Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations

TL;DR

This study reevaluates Ulysses pickup ion data to accurately determine the neutral interstellar hydrogen density at the solar wind termination shock, providing insights into local interstellar conditions.

Contribution

It introduces a robust analytical and kinetic modeling approach to estimate interstellar hydrogen density from pickup ion data, accounting for uncertainties.

Findings

Neutral interstellar hydrogen density at the termination shock: 0.087±0.022 cm⁻³

Method demonstrates robustness against parameter uncertainties

Implications for hydrogen density and ionization in the local interstellar cloud

Abstract

By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements near 5 AU close to the ecliptic right after the previous solar minimum, this paper presents a determination of the neutral interstellar H density at the solar wind termination shock and implications for the density and ionization degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at the termination shock was determined from the local pickup ion production rate as obtained close to the cut-off in the distribution function at aphelion of Ulysses. As shown in an analytical treatment for the upwind axis and through kinetic modeling of the pickup ion production rate at the observer location, with variations in the ionization rate, radiation pressure, and the modeling of the particle behavior, this analysis turns out to be very robust against uncertainties in these parameters and the modeling. Analysis using current heliospheric parameters yields the H density at the termination shock equal to $0.087\pm0.022$ cm$^{-3}$, including observational and modeling uncertainties.