Variations of High-Energy Ions during Fast Plasma Flows and Dipolarization in the Plasma Sheet: Comparison Among Different Ion Species

Abstract:

We investigate the responses of different ion species (H⁺, He⁺, He⁺⁺, and O⁺) to fast plasma flows and local dipolarization in the plasma sheet in terms of energy density. We use energetic (9-210 keV) ion composition measurements made by the Geotail satellite at r = 10~31 R_E. The results are summarized as follows: (1) whereas the O⁺-to-H⁺ ratio decreases with earthward flow velocity, it increases with tailward flow velocity with Vx dependence steeper for perpendicular flows than for parallel flows; (2) for fast earthward flows, the energy density of each ion species increases without any clear preference for heavy ions; (3) for fast tailward flows the ion energy density increases initially, then it decreases to below pre-flow levels except for O⁺; (4) the O⁺-to-H⁺ ratio does not increase through local dipolarization irrespective of dipolarization amplitude, background B_Z, X distance, and V_X; (5) in general, the H⁺ and He⁺⁺ ions behave similarly. Result (1) can be attributed to radial transport along with the earthward increase of the background O⁺-to-H⁺ ratio. Results (2) and (4) indicate that ion energization associated with local dipolarization is not mass-dependent possibly because in the energy range of our interest the ions are not magnetized irrespective of species. In the tailward outflow region of reconnection, where the plasma sheet becomes thinner, the H⁺ ions escape along the field line more easily than the O⁺ ions, which possibly explains result (3). Result (5) suggests that the solar wind is the primary source of the high-energy H⁺ ions.