  The solar-wind magnetosphere interaction primarily occurs at altitudes where
the dipole component of Earth's magnetic field is dominating. The disturbances
that are created in this interaction propagate along magnetic field lines and
interact with the ionosphere-thermosphere system. At ionospheric altitudes, the
Earth's field deviates significantly from a dipole. North-South asymmetries in
the magnetic field imply that the magnetosphere ionosphere-thermosphere (M-I-T)
coupling is different in the two hemispheres. In this paper we review the
primary differences in the magnetic field at polar latitudes, and the
consequences that these have for the M-I-T coupling. We focus on two
interhemispheric differences which are thought to have the strongest effects:
1) A difference in the offset between magnetic and geographic poles in the
Northern and Southern Hemispheres, and 2) differences in the magnetic field
strength at magnetically conjugate regions. These asymmetries lead to
differences in plasma convection, neutral winds, total electron content, ion
outflow, ionospheric currents and auroral precipitation.
