Trigger delay in whistler precursors

Detailed dispersion analysis of about thirty precursor events showed that the associated long whistlers traveled in the plasmasphere, at L ∼ 2.5–3.8, and probably in the same duct as their precursors. Measurement of the f, t trigger points of the precursors showed that the time of triggering (t) was generally more than halfway between the initiating lightning flashes and the associated two-hop whistlers by an amount (Δt) that generally decreased with trigger frequency (f). Because of apparent objections to triggering by hybrid whistlers, three alternative eplanations are investigated. Since all these are subject to serious objections, the hybrid explanation is re-examined. It is shown that the hybrid signal is expected to be quite weak (and hence rarely, if ever, observed) and that weak trigger signals should give, according to R. A. Helliwell's 1967 theory, both a delay and a frequency shift that account for the observed trigger points quite well.