Interstellar Pickup Ion Observations to 38 AU

TL;DR

This study presents the first direct measurements of interstellar pickup ions in the solar wind from 22 to 38 AU, revealing their significant influence on solar wind dynamics and implications for the termination shock at 90 AU.

Contribution

It provides novel direct observations of pickup ions beyond 22 AU, quantifies their properties and trends, and discusses their impact on the heliosphere's outer regions.

Findings

Pickup ions dominate internal solar wind pressure by ~20 AU.

Pickup H+ temperature and pressure increase with distance, indicating additional heating.

Extrapolated pickup ion pressure at the termination shock is about 16% of solar wind dynamic pressure.

Abstract

We provide the first direct observations of interstellar H+ and He+ pickup ions in the solar wind from 22 AU to 38 AU. We use the Vasyliunas and Siscoe model functional form to quantify the pickup ion distributions, and while the fit parameters generally lie outside their physically expected ranges, this form allows fits that quantify variations in the pickup H+ properties with distance. By ~20 AU, the pickup ions already provide the dominant internal pressure in the solar wind. We determine the radial trends and extrapolate them to the termination shock at ~90 AU, where the pickup H+ to core solar wind density reaches ~0.14. The pickup H+ temperature and thermal pressure increase from 22-38 AU, indicating additional heating of the pickup ions. This produces very large extrapolated ratios of pickup H+ to solar wind temperature and pressure and an extrapolated ratio of the pickup ion pressure to the solar wind dynamic pressure at the termination shock of ~0.16. Such a large ratio has profound implications for moderating the termination shock and the overall outer heliospheric interaction. We also identify suprathermal tails in the H+ spectra and complex features in the He+ spectra, likely indicating variations in the pickup ion history and processing. Finally, we discover enhancements in both H+ and He+ populations just below their cutoff energies, which may be associated with enhanced local pickup. This study serves to document the release and as the citable reference of these pickup ion data for broad community use and analysis.