We performed computer simulations of electrostatic solitary waves (ESW), which correspond to broadband electrostatic noise (BEN), observed by Geotail spacecraft in the magnetotail. According to Geotail/WFC data analysis, ESW are composed of sequences of impulsive solitary waves, and their potentials are revealed to have uniform structures in the direction perpendicular to the ambient magnetic field. We performed a series of two-dimensional electrostatic particle simulations with a cold electron beam drifting against background hot electrons along the ambient magnetic field. In these simulations, spatial potentials are excited by the initial electron beam instability. They coalesce with each other and form potential troughs uniform in the direction perpendicular to the ambient magnetic field, as predicted by the Geotail observations. We varied the magnitude of the ambient magnetic field and found that this parameter critically affects the ESW formation process. These simulation results indicate that ESW are excited by the electron beam instability in the presence of the static magnetic field with a certain magnitude. ESW are also subject to the enhanced thermal fluctuations due to the limited number of superparticles in the simulations. ¿ 1998 American Geophysical Union