Quantification of the total ion transport in the near-Earth plasma sheet

Abstract. Recent studies strongly suggest that a majority of the observed O⁺ cusp outflows will eventually escape into the solar wind, rather than be transported to the plasma sheet. Therefore, an investigation of plasma sheet flows will add to these studies and give a more complete picture of magnetospheric ion dynamics. Specifically, it will provide a greater understanding of atmospheric loss. We have used Cluster spacecraft 4 to quantify the H⁺ and O⁺ total transports in the near-Earth plasma sheet, using data covering 2001–2005. The results show that both H⁺ and O⁺ have earthward net fluxes of the orders of 10²⁶ and 10²⁴ s⁻¹, respectively. The O⁺ plasma sheet return flux is 1 order of magnitude smaller than the O⁺ outflows observed in the cusps, strengthening the view that most ionospheric O⁺ outflows do escape. The H⁺ return flux is approximately the same as the ionospheric outflow, suggesting a stable budget of H⁺ in the magnetosphere. However, low-energy H⁺, not detectable by the ion spectrometer, is not considered in our study, leaving the complete magnetospheric H⁺ circulation an open question. Studying tailward flows separately reveals a total tailward O⁺ flux of about 0. 5 × 10²⁵ s⁻¹, which can be considered as a lower limit of the nightside auroral region O⁺ outflow. Lower velocity flows ( < 100 km s⁻¹) contribute most to the total transports, whereas the high-velocity flows contribute very little, suggesting that bursty bulk flows are not dominant in plasma sheet mass transport.