The phase relationship between variations of ionospheric total electron content (TEC) and ground-level ULF geomagnetic pulsations has been examined for the advection and compression mechanisms. To determine the causal mechanism, several earlier studies have examined the phase difference between oscillations of Doppler velocity in ionospherically reflected radio waves and simultaneous ULF geomagnetic pulsations. In most instances it was found that the phase relation varied from event to event. With the application of Euler's formula this study shows that in low geomagnetic and midgeomagnetic latitudes the phase differences between variations of TEC and ULF pulsations in the northward component of the geomagnetic field due to the advection and compression mechanisms are 0 degrees and 180 degrees, respectively. We also found that TEC variations tend to lead ionospheric Doppler velocity oscillations by 90 degrees. Furthermore, it is shown that the phase relationship between ionospheric Doppler velocity oscillations and ULF pulsations of the northward component of the geomagnetic field, caused by the advection and compression mechanisms, are functions of the scale length, frequency of ULF waves, and geomagnetic dip.
