The interplanetary medium in the Solar System consists of fully ionized solar wind emitted from the Sun and interstellar wind of neutral atoms. Unlike charged ones, electrically neutral particles freely enter and traverse the heliosphere inflated by the solar wind and inform on the physical conditions in the interstellar medium surrounding the heliosphere. The heliosphere modifies the interstellar plasma flow, decoupling it from the flow of the neutral atoms. Nevertheless, the collisions between neutral atoms and charged particles modify the properties of the interstellar atoms entering the heliosphere.

Our team included experts in observation, modeling, and theory of the filtration of interstellar neutral hydrogen atoms in the heliosphere. We aimed to interpret the recent observations of the interstellar neutral atoms in the context of modeling and theory of their transport. Interstellar atoms traveling through the heliosphere are ionized, e.g., by charge exchange with solar wind protons. These ions are picked up by the solar wind and form an energetic population known as pickup ions. The New Horizons mission, which is currently 50 times farther away than Earth from the Sun, measures those ions, allowing us to deduce how the density of the interstellar neutral hydrogen atoms changes with the distance from the Sun. We compared these results with models of their density distribution in the heliosphere. Some of these models calculate the global filtration in the outer boundaries of the heliosphere, while other ones provide detailed descriptions of the ionization processes close to the Sun.

The density profiles retrieved from the observations and predicted by the models differ significantly. Unlike the global models that predict a monotonic increase of interstellar hydrogen density as New Horizons moves away from the Sun, the observations suggest that the density may decrease beyond 30-40 au from the Sun. This discrepancy is not explained by time variations close to the Sun and thus may be related to the temporal evolution of the outer boundaries or physical conditions in the interstellar medium. Further observations by New Horizons may address this issue together with the development of more advanced models of the filtration of interstellar neutral atoms.