Interstellar Pickup Ion Observations to 38 AU

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.

arXiv comment31 pages, 16 figures, accepted for publication in The Astrophysical Journal Supplement Series

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Cited by in corpus (10)

• In Situ Observations of Preferential Pickup Ion Heating at an Interplanetary Shock
• The New Horizons Kuiper Belt Extended Mission
• Plasma pressures in the heliosheath from Cassini ENA and Voyager 2 measurements: Validation by the Voyager 2 heliopause crossing
• Interstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. The Large-scale Structures
• Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient
• Angular Scattering in Charge Exchange: Issues and Implications for Secondary Interstellar Hydrogen
• Energetic pickup proton population downstream of the termination shock as revealed by the IBEX-Hi data
• Interstellar Neutral Hydrogen in the Heliosphere: New Horizons Observations in the Context of Models
• Turbulence Driving by Interstellar Pickup Ions in the Outer Solar Wind
• Test particle energization of heavy ions in magnetohydrodynamic turbulence