GLOBAL ELECTRON-DENSITY DISTRIBUTION IN THE PLASMASPHERE DEDUCED FROMAKEBONO WAVE DATA AND THE IRI MODEL

Determination of global electron density distribution in the plasmasph ere has been attempted using the wave normal directions and delay time of Omega signals, and in-situ electron density observed on the Akebon o satellite. The present study is based on the assumption that the pla sma model is represented by a diffusive equilibrium (DE) model with te mperature gradient along a magnetic field line, and that the electron density at a reference altitude (1000 km) is dependent on L-value. Rel ative concentrations of ion constituents H+, He+, and O+ at the refere nce altitude are determined by fitting as either L-independent or L-de pendent. For the altitude range below the reference altitude, we have adopted a distribution which has one maximum density at the F2 peak an d one minimum density al the E valley, both being referred to the IRI model. The parameters related to these models are determined by the no n-linear least squares fitting method for the in-situ electron density , the wave normal direction and the delay time of Omega signals observ ed by Akebono. Wave normal directions and delay time at Akebono for th e model are determined by ray tracing. The validity of our plasmaspher ic model based on the DE model was checked by comparing it with the SU PIM (Sheffield University Plasmaspheric and Ionospheric Model), and we can conclude that our model is flexible enough to represent the gener al global distributions of the electron density in the plasmasphere, a lthough it is possible that in some cases, unrealistic distributions o f the plasma temperatures should be used in the model. The reliability of our fitting algorithm was also checked by computer simulations, an d we have confirmed that the global electron density distribution can be determined from the satellite wave data, without any significant er ror. Several plasmaspheric electron density distributions are shown as a result of applying our method to Akebono data. (C) 1997 Elsevier Sc ience Ltd.