Diurnal changes of phase and group velocity of VLF radio waves

Very-low-frequency radio-wave diurnal phase changes for frequencies between 10.2 and 26.1 kHz, inclusive, are tabulated and plotted as a function of frequency and path distance. Ninety measurements are listed, of which 25 are new data. By using a daytime theoretical phase velocity for a homogeneous, sharply bounded ionosphere at 70 km which agrees well with experimental data, it is possible to obtain a nighttime first-order-mode phase-velocity curve which does not change with frequency in a manner which is easy to predict. First-order-mode phase velocity curves are used to obtain estimates of first-order-mode group velocity. These calculations suggest that for frequencies less than about 16 kHz the nighttime group velocity is greater than the daytime group velocity. Above 16 kHz it appears that the nighttime group velocity is less than the daytime group velocity. Calculated average group velocity is in agreement with a recent experimental measurement at 17.8 kHz within the experimental error.