Non-uniform ionisation of the upper atmosphere due to the electromagnetic pulse from a horizontal lightning discharge

The purpose of this paper is to model numerically the ionisation created in the mesosphere above a strong horizontal (cloud-to-cloud) lightning discha rge, and thus to understand better the formation of elves and related colum niform (spatially structured) sprites. Such ionisation depends upon height, via the ratio of the VLF/LF wave electric field (radiated by the current i n the discharge) to the atmospheric neutral density. The simulation shows l ocal maxima on the order of 10 km apart due to the interference between dir ect waves radiated by the discharge current, waves reflected by the ground and waves reflected by the ionosphere. Parametric studies are carried out v arying the frequency, amplitude, height, length and duration of the current . For waves greater than or equal to 30 kHz generated by a current greater than or equal to 50 kA, localised peaks of electron density occur from 75 t o 85 km altitude, particularly for discharges longer than 15 km. From such peaks, time-varying "stalactites" on the bottom of the ionosphere, streamer s could develop and propagate downwards. These could become columniform spr ites in the presence of the quasi-electrostatic field due to the negative c harge remaining in the thundercloud after a positive cloud-to-ground discha rge. (C) 2001 Elsevier Science Ltd. All rights reserved.