The poleward and equatorward boundaries of the nightside auroral oval are scaled from DMSP auroral imagery and their response to the interplanetary magnetic field (IMF) is investigated. Although, as expected, the Bz component of the IMF is found to be the dominant factor in determining the latitude of the oval equatorward boundary, the correlation coefficients are not very high: the large variance results from the considerable scatter of points when the IMF is northward and large. By examining the oval boundaries from successive orbits, it is suggested that most of the expanded oval cases during such northward IMF periods occurred within several hours after periods of southward IMF. This indicates that the response of the oval to northward turnings (i.e., the contraction of the oval) takes place rather slowly, so that the contracted oval in its smallest size can be established only when the IMF has a northward component steadily for more than several hours. The observed oval equatorward boundary is mapped onto the equatorward plane of the magnetotail, and then plasma sheet processes are discussed with respect to present theories on the formation of the Alfv¿n layer as a result of the penetration of solar wind electric fields into the magnetotail.