Ion acoustic solitons at the acoustic speed in a warm negative ion plasma with superthermal electrons

Using the Sagdeev pseudopotential method, the existence of Korteweg-de Vries (KdV) and non-KdV solitons is investigated in a negative ion plasma comprising adiabatic positive and negative ions and kappa distributed electrons. For some plasma parameter values, the plasma model supports the coexistence of solitons of both polarities. Positive KdV solitons coexist with negative non-KdV solitons at low values of negative to positive ion density ratio, and positive non-KdV solitons coexist with negative KdV solitons at higher values. There is therefore a switch in polarity between positive KdV and negative KdV solitons at a critical value of negative to positive ion density ratio and a switch in polarity between negative non-KdV and positive non-KdV solitons at the same point. At the critical point, there is no soliton at the acoustic speed, although there is coexistence at larger Mach numbers. This confirms that the existence of a soliton at acoustic speed is not a necessary condition for the coexistence of solitons of both polarities. When electrons are strongly non-thermal and the ion temperatures are important, the coexistence region vanishes and the non-KdV solitons disappear with it. It was also found that there is a forbidden region in terms of negative (positive) ion temperatures when the negative (positive) ion temperature increases with the other plasma parameters held fixed.