Authors: Parisa Mostafavi (JHU/APL), Laxman Adhikari (UAH), John Richardson (MIT), Gary Zank (UAH), Pontus Brandt (JHU/APL), Robert Allen (SWRI), Merav Opher (BU), Ian Cohen (JHU/APL), Alan Stern (SWRI)

The solar wind undergoes a fundamental transition in the outer heliosphere as interstellar pickup ions (PUIs) become an increasingly important component of the plasma pressure and energy density. Observations from Voyager and New Horizons have shown that beyond ~10 AU the solar wind is no longer well described as a thermal core plasma, yet the consequences of this transition for planetary-scale interactions remain poorly understood. Uranus occupies a unique location in this regime transition: it lies deep enough in the outer heliosphere for PUIs to contribute substantially to the plasma pressure.

We investigate how interstellar PUIs modify the solar-wind interaction with Uranus using a two-fluid treatment in which the thermal solar wind and PUIs evolve as distinct pressure populations. We show that PUIs substantially increase the effective plasma beta and reduce the fast magnetosonic Mach number relative to classical core-solar-wind descriptions. Across the Uranian bow shock, PUI pressure becomes comparable to the downstream ram pressure in the magnetosheath, introducing a previously neglected pressure component into the force balance applied to the magnetopause. Including this revised pressure partition produces magnetopause standoff distances consistent with the compressed state observed by Voyager 2.

These results suggest that Uranus is not simply an outer analogue of inner-heliospheric magnetospheres, but a natural laboratory for studying how stellar-wind pressure transmission changes as the heliosphere transitions from a thermal-core regime to a suprathermal-pressure-dominated plasma. More broadly, because PUIs originate from interstellar neutral hydrogen, this work highlights how the surrounding interstellar medium can influence shock formation, pressure balance, and star–planet coupling across planetary magnetospheres, astrospheres, and astrophysical plasma environments.