VLF phase and amplitude: daytime ionospheric parameters

Experimental observations of the daytime variations of VLF phase and amplitude over a variety of long subionospheric paths have been found to be satisfactorily modelled with a D-region ionosphere, described by the two traditional parameters, H′ and β (being measures of the ionospheric height and the rate of increase of electron density with height, respectively). This VLF radio modelling uses the NOSC Earth–ionosphere waveguide programs but with an experimentally deduced dependence of these two ionospheric parameters on solar zenith angle. Phase and amplitude measurements from several VLF Omega and MSK stations were compared with calculations from the programs LWPC and Modefinder using values of H′ and β determined previously from amplitude only data. This led to refined curves for the diurnal variations of H′ and β which, when used in these programs, give not only calculated amplitudes but also, for the first time, calculated phase variations that agree well with a series of observations at Dunedin, New Zealand, of VLF signals from Omega Japan, Omega Hawaii, NPM (Hawaii) and NLK (Seattle) covering a frequency range of 10–25 kHz.