Several papers have been written about the observed relationship between ground based magnetometer pulsation measurements and simultaneous oscillations of Doppler frequency shifts in ionospherically reflected radio frequency echoes. In this paper we derive the mechanisms which relate the observations for the vertical incidence case at low to mid latitudes. We investigate the effects of oscillating electric and magnetic fields at ionospheric heights on the phase path of the reflected radio wave, which gives rise to the Doppler shifts. We identify three main mechanisms at work in the ionosphere, which are applicable to all latitudes; however, our numerical computations do not take account of certain high-latitude effects. By quantifying the electric and magnetic pulsation fields for a partially reflected downcoming Alfv¿n wave, we derive a quantitative phase and amplitude relationship between the rate of change of phase path on Doppler velocity and the pulsation magnetic field measured on the ground. A surprising result is that the dominant mechanism is not necessarily the vertical component of bulk plasma movement, as has been previously suggested. In many cases, the dominant mechanism is compression and rarefaction of plasma frozen onto the field lines as they oscillate under the action of the field-aligned component of the pulsation magnetic field. ¿ American Geophysical Union 1988