On gravity waves and the electrodynamics of the mid-latitude ionosphere

This thesis examines the coupling of thermospheric gravity waves to ionospheric disturbances using experimental observations and theoretical descriptions of the nighttime, mid-latitude F region. Previously, these disturbances have been separated into two types: passive ionospheric responses to field-aligned neutral winds known as traveling ionospheric disturbances and active irregularities caused by plasma instabilities. This work finds, however, that this distinction breaks down in the presence of the nighttime F region dynamo current. The dynamo current system substantially alters the ionospheric response to gravity wave forcing both parallel and perpendicular to the Earth's magnetic field. Most disturbances are damped, but a few which propagate in a narrow range of azimuths equatorward of geomagnetic west are greatly enhanced. This azimuthal filtering is a long-term feature of ionospheric disturbance observations. On rare occasions, these disturbances erupt into plumes of upwelling plasma known as mid-latitude spread F.; The thesis draws on detailed observations of disturbances over the Arecibo Observatory on January 26-27, 1993 using all-sky images of 6300 A airglow emissions, incoherent scatter measurements of electron density and ion velocities, and ionosonde measurements of the conjugate hemisphere. These disturbances are shown to have been electrodynamically driven and to have grown from an initial gravity wave seed. Analytical investigations and numerical simulations of gravity wave coupling to the Perkins instability show that the overall morphology of the instability agrees well with these and other observations of mid-latitude nighttime disturbances and that the instability is capable of feeding energy back into the gravity wave seed, potentially altering the distribution of thermospheric gravity waves.