  Observation of low- and high-frequency backward waves in the nonlinear regime
of the Buneman instability is reported. Intense low-frequency backward waves
propagating in the direction opposite to the electron drift (with respect to
the ion population) of ions and electrons are found. The excitation of these
waves is explained based on the linear theory for the stability of the electron
velocity distribution function that is modified by nonlinear effects. In the
nonlinear regime, the electron distribution exhibits a wide plateau formed by
electron hole trapping and extends into the negative velocity region. It is
shown that within the linear approach, the backward waves correspond to the
weakly unstable or marginally stable modes generated by the large population of
particles with negative velocities.
