Interplanetary Causes and Impacts of the 2024 May Superstorm on the Geosphere: An Overview

The recent superstorm of 2024 May 10-11 is the second largest geomagnetic storm in the space age and the only one that has simultaneous interplanetary data (there were no interplanetary data for the 1989 March storm). The May superstorm was characterized by a sudden impulse (SI+) amplitude of +88 nT, followed by a three-step storm main phase development which had a total duration of ~9 hr. The cause of the first storm main phase with a peak SYM-H intensity of -183 nT was a fast forward interplanetary shock (magnetosonic Mach number Mms ~7.2) and an interplanetary sheath with southward interplanetary magnetic field component Bs of ~40 nT. The cause of the second storm main phase with a SYM-H intensity of -354 nT was a deepening of the sheath Bs to ~43 nT. A magnetosonic wave (Mms ~0.6) compressed the sheath to a high magnetic field strength of ~71 nT. Intensified Bs of ~48 nT was the cause of the third and most intense storm main phase with a SYM-H intensity of -518 nT. Three magnetic cloud events with Bs fields of ~25-40 nT occurred in the storm recovery phase, lengthening the recovery to ~2.8 days. At geosynchronous orbit, ~76 keV to ~1.5 MeV electrons exhibited ~1-3 orders of magnitude flux decreases following the shock/sheath impingement onto the magnetosphere. The cosmic ray decreases at Dome C, Antarctica (effective vertical cutoff rigidity <0.01 GV) and Oulu, Finland (rigidity ~0.8 GV) were ~17% and ~11%, respectively relative to quite time values. Strong ionospheric current flows resulted in extreme geomagnetically induced currents of ~30-40 A in the sub-auroral region. The storm period is characterized by strong polar region field-aligned currents, with ~10 times intensification during the main phase, and equatorward expansion down to ~50 deg geomagnetic (altitude-adjusted) latitude.