Previous theoretical models showed that, in the middle geomagnetic latitudes the oscillations in the Doppler frequency shift lag and lead the H component of ULF pulsations field on the ground, owing to the advection and compression mechanisms by 90°, respectively. On March 24, 1991, measurements obtained from a CW-HF Doppler sounding system and a fluxgate magnetometer show phase differences of 15°-77° between the Doppler frequency shift oscillations and the Hcomponent of ULF pulsations field at ground level, which indicates that the Doppler velocity arises from predominant changes due to the compression mechanism.

Applying perturbation theory to the Appelton-Hartree formulas, a detailed derivation of ionospheric parameters for ionospherically reflected echoes shows that: 1) oscillations of Doppler velocity, virtual height and echo amplitude have the same power spectrum; 2) the temporal variations in Doppler velocity lead or lag those in phase height by 90°; and 3) the phase difference between the variations in echo amplitude and those in virtual height is either 0° or 180°. The analysis of experimental data obtained using a digital HF ionosonde (Dynasonde) confirms that oscillations in Doppler velocity, virtual height and echo amplitude have nearly identical power spectra and the phase difference is in good agreement with the derived theory. It is concluded that several different parameters measured by or derived from ionospheric soundings can be used to study ionospheric variations.

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