The area of the polar cap, i.e. the region of open magnetic field lines or, more practically, the region inside the auroral-oval auroral distribution can be a good indicator of the amount of energy in the magnetotail available for magnetospheric substorms. It is shown that the polar cap is not a simple circle during the episodic development of substorms but expands and contracts non-uniformly, depending strongly upon magnetic local time. Variations of the polar cap area under such circumstances represent merely the average around the oval of different time changes in the polar cap boundary location at different local times, in response to solar wind conditions and the substorm phases. It is demonstrated that this non-uniform expansion/contraction of the polar cap can be accounted for in terms of the relative strength of the two constituents of the solar wind-magnetosphere coupling: the directly-driven and unloading processes.
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