Accurate Modeling of Ionospheric Electromagnetic Fields Generated by a Low Altitude VLF Transmitter

摘要

The goal of this project is to accurately predict the high altitude wave energy generated by low altitude VLF sources. We applied full-wave finite difference numerical models of the electromagnetic fields in both time and frequency domain to compute the VLF energy injected through an arbitrary and therefore realistic ionosphere for a source located anywhere on the globe. A 2D finite difference time domain (FDTD) code has been validated in various methods and proved to converge to the correct answer. A complete convergence test shows the conditions under which the code generated quantitatively correct results. Comparisons with results from this time domain code show that our frequency domain code gives answers in quantitative agreement but is about 40 times faster for single frequency computations. With these two models developed, we analyze in detail the power flux at 120 km altitude produced by the NML ground transmitter. To further study the dependence of wave energy on Earth's magnetic field, we also analyze the high altitude wave energy and ionospheric absorption for transmitters located at all latitudes. Our results can be used to predict the high altitude VLF power produced by a nearly arbitrary ground-level VLF transmitter.