SIGNATURE OF BURST PARTICLE-PRECIPITATION ON VLF SIGNALS PROPAGATING IN THE ANTARCTIC EARTH-IONOSPHERE WAVE-GUIDE

The burst precipitation of energetic electrons (greater-than-or-equal-to 40 keV), induced by interactions with lightning-generated whistler mode waves, has been observed to cause phase and amplitude perturbations on subionospheric VLF signals ("Trimpi" events). With a knowledge of the propagation characteristics of the subionospheiic signal, analysis of the perturbation details can lead to estimates of the energy, extent, and location of the precipitation. Trimpi events have been observed on VLF signals propagating at high latitudes (L greater-than-or-equal-to 4) over Antarctica, on 3.79-kHz signals transmitted from the horizontal dipole at Siple station. A mode theory computer model for propagation of VLF waves in the Antarctic Earth-ionosphere waveguide is used to illustrate the characteristics of 3.79-kHz signals as they propagate from Siple toward VLF receivers at Halley and South Pole stations. To simulate the effects of precipitation, localized depressions in the ionospheric reflection height are introduced over the great circle propagation paths in the model, and it is seen that, while the amplitude and phase perturbations of some specific events are accurately reproduced, large positive amplitude (up to 6 dB) Trimpi events at Halley cannot be reproduced. Calculations are presented which show that signals echoing from precipitation patches located away from the great circle path could be the cause of such signatures.