Interstellar Neutral Hydrogen in the Heliosphere: New Horizons Observations in the Context of Models

Abstract

Interstellar neutral (ISN) hydrogen is the most abundant species in the outer heliosheath and the very local interstellar medium (VLISM). Charge exchange collisions in the outer heliosheath result in filtration, reducing the ISN hydrogen density inside the heliosphere. Additionally, these atoms are intensively ionized close to the Sun, resulting in a substantial reduction of their density within a few au from the Sun. The products of this ionization - pickup ions (PUIs) - are detected by charged particle detectors. The Solar Wind Around Pluto (SWAP) instrument on New Horizons provides, for the first time, PUI observations from the distant heliosphere. We analyze the observations collected between 22 and 52 au from the Sun to find the ISN hydrogen density profile and compare the results with predictions from global heliosphere models. We conclude that the density profile derived from the observations is inconsistent with steady-state model predictions. 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 VLISM conditions. Furthermore, we show that the cold and hot models of ISN hydrogen distribution are not a good approximation closer to the termination shock. Therefore, we recommend a new fiduciary point based on the available New Horizons observations at 40 au from the Sun, at ecliptic direction (285.62°, 1.94°), where the ISN hydrogen density is 0.11 cm${}^{-3}$. The continued operation of New Horizons should give better insight into the source of the discussed discrepancy.

Author Accepted Manuscript. Accepted for publication in The Astrophysical Journal Letters. 11 pages, 4 figures, 1 table

References in corpus (24)

• The effects of a kappa-distribution in the heliosheath on the global heliosphere and ENA flux at 1 AU
• The New Horizons Pluto Kuiper belt Mission: An Overview with Historical Context
• The Solar Wind Around Pluto (SWAP) Instrument Aboard New Horizons
• Probing Heliospheric Asymmetries with an MHD-kinetic Model
• Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations
• Interstellar Pickup Ion Observations to 38 AU
• Magnitude and direction of the local interstellar magnetic field inferred from Voyager 1 and 2 interstellar data and global heliospheric model
• Comparing various multi-component global heliosphere models
• Sun-Heliosphere Observation-based Ionization Rates Model
• Interstellar neutral helium in the heliosphere from IBEX observations. II. The Warsaw Test Particle Model (WTPM)
• Neutral H density at the termination shock: a consolidation of recent results
• Neutral interstellar hydrogen in the inner heliosphere under the influence of wavelength-dependent solar radiation pressure
• Interstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. Ionization Rates For H, O, Ne, And He
• Plasma pressures in the heliosheath from Cassini ENA and Voyager 2 measurements: Validation by the Voyager 2 heliopause crossing
• Evolution of the Solar Lyman-Alpha line profile during the solar cycle
• The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity
• Interstellar Neutral Gas Species And Their Pickup Ions Inside The Heliospheric Termination Shock. The Large-scale Structures
• A new 3D solar wind speed and density model based on interplanetary scintillation
• Evolution of the solar Ly-$\alpha$ line profile during the solar cycle. II. How accurate is the present radiation pressure paradigm for interstellar neutral H in the heliosphere?
• Update of the Solar Lyman-Alpha Profile Line Model
• Interplanetary Lyman $\alpha$ line profiles: variations with solar activity cycle
• Angular Scattering in Charge Exchange: Issues and Implications for Secondary Interstellar Hydrogen
• On the sensitivity of heliosphere models to the uncertainty of the low-energy charge exchange cross section
• Energetic neutral atoms from the heliosheath as an additional population of neutral hydrogen in the inner heliosphere

Cited by in corpus (2)

• Production and Loss Processes of Hydrogen Energetic Neutral Atoms in the Heliosphere from 5 eV to 500 keV
• Gravitational Ionization by Schwarzschild Primordial Black Holes