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Polar caps are high latitude regions on both hemispheres with open magnetic field lines connecting directly to the interplanetary magnetic field. The openess of the field lines inside the caps can be verified by the existence of polar rain. At high altitudes, field lines originating from the polar cap form the tail lobe region in the magnetotail. Lobe is separated from the plasma sheet by plasma sheet boundary layer (PSBL).
Phenomena typical for polar cap are, for example
The diameter of the polar cap can be used to estimate the energy flux into the magnetosphere via dayside reconnection. The diameter can be measured by using low altitude polar satellites or, from the ground, with a special polar cap index.