Auroral-oval activity during the intense magnetic storm of May 2024 – Overcoming the underestimation of the AE index

We analyzed the activity in the auroral oval during the intense magnetic storm on 10 and 11 May 2024, as seen in records from magnetic observatories available in the INTERMAGNET database. During intense geomagnetic disturbances, the auroral oval expanded into sub-auroral regions with lower geomagnetic latitudes than those of the observatories standardly used to calculate the indices AU , AL , and AE . We calculated adjusted versions of the auroral electrojet indices according to the standard methodology, except that we considered data from observatories located under the auroral oval during the magnetic storm. We found that the standard AE index significantly underestimated the auroral-oval activity from midnight to early morning UT on 11 May 2024. We also compared the standard AE index and our adjusted index with the modified index of other authors who proposed a correction to the AE index through an empirical model based on solar wind parameters. We pointed out the limitation of using such an empirically calculated modified auroral electrojet index for this particular intense geomagnetic event. Another auroral activity index SME showed a similar profile during the disturbed period as AE a but reached higher values. The AE a index might underestimate the auroral activity due to the saturation of the Kp index, which we used for estimating the position of the auroral oval boundary. We indicated the direction of further research focused on dealing with the consequences of Kp index saturation on our method, with the help of an open-ended index such as Hp 30 or Hp 60 .