Periodic field strength variations which were observed during sunrise on VLF signals propagated over long paths are discussed. The stepwise phase increase and the minima of the signal amplitude may be explained by the waveguide-mode theory; they are caused by interference between the first and the converted second mode. Owing to a weak second mode at lower frequencies, the sunrise effect at lower frequencies in general is only weakly developed. The reversed stepwise phase varia· tion observed at higher frequencies may be explained by a strong converted second mode whose amplitude exceeds that of the first mode.

Diurnal phase changes of VLF signals propagated over long paths (d > 5000 km) are discussed. Assuming the model of single‐mode propagation, the annual mean phase change corresponds to the value expected from the theory. The strong scatter of the individual values may possibly be explained by a nonnegligible second mode during nighttime propagation. In the interpretation of the diurnal phase changes of VLF signals propagated over arctic paths, the dependence of the ionization in the lower ionosphere on the solar zenith distance has to be considered.