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We examine the electron-ion streaming instabilities driven by driftvelocities of the order of the electron thermal velocity in a nonmagnetizedplasma by one-dimensional electrostatic particle-in-cell code which adopts anion-to-electron mass ratio of 1600. An initial state is set up where the ionbulk speed is zero while the electrons drift relative to ions, and whereelectrons are much hotter. We examine in detail four runs where drift velocityis systematically varied from lower than to larger than the electron thermalvelocity. In all runs the Langmuir waves with Doppler-shifted frequenciesdominate early on when streaming instabilities are too weak to discern. Andthen intense ion-acoustic waves or Buneman instabilities appear, which tend tobe accompanied by localized electron and ion beams. Ion-acoustic modes andBuneman modes co-exist in the system when the initial drift velocity is justover the electron thermal speed. Beam modes are excited when the localizedbeams with large enough velocities appear. In the developed stage ofinstabilities, the direction in which density depressions propagate is alwaysopposite to that of the localized ion beams. When the initial drift velocity isclose to the electron thermal speed, categorizing the relevant instabilities isnot easy, and one needs to examine in detail the wave dispersion diagrams atvarious stages of the evolution of the system.

The electron-ion streaming instabilities driven by drift velocities of the order of electron thermal velocity in a nonmagnetized plasma