Numerical Study of Electrostatic Ion Beam-Density Drift Instabilities in the Geomagnetic Tail

The mechanism of the generation of the broadband electrostatic noises (BEN) in the plasma sheet boundary layer is studied in this paper. The model plasma consists of warm background electrons, warm earthward ion beams and cold tailward ion beams. It is shown that electrostatic ion beam-density drift instabilities can be excited in the broad range of the frequency, the peak growth rate is found in the low frequency region at the large propagation angle, and the considerable growth rate in the high frequency region at the small propagation angle. The direction for the peak growth rate is determined by the ratio of the ion beam velocity to the density gradient drift velocity. The results are consistent with observations of the magnetotail BEN.