Rogue wave in a collisionless, unmagnetized electronegative plasma is investigated. For this purpose, the basic set of fluid equations is reduced to the Korteweg-de Vries (KdV) equation. However, when the frequency of the carrier wave is much smaller than the ion plasma frequency then the KdV equation is also used to study the nonlinear evolution of modulationally unstable modified ionacoustic wavepackets through the derivation of the nonlinear Schrödinger (NLS) equation. In order to show that the characteristics of the rogue wave is influenced by the plasma parameters, the relevant numerical analysis of the NLS equation is presented. The relevance of our investigation to the Titan's atmosphere is discussed.

Nonlinear ion-acoustic structures are investigated in an unmagnetized, four-component plasma consisting of warm ions, superthermal electrons and positrons, as well as stationary charged dust impurities. The basic set of fluid equations is reduced to modified Korteweg-de Vries equation. The latter admits both solitary waves and double layers solutions. Numerical calculations indicate that these nonlinear structures cannot exist for all physical parameters. Therefore, the existence regions for both solitary and double layers excitations have been defined precisely. Furthermore, the effects of temperature ratios of ions-to-electrons and electrons-to-positrons, positrons and dust concentrations, as well as superthermal parameters on the profiles of the nonlinear structures are investigated. Also, the acceleration and deceleration of plasma species have been highlight. It is emphasized that the present investigation may be helpful in better understanding of nonlinear structures which propagate in astrophysical environments, such as in interstellar medium.

Properties of three-dimensional ion-acoustic solitary and shock waves accompining electron-positron-ion magnetoplasma with high-energy (superthermal) electrons and positrons are investigated. For this purpose, a ZakharovKuznetsov-Burgers (ZKB) equation is derived from the ion continuity equation, ion momentum equation with kinematic viscosity among ions fluid, electrons, and positrons having kappa distribution together with the Poisson equation. The dependence of the solitary and shock excitations characteristics on the parameter measuring the superthermality κ, the ion gyrofrequency , the unperturbed positrons-to-ions density ratio ν, the viscosity parameter η, the direction cosine , the ion-to-electron temperature ratio σ i , and the electron-topositron temperature ratio σ p have been investigated. Moreover, it is found that the parameters κ, , ν, η, and lead to accelerate the particles, whereas the parameters σ i and σ p would lead to decelerate them. Numerical calculations reveal that the nonlinear pulses polarity are always positive. This study could be useful to understand the nonlinear electrostatic excitations in interstellar medium.