Ion Landau damping of ion-acoustic solitons via nonlinear saturation and electron-mediated energy transfer

We investigate ion-acoustic soliton propagation in warm-ion plasmas using fully kinetic particle-in-cell simulations with temperature ratios Ti/Te ranging from 0.01 to 1.0 and initial amplitudes from 0.1 to 0.8. Linear kinetic theory predicts that ion Landau damping should cause complete soliton decay in warm-ion regimes where ions possess sufficient thermal velocity to resonate with the wave. However, our simulations reveal that solitons reach quasi-steady saturated states at approximately 1% above background density—well above typical detection thresholds—rather than decaying to zero as predicted. Comparison with linear theory shows a counterintuitive pattern: agreement worsens as amplitude decreases, suggesting nonlinear effects whose relative importance grows at smaller amplitudes. Phase space analysis reveals substantial trapped electron populations within the soliton potential well, and energy accounting shows that the majority of ion energy gain correlates with electron energy loss rather than field energy dissipation. The trapped electrons exhibit behavior consistent with adiabatic expansion, cooling as the soliton potential well broadens while apparently transferring energy to surrounding ions. Ion phase space analysis reveals an additional energy transfer channel: ions reflected at the soliton potential barrier form a coherent accelerated beam propagating ahead of the soliton. We propose that these nonlinear kinetic effects—electron-mediated energy transfer and ion reflection—enable soliton persistence by providing pathways that differ fundamentally from classical linear Landau damping. If correct, this mechanism may have implications for space debris detection schemes relying on soliton signatures in low Earth orbit plasmas.