Nonlinear electrostatic structures associated with drift ion-acoustic dynamics are investigated in Saturn’s inner magnetosphere using plasma parameters constrained by Cassini observations between \(12\,R_{\textrm{s}}\) and \(15.2\,R_{\textrm{s}}\) for a background magnetic field of \(B_{0}\simeq 0.3~G\) . Accordingly, the current plasma system is modeled as a hydrogen ion background permeated by coexisting cold and hot superthermal electron populations, each described by a \(\kappa\) -type distribution consistent with in situ measurements. Within this framework, a planar Gardner-like evolution equation is obtained in a remarkably direct way by means of the drift approximation, which naturally connects the Korteweg-de Vries-like (KdV), modified KdV-like (mKdV), and Gardner limits without invoking the reductive perturbation technique or successive changes of stretched coordinates. The resulting family of equations provides a unified description of solitary waves and double layers, and makes it possible to identify the existence domains and polarity of nonlinear drift ion-acoustic structures under realistic Saturnian plasma conditions. The analysis demonstrates that an outward increase in radial distance, accompanied by enhanced cold-electron content and modified superthermality, systematically amplifies the electrostatic potential and slightly broadens the associated Gardner double layers. For the drift-modified Gardner-like, KdV-like, and mKdV-like equations, the supported solitary structures can be either compressive or rarefactive, depending on the detailed balance between quadratic-cubic nonlinearities and dispersion, and they exist only within a restricted window of oblique propagation, represented here by \(\alpha =0.5\) . Outside this angular interval, coherent solitary solutions are suppressed, which underlines the importance of propagation geometry in determining the accessibility of nonlinear states. The predicted dependence of amplitude and width on the cold-to-hot electron density ratio, temperature profiles, and spectral index \(\kappa _{c}\) yields a set of observationally testable signatures for localized electrostatic solitary waves and double layers in Saturn’s magnetospheric plasma and, more broadly, in planetary magnetospheres with multi-temperature, superthermal electron populations.
