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

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.