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

TL;DR

This study provides direct in situ evidence that pickup ions are preferentially heated at an interplanetary shock, with PUIs carrying a significant portion of the downstream energy flux, highlighting their role in shock physics.

Contribution

First direct observation of preferential heating of pickup ions at an interplanetary shock using in situ measurements from New Horizons.

Findings

H+ PUIs are only a few percent of proton density but dominate pressure.

H+ PUIs are preferentially heated at the shock.

PUIs carry nearly half of the downstream energy flux.

Abstract

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 be preferentially heated at quasi-perpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H+) PUIs at an interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at ~34 au from the Sun. At 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. H+ SWIs lose ~85% of their energy flux across the shock and H+ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H+ PUIs is approximately six times that of H+ SWIs. We find that H+ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.