Electrodynamic and Ionospheric Puzzles of the 10–11 May 2024 Geomagnetic Superstorm

Abstract On 10 May 2024, a powerful coronal mass ejection arrived at Earth at 17:05UT and caused a major geomagnetic storm. With the minimum SYM‐H excursion of −497 nT (5‐min data), this storm is the largest geomagnetic disturbance since March 1989, and can be categorized as a superstorm. In this work, by using ground‐based and space‐borne instruments, we focus on unusual aspects of the electrodynamic and ionospheric response to the May 2024 storm at middle and low latitudes. Between the storm onset at 17:05UT, and until ∼19:40UT, we observed signatures of strong unshielded prompt penetration electric fields (PPEF), which caused an increase of the equatorial ExB drifts up to 95 m/s. This led to the occurrence of a strong ionospheric super‐fountain effect. The local pre‐noon sector was the first to respond to the PPEF, with a very rapid increase of the ionization and the EIA development in the local morning sector. Whereas, in the afternoon‐evening sector the ionosphere responded with ∼2 hr of delay, and the response continued to intensify even after the equatorial ExB drifts had dropped to undisturbed values. The development of such a powerful super‐fountain effect without or with little electrodynamic forcing is difficult to explain, but it could be due to storm‐time meridional and zonal thermospheric winds. During the early recovery phase of the storm, a second positive ionospheric storm occurred over the Australian‐West Pacific region in the local late afternoon to pre‐midnight sector, driven by another ionospheric uplift associated with the occurrence of smaller‐amplitude equatorial upward ExB drifts.