A note on lower ionosphere equivalent height diurnal variation

Five months of very-low-frequency phase measurements along the propagation paths Balboa (Panama)-Sao Paulo and Criggion (GB)-São Paulo, were studied in order to find apparent and equivalent diurnal height variation for the upper boundary of the earth-ionosphere waveguide. The results suggest that an explanation of the apparent height variation dependence on the wavelength and propagation path may be found if the second order mode of propagation is considered during nighttime and the diurnal change of the ionospheric conductivity gradient is taken into account. The results fit well with an “average ionosphere” model presenting nighttime conductivity gradient of 0.5 km^-1 and daytime gradient of 0.3 km^-1. As a consequence, the equivalent diurnal height variation found is situated near 10 km, the assumption of a single waveguide configuration for both propagation paths being valid.