Insight Into the Comparatively Weak Geo‐Effectiveness of the May 2024 ICME

Abstract The unusual ICME (interplanetary coronal mass ejection) of 10–11 May 2024, classified at the highest G5‐level in the American Geological Survey, produced the 2nd most intense geomagnetic storm in space age with revised intensity −606 nT. This study shows that some aspects of geo‐effectiveness of the May 2024 ICME are comparatively weak and explains the weakness for the first time. The May 2024 ICME caused only minor technological problems, whereas the ICMEs on 13 March 1989, 6 November 2001, and 30 October 2003 caused power outages. Prompt penetration electric field, F 3 layer, and ionosphere‐thermosphere storms were also weak on 10–11 May 2024. The differences seem due to the differences in the effective impact strength of the ICMEs, which involve the impulsive change ΔIEFy in the Y‐component of the interplanetary electric field (IEFy) at the ICME front and average IEFy in the ICME body. The May 2024 ICME is less geoeffective probably because it has large and rapid fluctuations at its front, so that, similar to other ICMEs, its ΔIEFy <4 mV/m. The highly geoeffective ICMEs causing even power outages have strong ΔIEFy >15 mV/m. The average IEFy in the body of the respective ICMEs is <20 and more than 20 mV/m.