Causes of the frequency shift of whistler-mode signals

Whistler-mode signals received at Wellington, New Zealand, from NLK in Seattle are typically doppler shifted ~0.1 Hz from the transmitter frequency, implying a changing phase path. At least during quiet times, this is shown to be primarily caused by (inward) drifts of the whistler duct rather than by changing electron density along the path. The mechanism is established by measuring the ratio of the changes in phase and group times for the path during the nights studied. On one of these nights, the drift is further confirmed by measurements on natural whistlers.