  We examine the electron-ion streaming instabilities driven by drift
velocities of the order of the electron thermal velocity in a nonmagnetized
plasma by one-dimensional electrostatic particle-in-cell code which adopts an
ion-to-electron mass ratio of 1600. An initial state is set up where the ion
bulk speed is zero while the electrons drift relative to ions, and where
electrons are much hotter. We examine in detail four runs where drift velocity
is systematically varied from lower than to larger than the electron thermal
velocity. In all runs the Langmuir waves with Doppler-shifted frequencies
dominate early on when streaming instabilities are too weak to discern. And
then intense ion-acoustic waves or Buneman instabilities appear, which tend to
be accompanied by localized electron and ion beams. Ion-acoustic modes and
Buneman modes co-exist in the system when the initial drift velocity is just
over the electron thermal speed. Beam modes are excited when the localized
beams with large enough velocities appear. In the developed stage of
instabilities, the direction in which density depressions propagate is always
opposite to that of the localized ion beams. When the initial drift velocity is
close to the electron thermal speed, categorizing the relevant instabilities is
not easy, and one needs to examine in detail the wave dispersion diagrams at
various stages of the evolution of the system.
