The asymmetric transport of the earth's polar outflows controlled by the interplanetary magnetic field Bx

The polar outflows, as an important plasma source of the earth's magnetosphere, usually exhibit significant north-south asymmetries, which can strongly affect the plasma distributions in the magnetotail lobe and perhaps contribute to the substorm triggering. But the mechanism of the asymmetric transport of these outflows is still unclear. In this letter, 3-D global magnetohydrodynamic (MHD) simulations are performed to investigate the development of the polar outflows after their escapes from the inner boundary under influences of the interplanetary magnetic field (IMF) B_x. It is found that the velocity of northern polar outflows is much stronger than the south. We suggest that the IMF B_x causes the north-south asymmetries in the magnetospheric configuration, and subsequently, great differences of the force and mass distributions appear between the two hemispheres, which lead to the significant north-south asymmetries in the transport of the polar outflows. We also discuss the differences in the acceleration mechanisms of the polar outflows between the northward and southward IMF cases.