We describe wave observations carried out on board Galileo during the crossing of a flux rope in the Earth magnetotail. Waveform observations give evidence for transient electrostatic structures. Numerical simulations of the evolution of counterstreaming electron beams with a core of hotter ions have been performed with parameters consistent with Galileo observations during the flux rope crossing. A strongly unstable mode, with a zero real part of the frequency is found to develop and to lead to the generation of coherent electrostatic structures with velocities between ¿vt, the electron thermal velocity. The characteristics of the simulated structures are shown to be similar to those observed in the magnetotail.¿ 1997 American Geophysical Union