The trimpi effect in antarctica: observations and models

Abstract

LEP (lightning electron precipitation) bursts can be detected on the ground because they cause transient perturbations (known as ‘Trimpi events’) in the received amplitude and phase of subionospherically propagating radio waves, typically at VLF. The occurrence of mid-latitude Trimpis at Halley (events observed on five nights per month at equinox) is similar to other ( L $ ̃ 4 ) stations e.g. Siple and Eights, and much less than for lower latitude stations such as Palmer or Faraday ( L $ ̃ 2.5 ), nearer to the active LEP belt at 2 ⩽ L ⩽ 3. Activity is mainly on approximately north— south paths from NSS and NAA. In one case study of simultaneous Trimpi activity on the NSS-Halley and NAA-Halley paths, events were not synchronised, implying precipitation regions ⩽ 200–300 km in east-west extent. The maximum amplitude and phase excursions of a sequence of events on the NSS-Halley path oscillated sinusoidally in quadrature ; this was consistent with the echo Trimpi model of Dowden and Adams, assuming an echo signal 15 dB down on the direct signal, with the velocity of the precipitation region perpendicular to the propagation path changing the phase path difference between direct and echo signals by one wavelength (14 km at 21.4 kHz) in about 23 min. Burst precipitation of energetic electrons at high latitudes, close to, and poleward of, the plasmapause, can be well studied by means of the Trimpi effect using the Siple VLF transmitter and a network of VLF receivers in Antarctica (Siple, Faraday/Palmer, Halley, and South Pole). A high-latitude, low VLF waveguide mode propagation computer model for use in such studies has been developed, and tested against observations of controlled transmissions from a dipole antenna of variable (simulated) orientation. Published observations of Trimpi events in this high-latitude region are well reproduced by the model. A new experiment— OPALnet— designed to investigate LEP through its effects on the phase and amplitude of signals from the Omega VLF navigational network, is described. Its novel features permit multi-frequency observations on one path, multi-path observations at one frequency, and ‘VLF imaging’ using a grid of intersection paths.