Abstract
A 3‐D finite difference time domain model is used to simulate the lightning electromagnetic pulse (EMP) and its interaction with the lower ionosphere. Results agree with the frequently observed, doughnut‐shaped optical signature of elves but show that the structure exhibits asymmetry due to the presence of Earth's ambient magnetic field. Furthermore, in‐cloud (horizontal) lightning channels produce observable optical emissions without the doughnut shape and, in fact, produce a much stronger optical output for the same channel current. Electron density perturbations associated with elves are also calculated, with contributions from attachment and ionization. Results presented as a function of parameters such as magnetic field direction, dipole current orientation, altitude and amplitude, and ambient ionospheric density profile demonstrate the highly nonlinear nature of the EMP‐ionosphere interaction. Ionospheric effects of a sequence of in‐cloud discharges are calculated, simulating a burst of in‐cloud lightning activity and resulting in large density changes in the overlying ionosphere.
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