Dynamics of Energetic Heliospheric Ions in Pluto's Induced Magnetosphere

We present a model of the interaction between energetic heliospheric ions and Pluto's induced magnetosphere. The electromagnetic fields near the dwarf planet are highly non-uniform, displaying extended signatures of pile-up and draping. While the induced magnetosphere possesses a downstream extension above 100 Pluto radii, the weak interplanetary magnetic field in the outer heliosphere leads energetic ions to gyrate on comparable length scales. We obtain the three-dimensional structure of the fields near Pluto using a hybrid model, and a particle tracing tool is applied to study the dynamics of energetic ions traveling through these fields. For multiple initial energies, we compute the ion fluxes through a plane detector downstream of Pluto. Our results are as follows: (a) Deflection by Pluto's induced magnetosphere causes highly non-uniform perturbations in the flux pattern of energetic ions at its downstream side. These patterns include regions where the fluxes are increased or reduced by up to 40%, compared to the values in uniform fields. (b) Consistent with findings from New Horizons, the modeled perturbations gradually diminish with distance downstream of the dwarf planet out to 200 Pluto radii. (c) The deflection of the energetic ions mainly occurs within regions of Pluto's induced magnetosphere where the magnetic field is significantly enhanced, thereby causing a localized reduction in gyroradii. (d) The magnitude of the depletion in flux in our steady-state model is weaker than seen by New Horizons; this may suggest that time-dependent processes in Pluto's wake (e.g., bi-ion waves) play a major role in deflecting these ions.