Maxengana, Mihlali2024-11-062024-11-062024https://hdl.handle.net/10566/17574The effect of ion beams on linear and nonlinear low frequency electrostatic ion-acoustic waves is examined in multi-species plasma models, which are regularly observed in the terrestrial magnetosphere. These models comprise two thermal (adiabatic) ions and one or two Boltzmann electron species, with different temperatures in the latter case. One or both ions are regarded as drifting (beam) species. The linear study reveals the existence of two linear ion-acoustic modes: slow and fast ion-acoustic modes. The slow ion-acoustic modes emerge in models with at least one beam, while fast modes occur in beam and beamless plasma models. The slow ion-acoustic modes are found to be highly sensitive to variations in ion beam speed, becoming unstable to the ion beam instability for intermediate speeds. In the nonlinear regime, the ion-acoustic modes evolve into nonlinear structures such as solitons and double layers, which are pulse like disturbances in potential, commonly known as electrostatic solitary waves. A critical value of the beam speed must be exceeded for the existence of the slow ion-acoustic solitons of which the one slow mode propagates above and the second below the critical acoustic speed associated with the corresponding linear wave.enIon-acousticBoltzmann electronIon plasma frequencyElectrostatic wavesSpace plasmasThesis