Two-dimensional frequency domain modeling of lightning EMP-induced perturbations to VLF transmitter signals

Abstract

The lightning electromagnetic pulse creates observable modifications to the overlying D region ionosphere, exhibited optically as elves. Recent work, both experimental and theoretical, has shown that elves are accompanied by considerable electron density disturbances of up to a factor of 2 increase in local density. We investigate the possibility that these density disturbances are observed by subionospheric VLF transmitter signals as the so-called early VLF events. We use a finite difference frequency domain model to simulate the VLF transmitter signal propagating subionospherically to a VLF receiver, under ambient conditions and through a disturbed region. We show that modeled electron density disturbances, consistent with optical observations of elves, yield small but detectable perturbations to the VLF transmitter signal and may explain at least some observed early VLF events. We further show that sequences of in-cloud lightning pulses, as in spider lightning, may yield considerably higher density disturbances and similarly more prominent VLF transmitter perturbations. In this way, the model herein supports previously reported correlations between sferic bursts and early VLF events.