We examine temporal and spatial variations of the distance between the electron precipitation boundaries across the polar cap; the Defense Meterological Satellite Program satellite data, within a few hours of the dawn-dusk and noon-midnight meridians, are used. It is found that changes in the distance between the precipitation boundaries across the polar region in both the dawn-dusk and noon-midnight sectors tend to follow grossly changes of the AE index. On the basis of the simultaneous ISEE 3 data, it is found that the increase of the distance begins at about the time when the southward turning of the interplanetary magnetic field reaches the magnetosphere, but prior to the corresponding increase of the AE index. The period of the maximum distance approximately coincides with the period of the maximum AE value. The decrease of the distance begins at about the same time as the substrom activity begins to subside, but it continues well after the AE index has reduced to very small values. These findings are discussed in terms of changes of the total open magnetic flux by assuming that the precipitation boundaries coincide approximately with the boundaries of the open field line region.