Authors: Chin-Chun Wu (Naval Research laboratory); Kan Liou ( SPACE SCIENCE CONSULTING, LLC); Brian E. Wood (Naval Research laboratory)
The 10-11 May 2024 grand geomagnetic storm (minimum Dst, Dstmin = -406 nT) is ranked as the 7th largest geomagnetic event on record. The solar source(s) of the May storm is still unsettled. It was associated with five halo coronal mass ejections (CMEs) erupting during May 8−9, but the Wind spacecraft only recorded an interplanetary shock at 16:32 UT on May 10 and a likely second one at 21:36 UT on the same day. In addition, the five CMEs were associated with moderate X-ray flares (M8.7−X2.2) and initial speeds of only ~530−1280 km/s, yet they produced a very intense geomagnetic storm. The present study employs a global magnetohydrodynamic simulation model [Wu et al. 2020] to address this puzzle. The CMEs, simulated with pressure pulses, were injected into the inner boundary (18 Rs) of simulation domains with speeds estimated from the white-light coronagraph images. An error range of 1.0−18.5 hours is found for the shock arrival time (SAT) when individual CMEs are simulated, and an error range of 0.2−6.0 hours for the SAT when the five CMEs are simulated together. The latter simulation reproduces the radial velocity profile in much better agreement with the Wind observations than any individual one does, suggesting that merging of the CMEs is responsible for the May geomagnetic storm.