Observations of the electron number density have been successfully carried out by the NEI instrument on board the Ohzora (EXOS-C) satellite after the launch on February 14, 1984. The NEI instrument was designed to measure the effective capacitance of the rod type probe whose length is 45cm with diameter of 2cm in the swept frequency range from 0.1MHz to 16MHz.The results of the initial phase observation show complicated features of the polar ionosphere relating to the auroral particle precipitation and also the large scale movement of the ionospheric plasma. The trough region in the ionospheric level frequently observed in clearly defined form and the remarkable enhancements of the ionization are sometimes observed at the low latitude side of the boundary. Usually the ionosphere around the auroral oval region shows ionization enhancement and the small scale irregularity is predominant. The precipitating low energy particles in the auroral region, excite intense plasma waves. The electrostatic component with large amplitude and also charging of the probe clearly appear in the detected impedance values versus the swept frequency. The polar cap ionosphere which usually shows lower density level than the auroral oval region, indicate sometimes a very large electron density enhancement exceeding auroral oval ionization even in the night side.
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Takahashi et al. (1985) studied this question.