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 light ning-induced electron precipitation events. Results indicate that the scattering is generally independent of the conductivity and permittivi ty of the Ear-th's surface immediately beneath the disturbed region ex cept for extremely low conductivities such as that found over deep ice caps. Thus the scattered signal is principally a function of the iono spheric perturbation. For typical disturbances characterized by altitu de 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 scat ters within a fairly narrow angular region (15-dB beam width of +/- 7- degrees for a disturbance radius of 100 km) centered on the forward sc atter direction. Thus moderate- to large-scale disturbances (radius 50 -200 km) must be located within < 250 km of a moderate-length path (30 00-16,000 km) in order to scatter a measurable signal to the receiver. These two findings suggest that the scattered signals can be used wit h confidence as a diagnostic tool to determine the characteristics of the energetic electron precipitation.