Formation of Minor-Ion Charge States in the Fast Solar Wind: Roles of Differential Flow Speeds of Ions of the Same Element

To investigate the possibility of differential flow speeds between ions of the same element and their roles in the determination of ionic fractions, this paper extends our latest minor-ion model to a five-fluid model, describing the behavior of five species of minor ions of one given element in the fast solar wind. We solve the five sets of mass, momentum, and energy equations simultaneously to include the effects of ionization, recombination, and heating. The five species of minor ions are taken as test particles flowing in a background plasma consisting of electrons, protons, and alpha particles. The parameters of the background gas are calculated using a previous three-fluid wave-driven magnetohydrodynamic model for the fast solar wind. These background parameters are modeled as closely as possible to observed values. Using this background of fast solar wind parameters, the five-fluid minor-ion model has no problem reproducing the frozen-in charge-state distributions observed in the fast solar wind for C and O ions, while the modeled ionic fractions of Si, Mg, and Fe show significant shifts to lower charge states compared with the observed values. It is found that the majority of C and O ions are frozen-in below 1.2 solar radii, while most Si, Mg, and Fe ions are frozen-in beyond 1.3-1.5 solar radii. Comparing the cases with and without differential flows shows that even though differential flow speeds between ions of the same element do develop beyond a certain heliocentric distance (e.g., 1.2 solar radii for Si ions), they cannot account for the high ion charge states observed in situ.