Satellite observations of equatorial phenomena and defocusing of VLF electromagnetic waves

Amplitude measurements of whistlers and signals from VLF transmitters have been made with the low altitude, polar-orbiting OGO 4 satellite. Two aspects of these measurements related to the behavior of the waves near the magnetic equator are described and interpreted: (1) Daytime spectrograms taken near the magnetic equator show a remarkable high-frequency cutoff in the amplitude of the VLF whistler waves. The cutoff frequency decreases as the satellite approaches the magnetic equator, and sometimes all signals drop below the equipment threshold. This feature is also present when the signals from different VLF transmitters are simultaneously observed. It is shown that the above phenomenon is explained primarily by absorption in the D and E regions of the ionosphere and by the daytime ray trajectories near the equator. (2) During the night, absorption becomes relatively small, but nevertheless an abrupt amplitude cutoff of signals from VLF transmitters may still occur. This nighttime cutoff is explained primarily by defocusing near the equator, which is enhanced for nighttime ionization profiles. The defocusing of the VLF waves depends strongly upon the rapid change of ionization-gradient occurring between 500 and 1000 km of height and also upon the curvature of the earth’s magnetic field around the magnetic equator.