The geomagnetic and ionospheric effects of the May 2024 Mother’s Day superstorm over the Mediterranean sector

On 8 May 2024, the solar active region AR13664 started releasing a series of intense solar flares. Those of class X released between 9 and 11 May 2024 gave rise to a chain of fast Coronal Mass Ejections (CMEs) that proved to be geoeffective. The Storm Sudden Commencement (SSC) of the resulting geomagnetic storm was registered on 10 May 2024 and it is, to date, the strongest event since November 2003. The May 2024 storm, named hereafter Mother’s Day storm, peaked with a Dst of -412 nT and exhibited almost no substorm signatures in the recovery phase.

This study deals with the Space Weather effects that the Mother’s Day storm had on the Mediterranean sector, with a special focus on Italy. Istituto Nazionale di Geofisica e Vulcanologia (INGV) operational manages and monitors a dense network of GNSS receivers (including scintillation receivers), ionosondes and magnetometers in the Mediterranean area, which facilitated a detailed characterization of the storm effects.

Geomagnetic observatories located in Italy recorded a SSC with a rise time of only 3 minutes and a maximum variation of around 600 nT. The most notable ionospheric effect following the arrival of the disturbance was a significant decrease in plasma density on 11 May, resulting in a pronounced negative ionospheric storm registered on both foF2 and Total Electron Content (TEC). These negative ionospheric phases were ascribed to neutral composition changes and, specifically, to a decrease of the [O]/[N2] ratio. The IRI UP IONORING data-assimilation procedure, recently developed to nowcast the critical F2-layer frequency (foF2) over Italy, proved to be quite reliable during this extreme event. Relevant outcomes of the work relate to the Rate of TEC change Index (ROTI), which showed unusually high spatially distributed values on the nights of 10 and 11 May. The ROTI enhancements on 10 May might be linked to Stable Auroral Red (SAR) arcs and an equatorward displacement of the ionospheric trough. Differently, the ROTI enhancements on 11 May might be triggered by a joint action of low-latitude plasma pushed poleward by the pre-reversal enhancement (PRE) in the post-sunset hours and wave-like perturbations propagating from the north.

The storm attracted also the general public’s attention to Space Weather effects, including mid-latitude visible phenomena like SAR arcs. This presentation outlines also the monitoring report of the Space Weather Monitoring Group (SWMG) of the INGV Environment Department and its effort to timely disseminate information about this exceptional event.