Authors: Bishwas L. Shrestha (Princeton University)

Interplanetary shocks driven by solar wind structures propagate outward, evolving continuously as they sweep through the outer heliosphere. As PUIs travel outward with the expanding solar wind, they are heated by passing interplanetary shocks and sometimes form high-energy power-law tails in the downstream. PUIs are preferentially heated at distant interplanetary shocks and dominate the internal pressure in the outer heliosphere. Observations from New Horizons have revealed unique properties of interplanetary shocks in the outer heliosphere, including a lack of a distinct solar wind density jump across shocks, while showing a distinct jump in the properties of the PUI. A detailed understanding of these phenomena is crucial as New Horizons approaches the heliospheric termination shock (HTS), where PUIs are believed to be preferentially heated.

This discussion focuses on the unique characteristics of shocks and PUIs in the outer heliosphere, including our current understanding and the knowledge gaps that remain. This includes our understanding from New Horizons and Voyager observations in the outer heliosphere, their connections with theory/modeling, and observation opportunities that can be enabled in the near future.