Field-aligned currents during northward IMF: Morphology and causes

We present the results of a global MHD simulation of solar wind magnetosphere interaction during northward IMF. In particular, we emphasize the effect of the IMF B y component on the reconnection geometry and the mapping along field lines to the polar ionosphere, through field-aligned currents. We find that the existence and geometry of the polar cap is closely connected to the IMF B y component. During strictly northward IMF the simulated magnetosphere can remain essentially closed because the solar wind field lines reconnect in both hemispheres, thereby creating newly reconnected closed dayside field lines. The existence of a small nonzero IMF B y component, however, effectively acts to open up the magnetosphere. When ∣B y ∣ <B z the position of the polar cap is strongly asymmetric with respect to the noon-midnight meridian, depending on the sign of B y . In the northern hemisphere for B y positive(negative) the polar cap is then located mainly in the dawnside (duskside), in close accordance with what have been observed using particle precipitation data or auroral observations. The simulated NBZ currents map to major portions of the magnetopause: the flanks and the mantle. They can exist both on open and closed field lines and are created by the shear of the newly reconnected field lines against the mantle field as they are convected tailward by the solar wind. When the IMF rotates from northward toward east, the magnetospheric mapping regions of the NBZ currents likewise rotates. However, the idea that the NBZ currents rotate to form the two sheets of FACs sandwiching the ionospheric DPY current is only partly confirmed by the simulation.