HEATING OF THE NIGHTTIME D-REGION BY VERY-LOW FREQUENCY TRANSMITTERS

VLF signals propagating in the Earth-ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low freq uency (VLF, 3-30 kHz) transmitters. Ionospheric cooling and heating ar e observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0-kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/ on episodes during December 1992, increasing the amplitude and retardi ng the phase of the 21.4-kHz NSS probe wave propagating from Annapolis , Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3 deg rees, respectively. In 6 of these 41 episodes, the amplitude of the 28 .5-kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpect ed due to the > 770 km distance between NAA and the NAU-Gander great c ircle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 8 50 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth -ionosphere waveguide. A three-dimensional model of wave absorption an d electron heating in a magnetized, weakly ionized plasma is used to c alculate the extent and shape of the collision frequency (i.e., electr on temperature) enhancement above a VLF transmitter. The enhancements 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. Heating by the NAA transmitter is predicted to increase the n ighttime D region electron temperature by as much as a factor of 3. Th e 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 w ave. The range of predicted scattered field amplitudes is in general c onsistent with the observed signal perturbations. Discrepancies in the predictions are attributed to lack of knowledge of the D region elect ron density profile along the probe wave great circle paths.