Lightning induced enhancements of D-region ionisation and whistler ducts

Abstract

A 3-D magnetospheric ray-tracing program has been constructed in which ray paths can be calculated for propagation in 3-D ducts having a Gaussian enhancement of electron density in both the geomagnetic meridian and geomagnetic longitude directions. Such calculations show a reduction in wave path excursions both in geomagnetic latitude (or L-value) and longitude as rays propagate upwards in a duct between low altitude (∼1000 km) and the equatorial plane. The change in wave energy flux between low altitude and the interaction region for wave–particle interactions and the cross- L extent of related precipitating electron fluxes can then be determined. The size of localised electron density enhancements in the D-region lightning induced enhancements (LIEs) which cause phase and amplitude perturbations on sub-ionosphere VLF propagation (Trimpi effect) can then also be estimated. LIE extents determined in this way appear to be generally about an order of magnitude smaller than estimates from Trimpi observations or low altitude satellite observations of LEP (lightning-induced electron precipitation). This discrepancy has been investigated by ray-tracing in a variety of 3D duct models, including ducts which vary in width and/or enhancement along their length. Calculations made for ducts with maximum enhancement and width in Δ L-value constant along their length imply that precipitation regions and LIEs would typically have smaller dimensions in L-value than the low altitude whistler duct widths measured by the spatial extent of the whistler-mode waves. Therefore, to account for this discrepancy in duct width, ray paths were also determined for ducts which vary in enhancement and/or width in Δ L-value along their length. Although a little better fit with experimental observations is obtained for such ducts, the results can still not explain significantly wider ducts in L-value in the interaction region than at low altitudes. Thus other possible explanations are considered; in particular that the non-linear resonant current in the interaction region near the equatorial plane will radiate wave power into modes with larger wavenormal angles to the geomagnetic field direction than the original trapped modes, and this will then result in a larger region from which precipitating electrons arise and hence larger LIEs than the actual duct width in the interaction region.