Triton's Variable Interaction with Neptune's Magnetosphere

During its encounter of Neptune, Voyager 2 observed geyser-like plumes emanating from the moon Triton. These plumes have been attributed to the presence of a potential liquid water subsurface ocean beneath the moon's icy crust. Due to the tilt between Neptune's magnetic and rotational axes, along with the large orbital obliquity of Triton, the moon's local magnetospheric environment strongly varies in time. The associated magnetic field oscillations drive currents within the conducting layers (e.g., a subsurface ocean or dense ionosphere) at the moon. However, the interaction between Triton and the Neptunian magnetospheric plasma also generates a system of currents that may obscure any signatures associated with induction at Triton. To model Triton's interaction with the ambient plasma and constrain the resulting current systems, we apply a hybrid (kinetic ions, fluid electrons) model including the moon's ionosphere and induced field. We consider two orientations between the ambient magnetic field and flow velocity to represent the extremes in the changes to the local electromagnetic field over a synodic rotation. For each, we first investigate the (analytical) magnetic signatures associated with the superposition of Triton's induced field and the magnetospheric field in the absence of any plasma effects. To study the effect of Triton's ionosphere on the currents, we model the interaction between the ionospheric and magnetospheric plasma in isolation from the moon's inductive response, before combining these effects to investigate the complex scenario of plasma interaction and induction. Finally, we explore the sensitivity of the plasma interaction to changes in the ambient plasma density and the strength of Triton's inductive response. Our findings illustrate that, despite plasma interaction signatures that dominate the magnetic field perturbations far from Triton (beyond ~3 Triton radii), the induced field is clearly discernible near the moon's surface (within ~3 Triton radii). In addition, we show that the orientation of the magnetospheric field and velocity vectors strongly affects Triton's plasma interaction: at times, it resembles the interactions at Callisto, Europa, or Titan; at others, Triton's interaction reveals unprecedented signatures that may be unique to moons of the ice giant planets.