POLAR WIND ION DYNAMICS IN THE MAGNETOTAIL

The circulation of polar wind ions from the high-latitude ionosphere to the plasma sheet is examined by means of three-dimensional particle codes. The simulations reveal the possible creation of high-altitude mirror points for particles traveling in the equatorial magnetotail. It is shown that this feature follows from large centrifugal decelerations in the course of the fast ExB transport, and not from nonadiabatic motion in the local field reversal. Such decelerations yield the confinement (here termed ''centrifugal trapping'') of populations with relatively small parallel speed near the tail midplane. It is also demonstrated that the net polar wind plasma supply to the plasma sheet critically depends upon the dynamics of ions which originate from the highest latitudes of the dayside ionosphere. During quiet times, these particles gain access to the distant tail where they experience durable nonadiabatic trapping. Accordingly, it is found that the polar wind forms a prominent source of plasma for the central nightside sector, yielding appreciable density levels (of the order of several tenths of ions cm-3) in the characteristic plasma sheet energy range (from hundreds of electron volts up to a few kiloelectron volts). A lesser contribution is obtained during active times, as the low-energy ions are confined to low L shells where they essentially travel in an adiabatic manner. Thus, in addition to emphasizing the role of ionospheric ion dynamics inside the neutral sheet, the trajectory simulations suggest that the polar wind efficiency in feeding the plasma sheet depends upon the time evolution of the particles, rather than from an increasing plasma supply from low to high Kp.