Spectral broadening of VLF radio signals traversing the ionosphere

Nearly monochromatic signals at 13.6 kHz±1 Hz injected from a ground‐based VLF transmitter can experience a bandwidth expansion as high as 1% (∼ 100 Hz) of the incident wave frequency as they traverse the ionosphere and reach satellite altitudes in the range of 600–3800 km (Bell et al., 1983). The off‐carrier components, having electrostatic nature, are believed to be induced lower hybrid wave modes. We investigate two different source mechanisms that can potentially result in the observed spectral broadening of injected monochromatic VLF waves. One is the nonlinear scattering of VLF signals by ionospheric density fluctuations that renders the nonlinear mode conversion of VLF waves into lower hybrid waves. These electrostatic modes result when the injected VLF waves are scattered by ionospheric density fluctuations with scale lengths less than in the upper ionosphere, where c , ω pe , Ω e , and ω o are the speed of light in vacuum, the plasma frequency, the electron cyclotron frequency, and the VLF wave frequency, respectively. In the absence of ionospheric irregularities, a second mechanism that involves a parametric instability can excite the lower hybrid waves (Lee and Kuo, 1984). This process tends to produce a spectrally broadened transmitted pulse with peaks at a discrete set of frequencies on both sides of the nominal carrier frequency. By contrast, the nonlinear scattering mechanism generates single‐peaked spectra centered at the carrier frequency. Both types of spectra were observed in the experiments. Therefore, the two suggested source mechanisms contribute additively to the observed spectral broadening of injected VLF waves.