Substorm theories: United they stand, divided they fall

The magnetospheric substorm expansion involves the intermittent release of energy stored in the geomagnetic tail as a result of the interaction of Earth's magnetic field with the solar wind. Consensus on the timing and mapping of substorm features has permitted a synthesis of substorm models. Within the synthesis model the mechanism for onset of substorm expansion is still unknown. Possible mechanisms are: growth of an ion tearing mode, current disruption by a cross‐field current instability, and magnetosphere‐ionosphere coupling. While the synthesis model is consistent with overall substorm morphology, including near‐Earth onset, none of the onset theories, taken individually, appear to account for substorm expansion onset. A grand synthesis with unification of the underlying onset theories appears necessary.