Trimpi perturbations from large ionisation enhancement patches

A number of increasingly sophisticated and realistic models have been developed in order to investigate the interaction between sub-ionospherically propagating VLF waves and regions of ionisation enhancement (LIE) in the D-region caused by lightning-induced electron precipitation enhancements (LEP). This LEP-produced LIE can result in phase and amplitude perturbations on received VLF radio signals that are referred to as Trimpis or more precisely, classic Trimpis, to distinguish them from ``early/fast Trimpis'' or ``VLF sprites'' which are not caused by LEP and are not considered here. It is important, for comparison with experimentally observed Trimpi e€ects, that the spatial extent of the D-region electron density (Ne) perturbation is modeled accurately. Here, it is argued that most previous modeling has used patch (LIE) sizes that are typically up to 100 km in both latitudinal and longitudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also appear that maximum DNe values assumed have often been too large, and the patches (LIEs) have been incorrectly modelled as circular rather than elliptical in horizontal extent. Consequently, in the present work, Trimpi perturbations are determined for LIEs with smaller maximum DNe, larger spatial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the whole rectangular corridor linking transmitter and receiver. The Trimpi modelling program is fully 3D, and takes account of modal mixing at the LIE. The underlying theory assumes weak Born scattering, but the code calculates a non-Born skin depth attenuation function for the LIE in question. The LIE is modelled as an electron density enhancement with a Gaussian pro®le in all coordinates. Results for a large elliptical LIE0200 600 km show that signi®cant Trimpis, 0ÿ0.4 dB in amplitude and 0+48 in phase are predicted, using modest maximum DNe values 0 1.5 el/cc. Such an electron density enhancement is well within the range that would be expected to result from experimentally observed ̄uxes of electron precipitation following wave particle interactions with whistler-mode waves. 7 2000 Elsevier Science Ltd. All rights reserved.