This paper provides a brief review of the understanding of the coupled solar wind–magnetosphere–ionosphere system that can be attained from observations of the size of the ionospheric polar caps from space and the ground. These measurements allow the occurrence and rate of dayside and nightside reconnection to be deduced. The former can be correlated with upstream interplanetary magnetic field observations to give an estimate of the effective length of the dayside reconnection X-line. The latter allows a preli- minary statistical study of flux closure during substorms and other, smaller nightside reconnection events.