The auroral boundary index calculated by Gussenhoven et al. [1983> on the basis of Defense Meteorological Satellite Program precipitating electron data is used to examine the dynamics of the auroral oval in the midnight sector. The equatorward boundary of the oval expands equatorward and contracts poleward corresponding to southward and northward turnings, respectively, of the interplanetary magnetic field (IMF). The time scales are found to be, on the average, 45 min for the expansion and 8 hours for the contraction. A general empirical formula to predict the oval location for any IMF variation is devised. Physical processes in the magnetosphere consistent with ''rapid'' expansion and ''slow'' contraction of the auroral oval are discussed.