Transition from standing to sonic Langmuir solitons

We discuss the problem of existence of Langmuir solitons when the assumption of quasineutrality breaks down. For moving solitons a discrete spectrum of eigenvalues is found. The forms of the solitons are shown to be significantly different from those reported in previous finite-amplitude theories. The existence lines are presented for various Mach numbers and values of nonlinear frequency shift. The transition from standing to sonic Langmuir solitons is explained.