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

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

Mar 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010jgra..11503102m&link_type=abstract

Journal of Geophysical Research, Volume 115, Issue A3, CiteID A03102

Physics

3

Interplanetary Physics: Pickup Ions, Interplanetary Physics, Interplanetary Physics: Heliosphere/Interstellar Medium Interactions, Interplanetary Physics: Solar Wind Plasma

Scientific paper

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.

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