Phase and amplitude perturbations on subionospheric signals produced by a moving patch of artificially heated ionosphere

The Ionospheric Heating Facility (1‐MW HF transmitter and steerable array) at Ramfjordmoen, Norway, was used to generate a localized and steerable discontinuity in the Earth‐ionosphere waveguide for scattering Omega signals. The phase of the scattered signal (“echo”) relative to the direct signal varied with position of the heated patch of ionosphere (the discontinuity) in proportion to the change in echo path length. Previous results by Barr et al. (1985), using the same technique, Omega signals and path, show very similar behavior when looked at in this way. In this respect, both experiments simulated the phase and amplitude perturbations (“trimpis”) produced by lightning‐induced (via electron precipitation) changes in the waveguide and show that measurements of the varying echo phase of trimpis can be used to track lightning‐induced perturbations. The steerable discontinuity can also be used as a waveguide probe to measure the phase velocity of the strongest waveguide mode received on the ground by such scattering.