A global two-scale height ionospheric conductivity profile is inferred from the mean electromagnetic-wave propagation velocities of seven Earth-ionosphere cavity resonances in the frequency range . The time-varying reflection height for the electromagneticwaves is monitored on a diurnal basis. This D-layer height exhibits an increase at solar quiet conditions associated with decreased ionisation at mesospheric altitudes from solar short-wave radiation during the solar cycle and the solar rotation period. It is concluded that it is possible to monitor the global ionospheric D-layer height variability with naturally occurring electromagnetic waves in the atmosphere. A small network of magnetometers may detect and locate large-scale remote areas of mesospheric conductivity profile perturbations from short-lived extraterrestrial ionisation sources in the South Atlantic geomagnetic field anomaly, which are an appreciated hazard to spacecraft operation.