Abstract
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.
Highlights
The study of the fate of ion outflow is interesting for the atmospheric evolution of Earth, as it hints to what extent interactions between the solar wind and the magnetosphere affect the planetary atmosphere
In this paper we present a statistical study of O+ transport in the dayside magnetosheath and its dependence on the interplanetary magnetic field (IMF) direction
The average flux as well as the occurrence rate for O+ in the dayside magnetosheath are both higher during southward IMF, and both contribute to the higher total escape flux, a result consistent with studies showing higher cusp fluxes during southward IMF
Summary
The study of the fate of ion outflow is interesting for the atmospheric evolution of Earth, as it hints to what extent interactions between the solar wind and the magnetosphere affect the planetary atmosphere. We can assume that all observed positive H+ parallel flows during small clock angles, |θ | = tan−1(|By|/Bz) < 30◦, take place along path B, on a magnetosheath field line that has reconnected in – at least – one of the lobes. 12 % of the observed H+ during northward IMF and small clock angles is positive, and the absolute bulk velocities are typically considerably smaller than for the negative flow. For large clock angles the y component of the IMF is dominant and the situation is quite different from the situation illustrated in Fig. 2 since reconnection tends to take place out on the lobe flanks and because ions flowing out along the newly reconnected field lines will be restricted to the same hemisphere. The data set has not been divided into two subsets like for the small clock-angle data since it turns out this is not necessary in order to interpret the numbers
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