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The geomagnetic storm of May 2024, which we call “Gannon storm”’ in memoriam of Jennifer L. Gannon, was such an intense event that the geomagnetic activity index Kp reached its upper limit value of 9 for the first time since October 2003. This paper introduces the new version (V. 3.0) of Hpo indices and evaluates geomagnetic activity during the Gannon storm. The Hpo indices, namely Hp30 and Hp60, are measures of geomagnetic activity, similar to the 3-hourly Kp index but with higher temporal resolutions of 30 min and 60 min, without an upper limit at 9, enabling them to assess geomagnetic activity during Kp=9 storms. In V. 3.0, an improved algorithm is implemented, and the indices are extended back to 1985. In the current dataset, the March 1989 storm is the strongest event by various measures based on the Hpo indices, while the Gannon storm ranks between 2nd to 5th.
Yamazaki et al. (2024) studied this question.
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