An explanation for tailward flows with positive Bz in the distant tail neutral sheet during quiet times

Abstract

ISEE‐3 and GEOTAIL observations have revealed that at x = −210 to −220 RE, the z‐component of magnetic field across the tail neutral sheet is positive, and the plasma flow is tailward during geomagnetically quiet times. Observations have also shown that the y‐ and z‐components of the interplanetary magnetic field (IMF) partially penetrate into the magnetotail for both positive and negative IMF Bz. We have evaluated the topology of the magnetospheric magnetic field expected from a partial penetration of the IMF when it has a positive z‐component. We find that, in addition to the closed and purely IMF field lines that are generally acknowledged to exist for a southward IMF Bz, there is a region at intermediate radial distances of open field lines that cross the tail neutral sheet before exiting the magnetosphere. This region lies between x ≈ −150 RE and x ≈ −325 RE for the representative IMF orientation and model that we have used. We suggest that the quiet‐time observations from ISEE‐3 and GEOTAIL of tailward flow and positive Bz across the distant tail neutral sheet were obtained during positive IMF Bz conditions and in the region of open field lines. Such an explanation would also account for the GEOTAIL observation that magnetosheath‐like plasma associated with positive Bz is common in the distant tail plasma sheet beyond 150 RE but not closer to the Earth.