Modon formation in the nonlinear development of the collisional drift wave instability

Nonlinear evolution of the collisional drift wave instability has been investigated by means of numerical simulations based on a model equation derived from a two‐fluid approximation. This approximation is reduced to the Hasegawa–Mima equation when the collision and viscosity are neglected. A crucial feature is that coherent structures are formed from a turbulent state. A scenario for a path to the self‐organized motion through the turbulent motion has been found as follows. The initial exponential growth of a linear instability is followed by a parametric instability that excites many modes. At the same time, the wave breaks up into small vortices. The inverse cascade of the wave energy then sets in and small vortices fuse into larger ones. A large modon is formed at the final stage of evolution.