Propagation characteristics of omega signals and their triggered emissions observed by EXOS-D satellite

Omega signals have been frequently observed by the Japanese satellite EXOS-D (Akebono) in the magnetosphere. The authors have statistically analyzed VLF-WBA (wide-band analyzer) data in a period of 6 months, which were acquired by Kagoshima (KSC) and Prince Albert (PA) satellite tracking stations. About a quarter of the data obtained at KSC contain Omega signals from Australia and about a sixth of the data at PA contain those from North Dakota. Ray tracing calculations for the Australian Omega signals are consistent with the results of observations. 10 cases of triggered emissions are also found in the analyzed data. Amplitude and phase characteristics along an Omega pulse are investigated for the triggering event and an apparent phase lead, corresponding to a frequency increase by about 10 Hz was found at the center frequency of signals after the triggering starts