Ray-based modeling of lightning-induced ionospheric effects on short range VLF skywave signals

At locations close (~100-km) to a VLF transmitter, observation of the sky wave signal from the transmitter becomes possible by aligning a proximate magnetic loop antenna to null the ground signal. Previous observations using such an arrangement [e.g., Rodriguez et al. 1992], and [Pasko et al., 2002] show a very high degree of temporal variability in received signal amplitude. Observation of VLF signatures of Early/Fast and Lightning-induced Electron Precipitation (LEP) events under these constraints show an unusual number of remarkably large events, more than ten times larger than a typical long-distance path, and often exhibit non-exponential recovery, or no recovery at all. Because of the large spatial extent of LEP events compared to the sky wave path distances, at least portions of the disturbed region must necessarily be nearly overhead the receiver and/or transmitter and we can employ simple ray tracing techniques to determine the evolution of the scattered field from the ionospheric disturbance over time, taking into account path-length difference, absorption, ground, and ionospheric reflection coefficients and multiple hops. We present several examples of characteristic VLF perturbations observed on short paths, and interpret these in terms of a simple ray-path-based model of the VLF scattering from the lower ionosphere.