Substorm expansion embedded in a global cycle of field-aligned currents and auroral electrojets

Abstract 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. This cycle involves two components of the auroral electrojets: the convection-driven DP-2 current and the expansion-phase DP-1 substorm current. The antisunward-equatorward phase, corresponding to intervals of dominant dayside reconnection, begins with DP-2 and can stepwise transition into DP-1. 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, providing insight into substorm evolution.