Lightning induced enhancements of D-region ionisation and whistler ducts

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 enha ncement of electron density in both the geomagnetic meridian and geomagneti c longitude directions. Such calculations show a reduction in wave path exc ursions both in geomagnetic latitude (or L-value) and longitude as rays pro pagate upwards in a duct between low altitude (similar to 1000 km) and the equatorial plane. The change in wave energy flux between low altitude and t he 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 induce d 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 mag nitude smaller than estimates from Trimpi observations or low altitude sate llite observations of LEP (lightning-induced electron precipitation). This discrepancy has been investigated by ray-tracing in a variety of 3D duct mo dels, including ducts which vary in width and/or enhancement along their le ngth. Calculations made for ducts with maximum enhancement and width in Del ta L-value constant along their length imply that precipitation regions and LIEs would typically have smaller dimensions in L-value than the low altit ude whistler duct widths measured by the spatial extent of the whistler mod e waves. Therefore, to account, for this discrepancy in duct width, ray pat hs were also determined for ducts which vary in enhancement and/or width in Delta L-value along their length. Although a little better fit with experi mental observations is obtained for such ducts. the results can still not e xplain significantly wider ducts in L-value in the interaction region than at low altitudes. Thus other possible explanations are considered; in parti cular that the non-linear resonant current in the interaction region near t he equatorial plane will radiate wave power into modes with larger wavenorm al angles to the geomagnetic field direction than the original trapped mode s, and this will then result in a larger region from which precipitating el ectrons arise and hence larger LIEs than the actual duct width in the inter action region. (C) 1999 Elsevier Science Ltd. All rights reserved.