Substorm Expansion Embedded in a Global Cycle of Field-Aligned Currents and Auroral Electrojets

Geomagnetic substorms transfer solar wind energy into the planetary magnetosphere and ionosphere, producing auroral displays and ground magnetic disturbances, particularly intense during the expansion phase. Despite decades of study, the mechanisms governing the expansion phase remain unresolved. Based on coordinated observations of storm-time intense substorms, we reveal that substorm expansion is temporally embedded within a global cycle of field-aligned currents and auroral electrojets, coupled to large-scale plasma convection. The cycle manifests as a coherent movement of current peaks across magnetic longitude and latitude—first antisunward and equatorward, then sunward and poleward—and coincides with enhanced sunward ionospheric convection. The antisunward–equatorward phase, corresponding to intervals of dominant dayside reconnection, begins with a convection-driven DP-2 current and can stepwise transition into a substorm-expansion DP-1 current. During the subsequent sunward–poleward phase, reflecting intervals of dominant nightside reconnection, DP-1 either persists from the earlier interval or develops within this phase. These observations show that expansion onset can occur under dominance of either dayside or nightside reconnection, while the full development of DP-1 generally involves nightside reconnection, offering new insight into substorm evolution—an objective central to the SMILE mission.