In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary Shock

Non-thermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed, that PUIs should mediate and be preferentially heated at quasi-perpendicular shocks, dominating the downstream plasma energy compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H⁺) PUIs and H⁺ SWIs at an interplanetary shock made by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at 34 au from the Sun. Upstream of this shock, H⁺ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H⁺ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. In the shock frame, H⁺ SWIs lose the majority of their energy density flux across the shock, similar to that observed by Voyager 2 at the SW termination shock, and H⁺ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy density flux of all H⁺ PUIs is approximately six times that of H⁺ SWIs. By estimating the energy of the magnetic field, electrons, and other particles, we find that H⁺ PUIs, including their suprathermal tail, contain almost half of the total downstream energy density flux in the shock frame.