In conventional drift wave theory, the density gradient kappan=d ln n/dx determines the linear phase velocity, and the (electron) temperature gradient kappaT=d ln T/dx gives rise to a nonlinear term which leads to the existence of soliton-type solutions and solitary waves, Lakhin et al. (1987, Phys. Lett. A119, 348; 1988, Plasma Phys. Contr. Fusion 30, 457) claimed that it is not kappaTbut essentially the derivative of the density gradient, d kappan/dx, that is relevant. This claim is refuted by means of an expansion scheme in epsilon =e Phi /T<<1, where Phi is the drift wave potential.
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