Characterizing the Martin Luther King Event: Sun to Earth Evolution of the January 19, 2026 Geomagnetic Storm

We present a comprehensive Sun-to-Earth analysis of the extreme space weather event of 18-19 January 2026, hereafter the Martin Luther King (MLK) storm, which produced a severe (G4) geomagnetic storm and one of the most intense solar radiation storms of Solar Cycle 25. The geomagnetic disturbance was driven by a fast Earth-directed coronal mass ejection (CME) associated with an X1.9-class flare from NOAA Active Region 14341. We combine multi-viewpoint remote-sensing observations, in situ measurements from Solar Orbiter and Wind, and physics-based magnetohydrodynamic simulations using the CORHEL-CME framework to reconstruct the eruption from the low corona to 1 au and identify the mechanisms governing its geoeffectiveness. Observations and simulations indicate that the eruption originated from a right-handed magnetic flux rope embedded in a complex magnetic environment dominated by a neighboring positive-polarity coronal hole. The simulations reproduce the observed CME morphology, asymmetric expansion, magnetic fields measured at both spacecraft, and Dst index evolution. The main discrepancy is attributed to filament material not included in the model. Favorable magnetic polarity alignment between the CME and coronal-hole field minimized magnetic erosion through interchange reconnection, enabling rapid escape and preservation of the strong southward magnetic field that drove the geomagnetic storm. The prolonged recovery phase resulted from interaction between the trailing CME and a following high-speed stream from the same coronal hole. These results demonstrate that storm intensity was primarily established by the coronal magnetic configuration, whereas storm duration was controlled by the ambient solar wind.