Aspects of wave-particle interactions at mid-latitudes
Arguably the most significant source of particle precipitation into the ionosphere at mid-latitudes (L ∼2–3), at least for electrons in the tens to hundreds of keV energy range, is that which arises from the interaction between trapped particles and whistler-mode waves in the magnetosphere. Quasisteady plasmaspheric hiss has long been postulated as a major class of waves contributing to the loss of radiation belt particles but recent studies have suggested that under some conditions, impulsive precipitation caused by interactions with lightning-generated whistlers may be equally important as a loss process. Such lightning-induced electron precipitation (LEP) and its ionospheric signature is the subject of this paper. Although LEP may be detected and studied by a variety of ground-based, balloon-borne and satellite-borne sensors, through optical emissions, X-ray production, enhanced conductivity, or the direct measurement of the precipitating particles themselves, a technique using ground-based narrow-band VLF receivers to measure the Trimpi effect (the transient perturbations in amplitude and/or phase of received narrow-band VLF transmissions) caused by LEP-associated ionisation enhancements has become increasingly popular due to its simple instrumentation and wide field of view. Most work has concentrated on the 2 < L < 3 region where typicalwhistler spectra, trapped electron energy distributions and magnetospheric plasma densities and magnetic field strengths are most favourable for the Trimpi effect. In order to use the technique to study in detail the characteristics and distribution of LEP (and its importance as a trapped-particle loss mechanism), using a network of intersecting transmitter-receiver great-circle paths (TRGCPs), a consensus on how to interpret the observational data is crucial, though this has recently been the subject of controversy. Whilst most studies suggest that only LEP within ∼200 km of the TRGCP gives rise to an observable Trimpi event, some have suggested that non-gaussian perturbations well off the TRGCP can be important. In this paper, the current state of LEP research is summarised and some recent results are presented, using data from a network of both widely and closely spaced observing sites in Antarctica.