Fig. 1

Conceptual circuit for the three phases of auroral substorms: the primary (S–M) dynamo is shown at the top. During the growth phase, the ionosphere cannot dissipate much increasing power ε, and thus, the magnetosphere accumulates magnetic energy in its inductive circuit. During the expansion phase, a new internal dynamo, the expansion phase dynamo (shown in red), is established as a result of unloading the accumulated magnetic energy and drives the unloading (UL) current; its 3-D circuit is shown in Fig. 6. The streaming current-carrying electrons along the magnetic field lines ionize the ionosphere, producing active auroras and increasing its conductivity during the expansion phase. The recovery phase is driven only by the S–M dynamo