Finite-difference time-domain simulation of the atmosphere's and ionosphere's electromagnetic responses to the lightning discharge

The purpose of this study is to simulate the electromagnetic response of the atmosphere and the ionosphere during and after the lightning discharge using the complete set of Maxwell's equations. A three-dimensional numerical model is used to calculate the transient electric field and Maxwell current that flows from the thunderstorm to the ionosphere during the lightning discharge process. The model contains an isotropic conductivity profile below 70 km and an anisotropic conductivity profile from 70 km to 150 km (different latitudes, day, and night have been considered), a time-varying thunderstorm (source function), and a perfectly conducting ground surface. Both vertical and horizontal transient electric fields in the ionosphere have been calculated. The results show that the relaxation time of the electric field due to the lightning discharge, which is longer than {dollar}varepsilonsb0/sigma{dollar}, will decrease with an increase of the altitude of observation and will have little change with the horizontal distance from the lightning discharge. The Maxwell current from the thundercloud spreads out to the ionosphere with a period of several milliseconds following the lightning discharge. This current flows along the directions both parallel and perpendicular to the geomagnetic field lines and mainly propagates horizontally above 70 km. A good agreement between the simulation results and the measurement data has been achieved. These results show the importance of the use of the complete set of Maxwell's equations and the inclusion of an anisotropic conductivity in the ionosphere in the lightning simulation. The conclusion of this thesis also may help to explain some of the long-standing discrepancies between measurements and theoretical predictions.