Observations of interstellar helium pickup ions in the inner heliosphere

Details Observations of interstellar helium pickup ions in the inner heliosphere Observations of interstellar helium pickup ions in the inner heliosphere

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh11c..07g&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #SH11C-07

Physics

[2151] Interplanetary Physics / Neutral Particles, [2152] Interplanetary Physics / Pickup Ions

Scientific paper

During the inner heliosphere transfer orbit of the MESSENGER spacecraft, the Fast Imaging Plasma Spectrometer (FIPS) measured the first pickup ion distributions at heliospheric distances of 0.3-0.75 AU. From several transits of MESSENGER through the interstellar helium gravitational focusing cone at 0.3 AU and 0.6 AU, we find an enhancement by a factor of 4 in measured pickup ion densities centered directly downwind from the direction of interstellar flow. These enhancements extend approximately 30 degrees around the cone center, consistent with corresponding observations made at 1 AU. For approximately time-stationary conditions from 2007 to 2009, we find a nearly 1/R dependence of pickup ion density in the inner heliosphere. We also present the first pitch-angle distributions of helium pickup ions in the inner heliosphere. Near the cone center at 0.3 AU, we observe a strong anti-sunward anisotropy with respect to the bulk solar wind flow, consistent with ions picked up in a nearly radial magnetic field and swept out into the heliosphere with inhibited pitch-angle scattering across 90 degrees. These data are compared with observations of He+ near 1 AU with the Solar Wind Ion Composition Spectrometer (SWICS) that is part of the ACE spacecraft. These measurements cover passes through the heliospheric focusing cone during the same time period. We compare FIPS and SWICS data with models of interstellar neutral density, ionization, and transport properties

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