THE VELOCITY DISTRIBUTION OF PICKUP He super(+) MEASURED AT 0.3 AU BY MESSENGER

• Published in: The Astrophysical journal Vol. 788; no. 2; pp. 1 - 13
• Main Authors: Gershman, Daniel J, Fisk, Lennard A, Gloeckler, George, Raines, Jim M, Slavin, James A, Zurbuchen, Thomas H, Solomon, Sean C
• Format: Journal Article
• Language: English
• Published: 20.06.2014
• Subjects: Interstellar; Mathematical models; MESSENGER Mission; MESSENGER Spacecraft; Pitch angle; Scattering; Spacecraft; Velocity distribution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/788/2/124

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 approximately 0.3 AU. Observations of He super(+) 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 super(+) as a function of magnetic pitch angle. Our measurements are most consistent with a disttibution 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 super(+) may not be 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 [lambda] sub(||) ~ 0.1 AU and a He super(+) production rate of ~0.05 m super(-3) s super(-1) within 0.3 AU.