The contrasts between the behaviour of the ionosphere-magnetosphere system in the two hemispheres provide very important insights into the mechanisms involved, and constraints on theoretical descriptions. For example, the transfer of solar wind energy into the magnetosphere and ionosphere normally differs for the northern and southern portions of the coupled magnetosphere/ionosphere system primarily because the Earth's magnetic axis is tilted both with respect to the ecliptic plane and the Earth's geographic axis. The magnetic tilt thus varies both with UT and with season. The effects of the imposed IMF are often anti-symmetric between the two hemispheres, whereas variations in solar wind pressure are symmetric. Thus, simultaneous observations in both hemispheres are needed for unambiguous interpretation. The conductivity of the ionosphere is normally very different in the two polar regions (because of the seasonal variation in solar UV illumination there), causing large differences in the intensity of field-aligned and horizontal polar ionospheric currents.
Ground-based observations, especially those from imaging instruments, also offer the opportunity to separate spatial from temporal changes in a way not possible with single spacecraft measurements. Therefore they are highly complementary to the GGS satellite observations.
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