Pickup hydrogen distributions in the solar wind at similar to 11 AU: Do we understand pickup ions in the outer heliosphere?

Academic Article


  • This study examines the detailed distribution of interstellar pickup hydrogen ions measured by the Surface Wave Process Program instrument on the New Horizons spacecraft en route to Pluto. The observations cover an 8 h interval on 15 October 2008 when the craft was at ∼11.3 AU, constituting the first detailed pickup ion measurements in the outer heliosphere, beyond ∼8 AU. We show the pickup ion cutoff to be roughly twice the solar wind speed, as expected; however, we also measure both a significant suprathermal tail above this “cutoff” and a pickup ion asymmetry between inward and outward ions of ∼4:1, very similar to that observed on Ulysses at <5 AU. This anisotropy could indicate some combination of a scattering mean free path that is much longer than previously realized, significantly more radial magnetic field than expected, some missing effect in the current models, and/or some unknown background in the low‐energy measurements. Alternately, these observations could indicate that we need to develop a new understanding of pickup ion transport throughout the heliosphere. Understanding these new observations is especially important because recent Voyager measurements have shown that these pickup ions play a critical role in the interaction at the heliosphere's termination shock. Thus, the observations reported here may be important for understanding the heliosphere's whole interstellar interaction.
  • Authors

  • McComas, DJ
  • Elliott, HA
  • Schwadron, Nathan
  • Status

    Publication Date

  • March 17, 2010
  • Published In

    Digital Object Identifier (doi)

    Start Page

  • n/a
  • End Page

  • n/a
  • Volume

  • 115
  • Issue

  • A3