Determination of Electron Density Profile in the Magnetosphere Deduced from Propagation Characteristics of VLF Wave

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Determination of plasma density distributions in the plasmasphere and ionosphere has been attempted using wave normal vector direction and delay time of Omega signals observed on the Akebono satellite as well as the electron density along the satellite trajectory.The VLF receivers on board the satellite have enabled us to determine the wave normal direction and delay time of Omega signals continuously for more than one hour. The wave normal direction and delay time at the satellite location can also be calculated by three dimensional ray tracing technique, if the electron density distributions in the plasmasphere is known. Our method is to determine plasma parameters of assumed functions which represent the plasmasphere and ionosphere, so as to fit observed data as mentioned above with those calculated by ray tracing by using the assumed model.The procedure to find parameters of several functions mentioned above is the non-linear least squarefitting method so as to minimize the differences of these quantities between the observed and the calculated by ray tracing.For the plasma density distribution a diffusive equilibrium (DE) model with a positive ion temperature gradient with altitude is assumed. For the electron density profile below 1,000km of altitude, the altitudes of the F2 peak and the valley just above the E layr are determined from the IRI model.The validity of our plasmaspheric model based on the DE model was checked by comparing with SUPIM (Sheffield University Plasmaspheric and Ionospheric Model) , and we can conclude that our model is flexible enough to represent the general global distributions of the electron density in the plasmasphere. The reliability of our fitting algorithm was also checked by computer simulations, and we have confirmed that the global electron density distribution can be determined from the satellite wave data, without any significant error.