TEMPORAL EVOLUTION OF VERY STRONG TRIMPIS OBSERVED AT DARWIN, AUSTRALIA

Very strong phase and amplitude perturbations (Trimpis) of the very st rong VLF transmission from NWC received in Darwin, Australia, enabled accurate measurement of the amplitude and phase of the scattered signa l and of the time variation of these. The amplitude of the scattered s ignal decays as the logarithm of time, quite at odds with the exponent ial decay observed on classic Trimpis. During the amplitude decay, the phase of the scattered signal decreased at a decreasing rate. This is shown to be consistent with scattering from a bundle of sprite-like, conducting columns extending some 50 km below the base of the ionosphe re.