THE SCATTERING OF VLF WAVES BY LOCALIZED IONOSPHERIC DISTURBANCES PRODUCED BY LIGHTNING-INDUCED ELECTRON-PRECIPITATION

A three-dimensional model of the scattering of VLF waves in the Earth-ionosphere waveguide by localized disturbances in the lower ionosphere is examined for typical disturbances expected to be produced by lightning-induced electron precipitation events. Results indicate that the scattering is generally independent of the conductivity and permittivity of the Ear-th's surface immediately beneath the disturbed region except for extremely low conductivities such as that found over deep ice caps. Thus the scattered signal is principally a function of the ionospheric perturbation. For typical disturbances characterized by altitude profiles of enhanced ionization expected for 1.4 less-than-or-equal-to L less-than-or-equal-to 3, most of the measurable wave energy scatters within a fairly narrow angular region (15-dB beam width of +/- 7-degrees for a disturbance radius of 100 km) centered on the forward scatter direction. Thus moderate- to large-scale disturbances (radius 50-200 km) must be located within < 250 km of a moderate-length path (3000-16,000 km) in order to scatter a measurable signal to the receiver. These two findings suggest that the scattered signals can be used with confidence as a diagnostic tool to determine the characteristics of the energetic electron precipitation.