General discussion of nonlinear electrostatic modes in multispecies plasmas

Parallel propagation of nonlinear electrostatic waves in multispecies plasm as is described via a Sagdeev-type pseudo-potential or by Korteweg-de Vries and modified Korteweg-de Vries equations for weak nonlinearities, almost r egardless of the assumptions about plasma compositions or pressure closures . We explore the reasons for that in some detail and show why this should b e the case.