O⁺ transport in the dayside magnetosheath and its dependence on the IMF direction

Abstract. Recent studies have shown that the escape of oxygen ions (O⁺) into the magnetosheath along open magnetic field lines from the terrestrial cusp and mantle is significant. We present a study of how O⁺ transport in the dayside magnetosheath depends on the interplanetary magnetic field (IMF) direction. There are clear asymmetries in the O⁺ flows for southward and northward IMF. The asymmetries can be understood in terms of the different magnetic topologies that arise due to differences in the location of the reconnection site, which depends on the IMF direction. During southward IMF, most of the observed magnetosheath O⁺ is transported downstream. In contrast, for northward IMF we observe O⁺ flowing both downstream and equatorward towards the opposite hemisphere. We observe evidence of dual-lobe reconnection occasionally taking place during strong northward IMF conditions, a mechanism that may trap O⁺ and bring it back into the magnetosphere. Its effect on the overall escape is however small: we estimate the upper limit of trapped O⁺ to be 5%, a small number considering that ion flux calculations are rough estimates. The total O⁺ escape flux is higher by about a factor of 2 during times of southward IMF, in agreement with earlier studies of O⁺ cusp outflow.