Pickup Ion Mediated Collisionless Shocks in the Outer Heliosphere

We are currently making the first detailed remote and in situ observations of pickup ion-mediated collisionless shocks anywhere in space. These observations are from the outer heliosphere, where charge exchange with incoming cold neutral atoms from the very-local interstellar medium (VLISM) has added copious interstellar pickup ions (PUIs) to the radially expanding solar wind. In the outer heliosphere, beyond ~20 au, the PUI thermal pressure dominates both the thermal pressure of the original solar wind and that of the interplanetary magnetic field; by the termination shock at ~100 au, this pressure is even becoming a significant fraction of the solar wind’s dynamic pressure. The PUIs are preferentially heated across the termination shock and produce most of the energetic neutral atoms (ENAs) from the heliosheath – the region between the termination shock and heliopause – through charge exchange with cold neutral atoms drifting in from the VLISM. Over the past decade and a half, we have begun directly sampling these PUIs both remotely through global observations of ENAs from the Interstellar Boundary Explorer (IBEX) and in situ with observations from the Solar Wind around Pluto (SWAP) instrument on New Horizons. This talk will summarize some of the observations and discoveries from these two source about pickup ion-mediated collisionless shocks in the outer heliosphere. Finally, we will also look forward to the promise of the even more advanced Interstellar Mapping and Acceleration Probe (IMAP), which is currently under development and slated to launch in 2025.