Ducted whistler propagation outside the plasmapause

A study has been made of the conditions under which lightning whistlers are observed after propagating along geomagnetic field aligned paths or “ducts” located outside the plasmapause. The study was based in part upon results previously obtained from Antarctic whistler recordings in 1963 and in part upon new data from 45 days of observations at Siple Station, Antarctica (L ≃ 4.3) in 1977 and in 1982. Propagation beyond the plasmapause was found, as expected, to be rare in comparison to propagation within the nearby outer plasmasphere. However, detectable propagation beyond the plasmapause near dawn was found to occur on at least one path on roughly one half of the days studied. The path equatorial radii of propagation tended to cluster in two locations, one at the plasmapause outer edge, and the other in a belt separated from the plasmapause by a region of low activity of order 0.5 RE in extent. The outer edge of the belt was at L ≃ 5.5–6. The probability of whistler detection at any L value outside the plasmapause up to ≃ 6–7 was found to increase with local time across the dayside of the Earth; maximum activity was observed after noon. Whistler activity beyond the plasmapause was least common in the 1800–2400 MLT sector. Spatial variations in lightning source activity may be responsible for large variations with longitude in whistler activity outside the plasmapause and for the many cases in which only a small fraction of whistlers within a given period exhibit evidence of plasmatrough propagation. Ionospheric processes, such as wave damping, defocusing in the mid‐latitude trough, scattering by 10 to 100‐m irregularities, and focusing within 50 to 100‐km blobs, can explain many features of the observed spatial distribution of whistler paths beyond the plasmapause and also the relatively low amplitude of the associated whistlers as compared to plasmasphere events. The large differences in whistler activity between afternoon and premidnight appear to be due to spatial differences in perturbing magnetospheric electric field activity and in electron density in the plasma trough region. The experimental findings serve to make a relatively sharp distinction between the plasmasphere and the region beyond from both the propagation and wave‐particle‐interaction points of view.