Responses of different ion species to fast plasma flows and local dipolarization in the plasma sheet

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 RE. 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 steeper Vx dependence 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 initially increases, then it decreases to below the preceding levels except for O+; (4) the O+-to-H+ ratio does not increase through local dipolarization irrespective of dipolarization amplitude, background Bz, X distance, and Vx; (5) in general, the H+ and He++ ions behave similarly. Result (1) can be attributed to radial transport in the presence of the earthward gradient of the background O+-to-H+ ratio. Results (2) and (4) suggest that ion energization at local dipolarization is not mass dependent in the energy range of our interest because the ions are not magnetized irrespective of species. Result (3) can be attributed to the thinning of the plasma sheet and the preferable field-aligned escape of the H+ ions on the tailward side of the reconnection site. Result (5) suggests that the solar wind is the primary source of the high-energy H+ ions.