| This thesis concerns the experimental verification of current theories explaining the formation of quasi-periodic field-aligned irregularities (QP) associated with mid-latitude sporadic E layers.;To address the gravity-wave modulation process common to both Woodman's and Tsunoda's theories, a detailed modelling study was performed with the full-wave model of Hickey et al. (2000). Both theories require short-wavelength, short-period gravity waves propagating in a region occupied by a ES layer, assumed to be organized by neutral winds according to wind-shear theory [Whitehead, 1961]. The results show that gravity waves with periods and wavelengths comparable to the required temporal and spatial quasi-periodicities are unlikely to exist at E region altitudes. In the majority of cases examined, the waves dissipated well below the mesosphere and had propagation directions that varied considerably in the presence of even moderate winds. This contradicts both the stringent requirements of the Woodman model and also causes difficulties for the less restrictive Tsunoda model.;In addition, the background polarization fields were determined from in situ electric field measurements.;The conclusion must be that such fields, if they do exist, are by no means ubiquitous. This result causes difficulty for the Tsunoda theory, which is predicated on a mechanism that implies large fields should be observed regularly during QP-like conditions.;To assess the Larsen model, a comprehensive analysis of E S and QP data was performed. Generation of Kelvin-Helmholtz (K-H) instabilities is likely whenever a neutral wind shear becomes unstable in the Richardson number sense. A large number of region wind studies indicate that large wind shears are common and often fulfill the requirements for instability. The very shear that is assumed to organize the ES layer, if unstable, would subsequently generate K-H structures. These K-H billows have been shown to exhibit primary horizontal wavelengths of approximately 8 times the shear layer depth. In the case of the observed shears during ES with vertical scales of 1--2 km, this gives horizontal wavelengths of approximately 8--16 km, which agrees well with the spatial separations observed for QP structures. (Abstract shortened by UMI.)... |
