1990 Volume 42 Issue 4 Pages 411-442
Plasma waves and sounder experiments (PWS) onboard the Akebono (EXOS-D) satellite have been successfully made after deployment of antenna and turn-on of the instruments on March 4, 1989. In natural plasma wave observations, the dynamic spectra of the plasma waves in the whole range of the orbits are observed in the PWS frequency range from 20kHz to 5MHz. The results are summarized as follows.
i) The upper hybrid emissions that cover a frequency range from the electron cyclotron frequency fC to the upper hybrid frequency fUHR for the case where fC is higher than the local plasma frequency fP, or cover from fP to fUHR for the case of fP>fC, are always observed in the altitude range from 1000km to apogee of the orbit of the satellite. From the global distribution of the upper hybrid frequency emissions, the structures of the plasmasphere are clarified.
ii) Auroral kilometric radiations are observed in the region associated with the auroral particle acceleration regions. Polarization and Poynting vectors of AKR are also precisely measured. The data show the possibility of studies on the structure of the acceleration regions in addition to the generation mechanism of AKR. The AKR data that have been obtained in the acceleration region of the auroral particles are especially important, suggesting the possible co-existence of the electrostatic plasma waves with the Z-mode waves.
iii) In the equatorial region of the plasmasphere the enhancement of the upper hybrid mode waves is discovered; the enhancement shows two possible plasma turbulences, the first is the enhancement of the microscopic instabilities that are identified only as the enhancements of the intensity of the upper hybrid emissions and called “equatorial enhancement of the wave turbulence (EPWAT)”; and the second is the enhancement of the irregular change of the upper hybrid frequency suggesting the turbulent variation of the plasma density that is called “equatorial enhancement of the density turbulence (EPDET)”. EPWAT's encircle the magnetic equator forming a disc within the altitude range of 1000km to 9000km, while EPDET's encircle the geographic equator within the altitude range of 1000km to 9000km, forming another disc.
iv) Sounder experiments of the PWS have also been carried out; the results show the principal resonances such as plasma resonances at fP, upper hybrid resonances at fUHR, and harmonics of the electron cyclotron frequency at nfC (n=1, 2, 3, ...). The sequence of diffuse plasma resonances fDn with the static plasma wave resonance fQn are also detected even in a high altitude range higher than 3000km. Very long ducted echoes of sounder transmitting pulses have also been detected even at the altitude of 6000km. All these results of the experiments reveal that the PWS/EXOS-D data provide a very promising key for studies of the plasma and plasma waves throughout the entire global ionosphere and plasmasphere, including the regions of auroral particles acceleration.
