HF SIDEBANDS IN THE IONOSPHERE
Doctor of Philosophy
Sidebands on the high frequency (HF) waves received from two transmitted waves separated by small frequency intervals were first observed by Gordon and Ganguly in January, 1984 at Arecibo, Puerto Rico. Papadopoulos proposed that HF sidebands would arise from the nonlinear interaction of an ELF wave with the two high frequency waves. The nonlinear mechanism for ELF and HF sideband generation is parametric decay of a high frequency radio wave into a low frequency compressional Alfven wave and a high frequency sideband (Papadopoulos et al., 1982). In 1984 Fejer proposed another nonlinear mechanism (phase modulation theory) for HF sideband generation. Fejer points out that the effect of the ponderomotive force is a reduction in the electron density and an increase in the phase delay which is greater for more powerful waves. Numerical solutions show that the phase delay would relate to the power density of the HF wave. This modulation index theory using the W.K.B. approximation shows that the amplitude of sidebands is related to: (1) the effective HF power; (2) the HF frequency; (3) the temperature of the electrons; (4) the scale height of the ionosphere; and (5) the difference frequency of the two pump waves. Observations of HF sidebands were again made in January, 1986. The 430 MHz incoherent radar provided the ionospheric background measurements. The modulation index theory provides the most complete interpretation of the observations.