Physics
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsh24a..04m&link_type=abstract
American Geophysical Union, Spring Meeting 2009, abstract #SH24A-04
Physics
2144 Interstellar Gas, 2151 Neutral Particles (7837), 2152 Pickup Ions, 2162 Solar Cycle Variations (7536), 2194 Instruments And Techniques
Scientific paper
The shape of pickup ion velocity distributions in the solar wind is typically used to infer the radial profile of the neutral source distributions sunward of the observing instrument. Since this radial profile is a direct function of the ionization rate relative to the neutral flow time, this information can also determine the ionization rate. However, this analysis also makes the significant assumption that the pickup distribution cools from the point of ionization in the expanding solar wind as an isotropic adiabatic gas. This isotropic-adiabatic assumption allows a simple mapping of the pickup intensity in velocity space to the ion production rate in radial position. However, the finding of substantial anisotropies of pickup ion distributions, in particular under radial interplanetary magnetic field conditions, and of large variations of pickup ion distributions that are correlated with magnetic field conditions, suggests that pitch-angle scattering of these particles is substantially inhibited in contrast to the simple theory. These observations prompt a re-evaluation of the existing models. Here, we explore the effect of different cooling functions on the shape of interstellar pickup He distributions under perpendicular magnetic field conditions. The shape also depends on the ionization rate through the radial neutral density profile. We show that pickup He observations over a solar activity cycle, during which the ionization rate will vary by factors of 3 - 4, can strongly constrain the actual cooling behavior of the pickup distribution and provide information on the extent of isotropization.
Bochsler Peter
Isenberg Philip A.
Moebius Eberhard
Saul Lukas
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