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-degrees, respectively. On March 24, 1991, measurements obtained from a CW-HF Doppler sounding system and a fluxgate magnetometer show phase differences of 15-degrees-77-degrees between the Doppler frequency shift oscillations and the H component of ULF pulsations field at ground level, which indicates that the Doppler velocity arises from predominant changes due to the compression mechanism.
