| This dissertation concerns the design and implementation of a controlled experiment aimed at determining the extent to which very-low-frequency (VLF, 3-30 kHz) transmitters operating worldwide modify the Earth's ionosphere. VLF signals propagating in the Earth-ionosphere waveguide are used to probe the heated nighttime D region of the ionosphere (50-90 km altitude) over three U. S. Navy VLF transmitters. Ionospheric cooling and heating are observed when a transmitter turns OFF and ON in the course of normal operations. Heating by the NAA transmitter (1000 kW radiated power) was observed by this method in 41 of 52 ON/OFF episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4 kHz NSS subionospheric probe wave measured at Gander, Newfoundland, by as much as 0.84 dB and 5.3{dollar}spcirc,{dollar} respectively. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier observations of VLF signals propagating in the Earth-ionosphere waveguide.; A three-dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent and shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The heated patches are annular with a geomagnetic north-south asymmetry and a radius at the outer half-maximum of the collision frequency enhancement of about 150 km. Increases up to a factor of 3 in the electron temperature are predicted to occur in the nighttime D region due to heating by the NAA transmitter. The calculated changes in the D region conductivity are used in a three-dimensional model of propagation in the Earth-ionosphere waveguide to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the perturbations of NSS-Gander observed during December 1992.; One possible consequence of electron heating, the modification of the nighttime D region electron number density, is investigated theoretically using a four-species model of the ion chemistry. The effects of a 100 kW (NAU), a 265 kW (NSS), and a 1000 kW (NAA) VLF transmitter are calculated for different ambient electron density profiles. The dominant effect of the heating on electron number density is a reduction in the D region electron density due to an increase in the three-body attachment rate of electrons to O{dollar}sb2.{dollar} Results indicate that the electron density is depleted by up to 26% at {dollar}sim{dollar}80 km altitude over a 1000 kW transmitter. |
