An Energetic Particle Valley in the Outer Heliosphere: Insights from Voyager and New Horizons and Implications for New Horizons’ Termination Shock Encounter

In the coming years, New Horizons (NH) is expected to exit the heliosphere by crossing the solar wind termination shock (TS) and make the first measurements of pick-up ions (PUIs) across the TS boundary. To date, the only working spacecraft to have crossed the TS are Voyager 1 and 2, with Voyager 1 encountering the TS on day of year (DOY) 351, 2004 at ~94 AU, and Voyager 2 undergoing multiple crossings between DOY 243 and 344, 2007 at ~83.6 AU. Although NH is approximately aligned in heliolongitude with Voyager 2, its trajectory lies near the heliographic equator, in contrast to the higher northern and southern heliolatitudes of Voyager 1 and 2, respectively.In this work, we analyze energetic particle observations (∼40–200 keV) from the Voyager Low Energy Charged Particle (LECP) instruments and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) onboard NH to characterize radial intensity variations in the outer heliosphere. Voyager 1 and 2 observations show a systematic decrease in energetic particle intensities with increasing heliocentric distance, followed by a recovery prior to their respective TS crossings, forming a heliospheric energetic particle “valley.” NH/PEPSSI observations from 5 to 60 AU exhibit a comparable radial decline but have yet to show the expected increase on the march toward the TS crossing.To mitigate temporal variability associated with solar cycle effects, all observations are normalized using near-Earth energetic particle measurements from IMP-8/EPE and ACE/EPAM. The combined radial profiles from Voyager and NH are well described by a double power-law with a break at~33 AU. The combined radial profiles from Voyager and NH are well described by a power-law dependence with a distinct break beyond ~33 AU. This break likely reflects a transition in the dominant transport and/or acceleration mechanisms operating in the inner and outer regions separated by this radial distance. The presence of this break across multiple heliolatitudes suggests a global heliospheric feature, potentially reflecting changes in particle transport, acceleration, or local plasma conditions in the outer heliosphere. By scaling the Voyager observations to the NH measurements, we estimate a NH TS crossing between 2027 (~68 AU) and 2034 (~83 AU).