Evidence for solar-wind triggering of substorm onset during the May 2024 superstorm: coordinated global observations and simulations

Substorms are often described by a loading—unloading cycle, where onset follows gradual loading of solar wind magnetic flux in the magnetosphere. Yet observations indicate that intense substorms can also be directly driven, though the underlying magnetospheric mechanism is unresolved. Here, combining global observations and simulations of the 10 May 2024 geomagnetic superstorm, we provide evidence that solar-wind—driven magnetospheric convection triggered an intense substorm. At 17:17 UT, a shock-compressed southward interplanetary magnetic field enhanced sunward convection and auroral currents, which rapidly extended to the nightside and initiated substorm expansion within six minutes. Simulations reproduce this response, revealing that dayside-driven convection of closed field lines depleted nightside flux and thinned the current sheet. This lowered the onset threshold and triggered substorm expansion with negligible flux loading. After 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.