Satellite observations have revealed electrostatic solitary structures in the Earth's auroral region. These structures have positive electrostatic potentials and move along the ambient magnetic field. In this paper we performed one-dimensional electrostatic particle simulations of electrostatic solitary waves (ESW) in plasma composed of three electron components: cold, hot, and beam electrons. First, the nearly monochromatic electrostatic acoustic waves are excited. When the amplitude of the electron acoustic (EA) waves is sufficiently large, part of hot and beam electrons are trapped by the electron acoustic waves. These waves coalesce each other during their nonlinear evolution, and at last the solitary structures with travel speed related to the beam velocity are formed at the quasi-equilibrium stage. These structures have positive potential signatures, and they seem to be stable. Electron density cavities for cold electron component are always accompanied with these structures. In addition, the corresponding electric fields have a bipolar structure, which has also been observed in the Earth's auroral region recently. The conditions for existence of such solitary structures are investigated through our simulations, and the comparisons between our simulated results and satellite observations in the Earth's auroral regions are also discussed.