ELF propagation in polar areas, measurements and theoretical field strength predictions

The influence of the ionosphere upon ELF propagation at 75 Hz is discussed using results for the propagation path from the U.S. Navy ELF Wisconsin Test Facility transmitter to Tromso, Norway. This 6 megameter path crosses the Greenland icecap and passes twice through the auroral regions. Full wave computations were made using a waveguide mode program. The propagation path was divided into eight segments taking into account in an approximate way the varying boundary conditions of the actual waveguide. Results are presented and discussed for conditions ranging from ambient to severely disturbed. Disturbed ionospheric conditions such as sporadic E-layers, relativistic electron precipitation, auroral events and solar proton events were modelled. Electron and ion density profiles were deduced from satellite observations in the vicinity of the path. The importance of the electron as well as the ion concentrations is discussed.