Cometary water-group ions in the region surrounding comet Giacobini-Zinner: Distribution functions and bulk parameter estimates

Abstract

The bulk parameters (number density and thermal energy density) of cometary water-group ions in the region surrounding comet Giacobini-Zinner have been derived using data from the EPAS instrument on the ICE spacecraft. The derivation is based on the assumption that the pick-up ion distribution function is isotropic in the frame of the bulk flow, an approximation which has previously been shown to be reasonable within ∼4 × 10 5 km of the comet nucleus along the spacecraft trajectory (corresponding to ± 5 h about closest approach). Bulk parameter determinations are consequently restricted to this region, which corresponds physically to the inner part of the region of cometary ion pick-up in the solar wind flow, and the interior region of slowed, mass-loaded flow. The transition between the pick-up and mass-loaded regions occurs at the cometary shock, which was traversed at a cometocentric distance of ∼ 1 × 10 5 km along the spacecraft track (±85 min about closest approach). Examination of the ion distribution functions in this region, transformed to the bulk flow frame, indicates the occurrence of a “flattened” distribution in the vicinity of the local pick-up speed, and a steeply falling tail at speeds above, which may be approximated as an exponential in ion speed. Bulk parameters derived by integrating over these distributions have been compared with simple theoretical estimates based on ionization of the neutral coma of the comet by solar u.v. radiation, by charge exchange, and by electron impact, giving a combined ionization time-scale of ∼ 10 6 s, followed by ion pick-up and isotropization in the solar wind flow. In the inner pick-up region the derived parameters are in good agreement with the theoretical estimates, within a factor of ∼ 2, showing that no major additional sources of ionization, or major energization processes, are required to account for the results. Comparison of the thermal pressure of the water-group pick-up ions with the solar wind thermal pressure and with the magnetic pressure, shows how the pick-up ion pressure grows from small values at large distances from the comet, to become the dominant pressure component in the region just upstream from the shock. As a consequence, the magnetosonic Mach number falls from values of ∼ 5 at large distances, down to ∼ 2.5 just upstream of the shock. In the mass-loaded region downstream from the shock the observed ions most probably correspond to the relatively energetic population which is produced in the fast-flow region upstream from the shock, and which is subsequently converted downsteam. Reasonable agreement between derived and theoretically-estimated bulk parameters is obtained in the outer part of the mass-loaded region on this basis. In the inner part of the mass-loaded region, however, within ∼6 × 10 4 km of the comet (±45 min about closest approach), the derived densities fall far below theoretical predictions, indicating that the energetic ions created in the upstream region must be actively removed from these stream-tubes by some process, at present undetermined.