The average relationship of the tail lobe magnetic energy density to the north-south component of the interplanetary magnetic field (IMF) and substorm expansion phase is investigated by using a superposed epoch analysis method. An enhanced average lobe energy density is observed to be associated with a southward component of the IMF prior to substorm breakup. Coincident with the expansion onset, the lobe field rapidly recovers, and the north-south component of the IMF turns northward. The average magnitude of the substorm-associated lobe field variation is measured to be ~14%, a value sifficiently large to enable the lobes to serve as the source of the energy dissipated during substorms.