The electron-ion beam instability is studied by one-dimensiong electrostatic particle-in-cell simulation. When the relative drift velocity between the electron and ion is suf- ficiently less than the electron thermal speed, the dominant mode is the Langmuir wave; the ion-acoustic instability is very weak; the Buneman instability is not excited. When the relative drift speed is equal to the electron thermal speed, the Langmuir wave, the ion-acoustic and the Buneman instability nearly exist simultaneously. The three instabilities now appear to have al- most equal intensities. When the relative drift speed exceeds the electron thermal speed, the ion-acoustic instability turns into the Buneman instability which appears to have much higher intensity than the Langmuir wave.
Magnetic spectrum of the electromagnetic ion cyclotron waves in the terrestrial plasma depletion layer （PDL） are sometimes observed to have a BIF （bifurcated） signature, where a diminution around 0.5Ωp with Ωp the proton gyrofrequency, occurs between two activity peaks in the spectrum. By one-dimensional hybrid simulations, the effect of relative drift velocities between protons and He2＋ on the magnetic spectral signatures in the PDL is studied. The results show that the relative drift velocity enhances the development of proton cyclotron waves and declines the development of helium cyclotron waves. The proton cyclotron waves are firstly excited, and followed by the excitation of helium cyclotron waves due to the increase in the relative drift velocity. Moreover, the boundary between two activity peaks gets obscure.
