Interstellar Pickup Ion Observations Halfway to the Termination Shock

In this study, we extend the prior interstellar pickup ion (PUI) observations from the Solar Wind Around Pluto (SWAP) instrument on New Horizons out to nearly 47 au—essentially halfway to the termination shock in the upwind direction. We also provide significantly improved analyses of these and prior observations, including incorporating a cooling index, α, to characterize the nonadiabatic heating of PUI distributions. We find that the vast majority (93.6%) of all distributions show additional heating above adiabatic cooling. Speed jumps indicate compressional waves and shocks with associated enhancements in core solar wind and PUI densities and temperatures. Interestingly, additional heating of the PUIs as indicated by a peak in the cooling index follows the jumps by about a week. We characterize nearly continuous solar wind and H⁺ PUI data over ∼22-47 au, producing radial gradients, "fiducial" values at 45 au—halfway to the nominal upstream termination shock—for direct comparison to models, and extrapolated values at the shock. These termination shock values are n_PUI = (4.1 ± 0.6) × 10^-4 cm^-3, T_PUI = (5.0 ± 0.4) × 10⁶ K, P_PUI = 30 ± 4 fPa, α = 2.9 ± 0.2, n_PUI/n_Total = 0.24 ± 0.02, T_PUI/T_SW = 716 ± 124, P_PUI/P_SW = 173 ± 32, P_PUI/P_{SW - Dyn} = 0.14 ± 0.01. The PUI thermal pressure exceeds by more than an order of magnitude the thermal solar wind and magnetic pressures in the outer heliosphere. SWAP provides the first and only direct observations of interstellar PUIs in the outer heliosphere, which are critical for both inferring the plasma conditions at the termination shock and understanding PUI-mediated shocks in general. This study examines these observations and serves as the citable reference for these critical data.