Results concerning the sunrise effect of VLF signals propagated over long paths

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 variation observed at higher frequencies may be explained by a strong converted second mode whose amplitude exceeds that of the first mode.