Solar‐Wind Triggering of Substorm Onset During the May 2024 Superstorm: Coordinated Global Observations and Simulations

Abstract Substorms are often described by a loading‐unloading cycle, where onset follows gradual accumulation of solar wind magnetic flux in the magnetosphere. Yet observations indicate that intense substorms can also be directly driven, though the underlying mechanism remains unresolved. For the first time, global observations strongly indicate that substorm triggering is linked to enhanced dayside‐driven convection and Region 1 FAC, supported by simulations. At 17:17UT during the May 2024 superstorm, a shock‐compressed southward interplanetary magnetic field enhanced sunward convection and auroral currents. These rapidly extended to the nightside, initiating substorm expansion within 6 min. Simulations reproduce this response, revealing that dayside‐driven convection of closed field lines depleted nightside flux and thinned the current sheet. This lowered onset threshold and triggered substorm expansion with negligible flux loading. Following onset, nightside flux loading became significant as a reconnection X‐line formed near 10 Earth radii, extended azimuthally, and supported a global substorm current wedge.