University of Michigan
Composition of the Inner Source of Pickup Ions and Mercury’s Plasma Environment: Results from MESSENGER’s first two Mercury flybys and the Cruise Phase in Between
During the 14 January and 6 October 2008 flybys of planet Mercury by the MESSENGER spacecraft, measurements with the Fast Imaging Plasma Spectrometer (FIPS) in the energy range of ~0.05 to ~15 keV/charge revealed for the first time the basic properties and spatial distributions of H+ and He++ as well as the composition of heavier ions in the different regions of Mercury’s magnetosphere. H+ density and temperature vary from 0.1 to 2 cm-3 and 5•105 to 5•106 K, respectively. In addition to singly charged heavy pickup ions and molecules, multiply charged ions and molecules were observed in the magnetosphere. During the cruise phase, between the two Mercury encounters, we observed an extended source of singly and multiply charged ions, with (K+ and, or Ca+), Fe+ and (FeO+ and, or CaS+) being the most abundant pickup ions among a composition that also includes C+, (Na+ and, or Mg+) and water group molecular ions. The solar corona can be excluded as a source for most these ions because of their composition. Rather, these ions are likely born within the inner heliosphere at distances less than ~0.3 AU. These new observations will be interpreted in the context of pickup ions measured by the Ulysses Solar Wind Ion Composition Spectrometer (SWICS) in the extended three-dimensional heliosphere, as well as in the plasma tails of comets.