Drifting of the solution pattern for the nonlinear Schrodinger equation

When a solution of the one-dimensional nonlinear Schrodinger equation is disturbed by a harmonic function of the space variable, the solution pattern may drift in space as time evolves. The value and the direction of the drifting velocity vary with time. To understand this phenomenon, a perturbation analysis is performed. The drifting velocity given by the analysis agrees well with the direct numerical result.