The observed diurnal phase changes of VLF radio waves show large deviations from the theoretical first-mode phase change. The main features of the deviations are explained by the presence of a second mode that is larger than the first mode at night but that is small during transitions and during daytime. The data indicate that for frequencies above 16 kHz, nighttime second-mode dominance extends out to several megameters from the transmitter, in agreement with theoretical predictions.
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