skip to main content

Title: The velocity distribution of pickup He{sup +} measured at 0.3 AU by MESSENGER

During its interplanetary 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 R ≈ 0.3 AU. Observations of He{sup +} interstellar pickup ions made by the Fast Imaging Plasma Spectrometer sensor on MESSENGER during these transits provide a glimpse into the structure of newly formed inner heliospheric pickup-ion distributions. This close to the Sun, these ions are picked up in a nearly radial interplanetary magnetic field. Compared with the near-Earth environment, pickup ions observed near 0.3 AU will not have had sufficient time to be energized substantially. Such an environment results in a nearly pristine velocity distribution function that should depend only on pickup-ion injection velocities (related to the interstellar gas), pitch-angle scattering, and cooling processes. From measured energy-per-charge spectra obtained during multiple spacecraft observational geometries, we have deduced the phase-space density of He{sup +} as a function of magnetic pitch angle. Our measurements are most consistent with a distribution that decreases nearly monotonically with increasing pitch angle, rather than the more commonly modeled isotropic or hemispherically symmetric forms. These results imply that pitch-angle scattering of He{sup +} may not bemore » instantaneous, as is often assumed, and instead may reflect the velocity distribution of initially injected particles. In a slow solar wind stream, we find a parallel-scattering mean free path of λ {sub ||} ∼ 0.1 AU and a He{sup +} production rate of ∼0.05 m{sup –3} s{sup –1} within 0.3 AU.« less
Authors:
 [1] ; ; ; ; ;  [2] ;  [3]
  1. Geospace Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
  2. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109 (United States)
  3. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 (United States)
Publication Date:
OSTI Identifier:
22356596
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 788; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATOMS; DATA ANALYSIS; DENSITY; DIFFUSION; DISTRIBUTION; DISTRIBUTION FUNCTIONS; HELIUM IONS; INCLINATION; INJECTION; INTERPLANETARY MAGNETIC FIELDS; MEAN FREE PATH; PHASE SPACE; SCATTERING; SOLAR WIND; SPECTRA; SPECTROMETERS; SUN; SYMMETRY; TRAJECTORIES; VELOCITY