SHINE 2013 Abstracts

First Name: Daniel
Middle Name: J
Last Name: Gershman
Email: djgersh@umich.edu
   
Principal Author Affiliation: University of Michigan
Second Authors and Affiliations
Lennard A. Fisk (UMich), George Gloeckler (UMich), Jim M. Raines (UMich), James A. Slavin (UMich), Thomas H. Zurbuchen (UMich), and Sean C. Solomon (Lamont-Doherty Earth Observatory)
Title of Poster/Presentation:
He+ Distribution Functions at 0.3 AU
   
Abstract:  
During its interplanetary cruise trajectory in 2007–2009, the MErcury Surface, Space ENvrionment, GEochemistry, and Ranging (MESSENGER) spacecraft passed through the gravitational focusing cone for interstellar helium multiple times at a heliocentric distance near R = 0.3 AU. Observations of He+ interstellar pickup ions made by the Fast Imaging Plasma Spectrometer (FIPS) sensor on MESSENGER during these transits provide a glimpse into the structure of newly formed pickup ion distributions. In addition to the more radial interplanetary magnetic field at 0.3 AU than farther from the Sun, ions at 0.3 AU will not have had sufficient time to substantially cool or be energized, resulting in a pristine distribution function that should be a function only of pickup ion injection velocities and pitch-angle scattering processes. From measured energy-per-charge spectra obtained during multiple spacecraft rotations in attitude, we have deduced the phase space density of He+ as a function of magnetic pitch angle. Our measurements are most consistent with a distribution that decreases nearly monotonically with pitch angle, rather than the more commonly modeled isotropic or hemispherically symmetric forms. In addition, there is a strong sunward–anti-sunward anisotropy, with a phase space density a factor of at least 3 greater for particles with pitch angles less than 90 degrees. These results imply that pitch-angle scattering of He+ may not be instantaneous and may result in structure that is a strong function of initial particle injection velocities.
Session: Other
Invited Paper: no
Student Paper: yes
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