ULF Doppler oscillations in the low latitude ionosphere

An array of magnetometers and Doppler sounders spanning L = 1.56–2.77 was used to examine perturbations in the daytime ionosphere driven by downgoing ULF waves. For frequencies away from the local field line resonance (FLR), the waves caused mostly vertical motions of the F-region plasma with amplitude ∼0.03–0.06 Hz/nT, with a phase delay of 30° to 40° at the ground. At the local resonant frequency and harmonics the amplitude and phase delay increased markedly. We modeled this by considering an admixture of ULF wave modes and oblique magnetic field geometry, and using actual ionospheric parameters. The model results agree well with observations when the downgoing wave mode varies smoothly from pure fast mode away from the resonance to mostly transverse mode at resonance. These results provide new information on the interaction between downgoing ULF waves and the ionospheric plasma.