The auroral boundary in terms of precipitating electrons is used to examine the influence of the By component of the interplanetary magnetic field (IMF) on the size of the auroral oval. It is found that when the earth's dipole tilt angle is positive (negative), the size of the auroral oval during periods of positive By is smaller (greater) than that during periods of negative By, and that the dependence occurs in the same way in both the northern and southern hemispheres. This finding implies that the size of the auroral oval is controlled by dayside reconnection, the rate of which depends strongly on the IMF By component and the dipole tilt angle. ¿ American Geophysical Union 1987