  We derive the nonlinear equations that describe coupled drift waves and ion
acoustic waves in a plasma. We show that when the coupling to ion acoustic
waves is negligible, the reduced nonlinear equation is a generalization of the
Hasegawa-Mima equation to the three-dimensional (3D) case. We find an exact
analytical solution of this equation in the form 3D soliton drift wave (3D
modon). By numerical simulations we study collisions between the modons and
show that the collisions can be fully elastic.
