The electron-ion beam instability is studied by one-dimensional electrostatic particle-in-cell simulation. When the relative drift velocity between the electron and ion is sufficiently 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 almost 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 terrestrialplasma depletion layer （PDL） are sometimes observed to have a BIF （bifurcated） signature,wherea diminution around 0.5Ωp with Ωp the proton gyrofrequency,occurs between two activity peaks inthe spectrum.By one-dimensional hybrid simulations,the effect of relative drift velocities betweenprotons and He2+ on the magnetic spectral signatures in the PDL is studied.The results showthat the relative drift velocity enhances the development of proton cyclotron waves and declinesthe development of helium cyclotron waves.The proton cyclotron waves are firstly excited,andfollowed by the excitation of helium cyclotron waves due to the increase in the relative driftvelocity.Moreover,the boundary between two activity peaks gets obscure.
