Numerical ray-trace calculations have been performed to investigate the whistler ducting characteristics of various field-aligned ionization density perturbations in the earth's phasmasphere. The results indicate that field-line curvature has a major influence on both the ducted ray trajectories and on the probability of escape during interhemispheric propagation. A specific comparison is made of the single-and multipleop ducting ablity of ''bell-shaped'' versus ''ledge-shaped'' ducts as a function of the density perturbation and invariant latitude. Modest (20%) ledge- and bell-shaped perturbations, as well as a wide variety of ''hybrid'' profiles are capable of effective whistler ducting in middle latitudes, with ledge-shaped profiles having considerably better multiple-hop ducting characteristics. furthermore, for a given duct the latitude range of starting locations from which lightning generated sferies can initially be trapped is considerably broader for ledge shaped profiles. Interhemispheric propagational losses from 20% ledge-shaped profiles in middle latitudes are between 5--12 db per hop compared to 8--20 db for similar bell-shaped profiles assuming specular reflection from the ionosphere. all transmitted downgoing rays consistently become ''unducted'' at altitudes near a few thousand kilometers due primarily to the ambient ionospheric topside plasma density gradients and magnetic field gradients and waves received on the ground should typically be observed at several degrees lower latitude than the ducting prodile.