O+ transport across the polar cap

The plasma sheet, inner magnetosphere, and high-latitude magnetosphere all contain significant amounts of O+ ions during active times. Singly charged oxygen ions unambiguously come from the ionosphere, making them an excellent tracer species. We test the cleft ion fountain theory, which asserts that O+ ions escape from the cleft, cross the polar cap, and then enter the plasma sheet. Statistical studies of O+ density in the cleft, high-altitude polar cap, and plasma sheet all indicate that the O+ density increases with increasing solar wind dynamic pressure. In order to examine O+ transport more directly, we use polar cap ion outflow measurements and the 2001 Tsyganenko magnetic field model driven with advanced composition explorer (ACE) solar wind parameters. We calculate the distance between the cleft and the ionospheric footpoints of magnetic field lines mapped from the polar spacecraft along the noon-midnight meridian. Using the observed outflow speed and the magnetic field line length we calculate the travel time for the ions. When we examine the distance from the cleft versus the O+ travel time for individual passes, the slope of the line is consistent with the measured ionospheric convection speed across the polar cap. We conclude that O+ ions emanating principally from the cleft are transported across the polar cap, and these O+ ions have access to the ring current and plasma sheet.