A detailed signature for individual substorms is sought in the interplanetary medium. Hourly values of interplanetary field and plasma parameters are correlated with hourly averages of the AE index. An interplanetary variable involving the southward component of the interplanetary field in the solar magnetospheric coordinate system is shown to be singularly important for the generation of substorms. The parameter best correlated with AE (0.8 correlation coefficient) is the integral or summation of Bz south over time for the hour preceding the AE hourly average. The magnitude of this integral appears to be linearly related to the hourly average of AE. The linearity suggests that the southward interplanetary field represents a continuing dynamic mechanism for the production of substorms rather than just being a trigger for the release of energy that has been stored in the magnetospheric tail. Furthermore, the additional energy that the southward component of the interplanetary field apparently puts into the tail is not accumulated for longer than about 1 hour before it appears as a substorm. A linear fit to AE that uses interplanetary parameters is obtained for two time intervals of data.