The dependence of substorm occurrence probability on the north--south component Bz of the interplanetary magnetic field (IMF) and on the size of the auroral oval is examined on the basis of two independent data sets (Isis 1 and 2 low-energy electron data and all-sky camera data from the Alaska meridan). The occurrence of substorms is identified by the electron precipitation pattern (Isis data), the auroral features in the Alaskan sector, and available ground magnetic records. The substorm occurrence frequency increases as the oval expands and as the Bz component of the IMF decreases. It increases from approximately 25% to 100% for Bz values of the IMF ranging from +7 to -5 nT; here Bz values are averaged for 1 hour preceding the time of satellite passage and the onset of substorms observed by the all-sky cameras. The occurrence probability of quiet times increases with an increasing value of the northward IMF. It is interesting to note that there are almost no periods without substorms when the IMF has a large southward component, but substorms do occur even when the IMF has a large northward component. Since there is a close relationship between the direction of the IMF and the size of the auroral oval, our finding that the occurrence frequency of substorms increases with the expansion of the auroral oval suggests that the substorm probability may be related to the amount of energy stored in the magnetotail.