Polar eyes the cusp

Measurements by the POLAR spacecraft show that the polar cusp is affected by the angle of the magnetic dipole axis to the direction of the solar wind flow, the dynamic pressure of the solar wind and the direction of the interplanetary magnetic field. The polar cusp moves equatorward with increasing southward interplanetary magnetic field and much less, but also in the equatorward direction, for increasingly northward magnetic field. The cusp moves to the afternoon side for positive Y components of the interplanetary magnetic field in the northern hemisphere and toward dawn in the southern hemisphere. Negative Y components, in the direction of the Earth's orbital motion, have the reverse effect on the cusp position. Higher dynamic pressure extends the cusp to greater local times away from noon and thickens it in the poleward direction. When the IMF is nearly due northward, reconnection tailward of the cusp can occur producing magnetosheath plasma on magnetic field lines that have two feet in the ionosphere. This process transfers momentum to the magnetospheric plasma, and creates the low-latitude boundary layer. Near the cusp the magnetic field lines connected to the Earth reverse rapidly across a thin current sheet that could be described as the magnetopause. However, there is a second current sheet further out, beyond which field lines are no longer connected to the ionosphere. D-shaped particle distributions in this region are consistent with reconnection tailward of the cusp. The distributions evolve to be more isotropic with time since reconnection. Perhaps surprisingly, the magnetosheath plasma moving toward the cusp reconnection site is slowed greatly on the field lines connected to the ionosphere.