Higher Density of Interstellar Hydrogen from Pickup Ion Observations in the Outer Heliosphere

Abstract:

Direct observations of interstellar neutral hydrogen atoms are challenging in the Earth proximity because most of these atoms are ionized along their trajectories from the very local interstellar medium to 1 au. The density of these atoms is decreased more than e-fold inside the ionization cavity extending to ~4 au in the upwind direction, and even more in the crosswind and downwind parts. As a result of this ionization, hydrogen atoms turn into pickup ions (PUIs), which accumulate in the expanding solar wind. The Solar Wind Around Pluto (SWAP) instrument on New Horizons provides, for the first time, extensive observations of PUIs in the outer heliosphere far beyond the ionization cavity. We analyze these observations out to nearly 40 au from the Sun, and we find that the interstellar hydrogen density in the upwind heliosphere far from the Sun of 0.127 cm^-3. This result is ~40% higher than the widely used consensus value of 0.09 cm^-3. Voyager observations of the slowdown of the solar wind also support this new value. Interstellar hydrogen atoms as the most abundant species in the heliosphere play an important role in many heliospheric processes. More hydrogen atoms imply higher charge-exchange rates of PUI creation and their subsequent neutralization to energetic neutral atoms (ENAs). Therefore, the new density estimate explains why the ENA fluxes from the heliosheath observed by IBEX are about two times higher than the fluxes predicted by numerical models. Based on this result and previous estimations of the filtration in the heliospheric boundaries, the hydrogen atom density in the unperturbed local interstellar medium is 0.195 cm^-3, which agrees with the astrophysical observations of the closest interstellar clouds.