Trimpi perturbations from large ionisation enhancement patches

A number of increasingly sophisticated and realistic models have been devel oped in order to investigate the interaction between sub-ionospherically pr opagating VLF waves and regions of ionisation enhancement (LIE1) in the D-r egion 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 precisel y, classic Trimpis, to distinguish them from "early/fast Trimpis" or "VLF s prites" which are not caused by LEP and are not considered here. II is impo rtant, for comparison with experimentally observed Trimpi effects, that the spatial extent of the D-region electron density (N-e) perturbation is mode led accurately. Here, it is argued that most previous modeling has used pat ch (LIE) sizes that are typically up to 100 km in both latitudinal and long itudinal extent, which are generally smaller than those that actually occur for real lightning induced electron precipitation events. It would also ap pear that maximum Delta N-e, values assumed have often been too large, and the patches (LIEs) have been incorrectly modelled as circular rather than e lliptical in horizontal extent. Consequently, in the present work, Trimpi p erturbations are determined for LIEs with smaller maximum diV,: larger spat ial extent and elliptical shape. Calculations of VLF Trimpis have been made as a function of the horizontal coordinates of the LIE centre, over the wh ole rectangular corridor linking transmitter and receiver. The Trimpi model ling 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 i s modelled as an electron density enhancement with a Gaussian profile in al l coordinates. Results for a large elliptical LIE similar to 200 x 600 km s how that significant Trimpis, similar to-0.4 dB in amplitude and similar to +4 degrees in phase are predicted, using modest maximum Delta N-e, values s imilar to 1.5 el/cc. Such an electron density enhancement is well within th e range that would be expected to result from experimentally observed fluxe s of electron precipitation following wave particle interactions with whist ler-mode waves. (C) 2000 Elsevier Science Ltd. All rights reserved.