Substorm Expansion Embedded in a Planetary-Scale Auroral Current Cycle

Abstract The substorm cycle is a fundamental mode of solar–planetary energy transfer, observed at Earth and other magnetized planets. It involves the storage of solar wind energy within planetary magnetospheres, followed by its rapid release into the planetary space environment. Yet, the global processes governing this cycle—particularly during the expansion phase of energy release—remain unresolved. Here we present coordinated observations of a series of intense substorms, revealing that substorm expansion is embedded in a previously unrecognized, planetary-scale cycle of auroral currents coupled to large-scale plasma convection. This cycle manifests as coherent spatial shifts of current peaks and convection centers across magnetic local time and latitude—first nightsideward and equatorward, then reversing toward the dayside and pole. Each cycle coincides with enhanced ionospheric convection directed toward the Sun. We interpret this cyclic behavior as global convection driven by alternating dominance of dayside and nightside magnetic reconnection. These findings uncover a reconnection-driven convection process that modulates intense substorm expansion on planetary scales. The results directly address a central objective of the SMILE mission: identifying what controls the substorm cycle.