Phase and amplitude perturbations on subionospheric signals explained in terms of echoes from lightning‐induced electron precipitation ionization patches

Abstract

Phase and amplitude perturbations (trimpis) on VLF signals propagating in the earth‐ionosphere waveguide are usually explained in terms of precipitation‐induced modification along the great circle path as though it were a one‐dimensional (1‐D) waveguide. The alternative 2‐D view presented here is that precipitation‐induced ionization patches, laterally displaced from the great circle path, produce echoes at the receiver slightly delayed relative to the direct signal. Interference between the direct and echo signals can produce both phase and amplitude trimpis of both signs. Single‐mode propagation is assumed here, since it is likely that the second mode amplitude exceeded that of the first by ∼12 dB. We apply this theory to a sequence of trimpis observed on the North West Cape (NWC)‐Dunedin path on July 10, 1987. This indicates a north–south drift of the precipitation region consistent with drift speeds of whistler ducts. The theory is also applied to events published in the literature.