Nonlinear electrostatic ion-acoustic "oscilliton" waves driven by charge non-neutrality effects

Abstract

Nonlinear "oscilliton" structures features a low-frequency (LF) solitary envelope, the amplitude of which is modulated violently by superimposed high-frequency (HF) oscillations. We have studied the charge non-neutrality effects on the excitation of electrostatic ion-acoustic (IA) oscillitons. A two-fluid, warm plasma model is employed, and a set of nonlinear self-similar equations is solved in a cylindrical geometry. Under charge-neutrality conditions, three conventional IA structures (namely, sinusoidal, sawtooth, and spicky/bipolar) are obtained. By contrast, under charge non-neutrality conditions, oscilliton structures are excited, where the LF envelope is in the sound-wave (SW) mode, while the HF ingredients include the IA mode and the ion-Langmiur (IL) mode. The amplitudes of the SW wave are violently modulated by the IA oscillations, whereas the upward sides of the IA amplitudes are modulated by the IL oscillations of smaller amplitudes, and the downward sides are modulated by hybrid IA/IL oscillations. The nonlinear oscillitons are found to be dependent not only upon the input parameters (e.g., the Mach number, the Debye length, and the initial temperature of particles), but on initial conditions as well.