Measurements Beyond 1 Au Are Necessary for Exploration of the Outer Heliosphere and Local Interstellar Medium

Heliospheric missions operating in the outer heliosphere and beyond are necessary for understanding the heliosphere and its interstellar neighborhoodInterstellar neutrals (ISNs), pickup ions (PUIs; ionized ISNs picked up by the local plasma fields), and energetic neutral atoms (ENAs; neutralized PUIs and solar wind ions) provide information about the interstellar medium, the global heliosphere, and processes at the transition region between solar and interstellar environments.These heliospheric data are usually collected from the vicinity of the Earth.Thus, they require careful and thoughtful interpretation of the observed signals and implementation of proper corrections for modulation by the solar environment.Interpretation is complicated because the solar wind and solar extreme ultraviolet (EUV) radiation ionize the incoming flux of interstellar particles measured near the Earth's orbit.The ionization rates vary depending on species and phase of solar activity.Hydrogen, the most abundant element, is the most prone to the modulation by the solar environment and undergoes the greatest losses inside the heliosphere.High ionization rates and radiation pressure create a density depletion region for ISN H at a few astronomical units from the Sun.The last decades provided strong arguments for measurements at least outside this cavity (beyond Jupiter's orbit) to mitigate the adverse solar environment losses for the fluxes of interstellar-born particles.In-situ heliospheric measurements far beyond 1 au should be a priority for the heliospheric community in the coming decades to make progress in data interpretation, improve quality of the observations, and advance our understanding of the heliosphere and its interstellar neighborhood.