| Man's influence on his environment is not confined to the immediate vicinity of the earth, but extends thousands of kilometers into space. Ground based studies show that electromagnetic signals from very-low-frequency (VLF) transmitters in the 0.1 to 1000 kW power range and radiation from electrical power systems regularly penetrate the ionosphere and travel to great distances in the magnetosphere. The manmade signals can interact with the trapped energetic particles in the earth's radiation belts. The interactions frequently stimulate VLF waves and can precipitate energetic particles into the ionosphere. The wave activity increases the background noise level of the magnetosphere, ionosphere and atmosphere. Precipitated particles stimulate x-ray and optical emissions and can modify the electrical properties of the lower ionosphere. The precipitated particles increase the electrical conductivity of the D and E regions, which increases the absorption of waves that pass through the ionosphere or those waves that are reflected from the lower layers. This enhanced conductivity also modifies the properties of the earth-ionosphere waveguide, which alters the phase of VLF signals in the vicinity of a particle-induced perturbation.;The present work focuses on theoretical and experimental studies of electromagnetic radiation from the world's electrical power systems. OGO-3 and OGO-4 satellite data have been used to show that low-level power line radiation (PLR) is prevalent throughout the ionosphere. The average value of all harmonics detected by OGO-4 in the frequency range of 0.2 to 1.6 kHz is 1.0 m(gamma) (10('-12)T, which is equivalent to 2.4 x 10('-12) W/m('2) for a refractive index of 50). Harmonic intensities in excess of 10.0 m(gamma) were observed less than 10% of the time. The average harmonic power density (W/m('2)) was 6 dB lower than the average noise power density (W/m('2)) of the ionosphere. Although weak, PLR or its effects have been detected in the high-altitude regions of the magnetosphere. It was found that one of the most commonly observed types of noise, known as chorus, occurred more frequently along geographic longitudes that contain large electrical power producers or consumers. Also, all well-defined chorus starting frequencies (10% of total cases) occurred at harmonics of the fundamental power frequency in that region. |
