A two-dimensional, explicit, electrostatic particle code is used to investigate the nonlinear behavior of electrostatic ion waves generated by an ion beam flowing through a thermal ion and electron background in a strongly magnetized plasma (&ohgr;ce≫&ohgr;pe where &ohgr;ce and &ohgr;pe are the electron gyrofrequency and the plasma frequency). To follow the nonlinear evolution of these ion waves, a long-lasting simulation is run with a large simulation grid: 128¿512&lgr;d. Beam ions are shown to generate oblique waves. The nonlinear beatings between these oblique waves produce purely transverse waves, which leads to a strong modulation of the density and of the electric potential in a direction transverse to the magnetic field. The transverse scale of these essentially field-aligned filaments is L⊥=10&rgr;i where &rgr;i is the ion Larmor radius of beam ions. Within these filaments, relatively stable field-aligned density and potential structures develop. The typical size, along the magnetic field, of these structures is L∥=10&lgr;d, the density is modulated by 30%, and the electric potential is as large as Te within these structures. Unlike the potential structures that develop in a two-component plasma with downgoing electrons, these structures move upward. These characteristics are in good agreement with the weak double layers recently detected by Viking. ¿ American Geophysical Union 1992