Supersolitons, flat-top solitons, and double layers in multi-component electron–positron–ion plasma with negative ions

A numerical investigation of ion-acoustic nonlinear structures is carried out in an unmagnetized multi-component electron–positron–ion plasma containing inertial positive and negative ions, superthermal electrons obeying a kappa distribution, and nonthermal positrons described by the Cairns distribution. Using the Sagdeev pseudopotential approach, the existence of ordinary solitons, flat-top solitons, double layers, and supersolitons is examined in detail. The effects of the spectral index k, negative ion density α, positron concentration αp, and nonthermal parameter β on the properties and existence regions of nonlinear structures are analyzed systematically. It is found that supersolitons are associated with multiple extrema in the Sagdeev pseudopotential and exist beyond the double-layer limit in the negative potential region. The corresponding electrostatic potential profiles, electric field structures, and phase-space trajectories confirm the complex nature of these nonlinear excitations. The analysis further reveals that increasing the negative ion concentration reduces the amplitude of supersolitons and leads to the formation of flat-top solitons, while stronger nonthermal effects enhance the nonlinearity of the plasma system. The obtained results may be useful for understanding nonlinear electrostatic structures in Saturn's magnetospheric plasma and related space plasma environments.