Coherent structures in the magnetotail triggered by counterstreaming electron beams

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 +/-upsilon(t), the electron thermal velocity. The characteristics of the simulated structures are shown to be similar to those observed in the magnetotail.