated during 1980 -1989 by the Arecibo incoherent scatter radar to study the behavior and physics of ionization irregularities. Low latitude ionization irregularities, known as sporadic-E and intermediate layers, undergo a regular daily descent, convergence, and dumping of ion layers controlled by the neutral tidal wind. A useful way of studying ion layers and their motion is by ion layer trajectory maps which consist of points representing the altitude and time of ionization layers. Two types of maps were used which assigned either a uniform layer intensity or a gray level/pseudo -color to indicate different layer intensities. Each has its own advantages and are often used together. Important aspects of layer formation are revealed by map analysis. During January, intermediate layers consistently appeared four times per day instead of the normal twice per day pattern. Simulation of ion trajectories based on the ion momentum equation, which includes both Lorentzian and collisional forces, shows that a combination of diurual, semidiurnal, and six-hour tides is necessary for such a feature to exist, whereas only diurnal and semidiurnal tides are needed to create the normal pattern. The six -hour period tide has not been previously reported. Extra or irregular layers appear frequently in layer trajectory maps, which can be simulated by the addition of gravity waves to the regular tidal wind system. Ion layer trajectory maps can thus indicate gravity wave activity in the upper atmosphere. Electric field effects are normally not a factor in low latitude ion layer formation because they are relatively weak and not commonly observed. Layer configurations during a geomagnetic storm, however, indicate that the electric field played an important role in controlling ion motion. This is supported by an ion trajectory simulation in which a normal tidal wind field and electric field are both active. The simulation shows that an electric field which points northeast, having a magnitude of a few mV/m, can drive ions upward out of the E region. The magnitude and direction of this nighttime electric field agree with F region drift measurements performed in the low latitude during disturbed conditions.