We present particle simulations of electrostatic solitary waves (ESW) observed by the Geotail spacecraft and recent spacecraft in the Earth's magnetosphere. Recent particle simulations have demonstrated that ESW correspond to Bernstein-Greene-Kruskal electron holes formed through nonlinear evolution of electron beam instabilities. Since an electron hole is a coherent electrostatic potential structure, electron beam instabilities were conventionally studied by electrostatic particle simulations. However, the Polar spacecraft and FAST spacecraft observed electromagnetic field signatures associated with ESW. To study interaction between coherent electrostatic potentials and electromagnetic waves, we extend the previous electrostatic particle model to an electromagnetic particle model. In the present two-dimensional simulations of an electron beam instability, electromagnetic field components are enhanced around two-dimensional electron holes. We found that the enhancement of electromagnetic fields is due to a current formed by electrons undergoing the E ¿ B0 drift, where the electric field is a perpendicular electrostatic field at the edge of a two-dimensional electron hole. An electromagnetic beam mode is excited by the current due to the drifting electrons moving with the electron hole. The amplitude ratio of the electric field to the magnetic field is estimated on the basis of the present simulation result, and it is in agreement with those of the Polar and FAST observations.