How the CME on 2023 April 21 Triggered the First Severe Geomagnetic Storm of Solar Cycle 25

The first severe (G4) geomagnetic storm of Solar Cycle 25 occurred on 2023 April 23–24, following the arrival of a coronal mass ejection (CME) on April 23. The characteristics of this CME, measured from coronagraphs (speed and mass), did not indicate that it would trigger such an intense geomagnetic storm. In this work, our aim is to understand why this CME led to such a geoeffective outcome. Our analysis spans from the source active region to the corona and inner heliosphere through 1 au using multiwavelength, multi-viewpoint remote sensing observations and in situ data. We find that rotation and possibly deflection of the CME resulted in an axial magnetic field nearly parallel to the ecliptic plane during the Earth encounter, which might explain the storm’s severity. Additionally, we find that imaging away from the Sun–Earth line is crucial in hindcasting the CME time of arrival at Earth. The position (0.39 au) and detailed images from the SoloHI telescope on board the Solar Orbiter mission, in combination with SOHO and STEREO images, helped decisively with the three-dimensional reconstruction of the CME.