Non-reciprocity in Omega propagation observed at middle latitudes

Initial efforts are reported in work to establish experimentally in as unambiguous a way as possible the direction and magnitude of VLF nonreciprocity at middle latitudes. Omega transmitters and propagation paths (Japan-Hawaii and Hawaii-North Dakota) are shown in relation to the equatorial region in which middle latitude propagation characteristics cease to apply for westward propagation at night. It is concluded that at middle latitudes, the diurnal phase change for west to east propagation is greater than for east to west over the same path for observations at Omega VLF frequencies. The magnitude of the effect increases at lower latitudes, being a 23 percent difference over a path from Japan to Hawaii as against a 9 percent difference over a path from Hawaii to North Dakota. The results can be interpreted as an increase in the effective reflection height of a VLF wave when propagating westward at night, a result consistent with nonreciprocal changes in the sunrise modal interference patterns observed between Omega Japan and Omega Hawaii.