Modeling the Detectability of Energetic Heliospheric Ions at Pluto During the New Horizons Flyby

We investigate the detectability of heliospheric helium ions at energies up to 100 keV by the New Horizons (NH) spacecraft during its flyby through Pluto's induced magnetosphere. The Pluto Energetic Particle Spectrometer Science Investigation energetic ion detector observed a reduction in their flux by an order of magnitude as the spacecraft passed through the non‐uniform electromagnetic fields near the dwarf planet. This is despite these ions gyrating on scales up to several hundred Pluto radii. To contextualize these observations, we incorporate the electromagnetic fields from a hybrid model into a novel tracing tool for energetic H and generate synthetic time series of their flux into the detector along the NH trajectory and several hypothetical, idealized flyby geometries. Our major results are: (a) The detectability of perturbations to H fluxes highly depends on PEPSSI's look direction. Even along the same flyby trajectory, different viewing geometries may reveal changes in helium flux by up to a factor of five or no perturbations whatsoever. (b) The substantial reductions in H flux seen by NH may largely stem from the detector's finite field‐of‐view, filtering the incoming particles in velocity space. Without this effect, the reduction in flux by the draped fields would not exceed a factor of two. (c) The modeled flux perturbations gradually decrease with energy and become indiscernible above 20 keV. This behavior is qualitatively consistent with energetic ion dynamics at other small solar system bodies. (d) Across the range of plausible interplanetary field orientations, the model persistently suggests weaker reductions in H flux than observed.