Predictions for New Horizons’ SWAP Measurements Downstream of the Heliospheric Termination Shock

Currently ∼62 au from the Sun, the New Horizons spacecraft is en route to the outer heliosphere boundaries. The first boundary it will encounter is the heliospheric termination shock (HTS), where the solar wind ion (SWI) and interstellar pickup ion (PUI) plasma mixture is slowed down to subsonic speeds, compressed, and heated. Some particles, mostly PUIs, undergo preferential acceleration at the HTS due to their higher energies and thus gain the capability to reflect from the shock and undergo, e.g., shock drift acceleration. This produces a tail in the downstream PUI energy distribution, with the potential for multiple power-law breaks. In anticipation of crossing the HTS, we have constructed a test particle model with synthetic turbulence to simulate New Horizons’ Solar Wind Around Pluto (SWAP) observations downstream of the HTS. SWAP can measure the energy/charge of multiple particle populations (thermal solar wind (SW) protons, alphas, and PUIs). Here, we calculate what SWAP might observe after it crosses the HTS. Our model shows that the count rate distribution will be very different from what is observed in the supersonic SW, with a hotter SW+PUI distribution and no sharp PUI cutoff. This will require a different method to quantify the moments of the SWIs and PUIs in the heliosheath. SWAP may be able to observe part of the PUI tail downstream of the HTS, if the upstream SW speed is ≲ 300 km s⁻¹. Moreover, if the shock compression ratio is also ≳ 2, SWAP may observe multiple populations of the accelerated PUI distribution.