Study on the Impulsive Plasma Wave Burst Phenomena Associated with Auroras

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Plasma Waves and Sounder (PWS) Experiment on board the Akebono (EXOS-D) satellite has been successfully carried out through the entire regions along the satellite orbit with initial perigee, apogee and inclination of 300 kin, 10,500 km and 68ﾟ, respectively ; the observation have also been made in the polar ionosphere covering from low to high altitude ranges. The high altitude observation in the polar region is aimed to explore the acceleration region of auroral particles ; in the low altitude polar ionosphere, on the other hand, observations have been made for sounding the energy dissipation region related to formation of auroras and ionospheric disturbances. Strong interactions of the auroral particles with ionospheric plasma with neutral gases makes disturbed states in entire altitude range of the auroral thermosphere.The present research work has been focused on the emissions of impulsive plasma wave burst (IPB) observed in the frequency range of the whistler mode waves in the low … More altitude polar ionosphere below 2,000 km by using electric field detecting antenna of plasma waves and sounder experiment (PWS) on-board the Akebono satellite. Within the research period form 1997 to 1998, the research activities have been directed to the theoretical research of the wave particle interactions in the polar ionosphere as well as data archiving and analysis of PWS data from the Akebono (EXOS-D) satellite.The results of the data analysis shows that the emissions of IPB are frequently observed indicating a group of intense impulses with electric field amplitude of several hundreds muV/m to several mV/m. The propagation directions identified by using the spin modulation effect on the electric fields detected with the two sets of dipole antennas showed that IPB's belong to the electrostatic whistler mode plasma waves whose wave number vectors have a large propagation angle from the ambient magnetic field. The origin of impulsive signatures of emissions can be attributed to the manifestation of the short lived nature of excited waves ; namely, the duration time is estimated to be in the range from 20 to 200 msec. These major characteristics of IPB are consistently interpreted with the theorecal approach to the whistler mode branch of the plasma waves, calculating the dispersion relation in the regime of electrostatic whistler mode waves under the condition of beam plasma instability related to the low energy electron beam in the F-region and topside ionosphere of the polar region. The short lived feature of IPB is understood as being resulted by severe damping effects of the electrostatic whistler mode plasma waves when the waves enter into the space of non free energy. The observation region of IPB is then limited in the region close to plasma instability that is possibly caused by the low energy electrons associated with auroras. Though the quest for origin of the impulsive generation of low energy electrons is deferred for future studies, it is inferred that the auroral particle precipitation into the polar ionosphere generates suprathermal electron beams forming successive groups with periodic modulation. Less