By the plasma wave sounder on board the Akebono satellite, the radio wave echoes, and plasma wave resonances have been observed through out the topside ionosphere and plasmasphere. The radio wave echoes had been observed even at altitude of 6,000 km showing feature of duct propagation. The plasma bulge structure were also confirmed in the high latitude topside ionosphere. The results of the sounder experiments show the principal resonances such as plasma resonances at fp, upper hybrid resonances at fUHR and harmonics of the electron cyclotron frequency at nfc; harmonic number n in the plasmasphere becomes higher than 10, sometimes. The sequence of the diffuse plasma resonance fDn changes the occurrence mechanism between the topside ionosphere and the plasmasphere with boundary regions around the altitude of 3,000 km. As a remarkable new finding, fD0 (n=0 case of the sequence of the diffuse plasma resonance) have been found. The occurrence of fD0 resonance gives confirmation to the mechanism of fDn generation as the instability of electrostatic electron cyclotron harmonic waves being associated with non linear wave particle interaction.