Contribution of energetic O+ ion escape into the magnetosheath to rapid recovery of the May 2024 superstorm observed by MMS

The spatial distribution of energetic O+ ions in the dayside outer and inner magnetosphere during the early recovery phase of the May 2024 superstorm is observed by MMS satellites over a 10-minute interval. During this short interval, the solar wind dynamic pressure sharply decreases from 40 nPa to 10 nPa, leading to the magnetopause expanding sunward. O⁺-rich plasma is found in the dayside magnetosheath with high O⁺ ion number density, N_{max_O+} ~ 4.8 cm^-3, and the number density ratio of O⁺ to H⁺ is about 0.1. O⁺ ions in the magnetosheath have energies in the range 3-40 keV. O⁺ ions, escaping from the ring current, are perpendicularly accelerated by the intense electric field, E_x ~ 50 mV/m, at the dayside magnetopause with high reconnection outflow ~260 km/s into the magnetosheath. The escape of energetic O⁺ ions, with high number density and temperature, from the ring current into the magnetosheath results in the rapid decay of the ring current energy flux during the early recovery phase of this superstorm. This O⁺ ion escape can cause the SYM_H index to recover by 16 nT. Our study provides evidence for a high-energy O⁺ ion flux in the magnetosheath, which drives the efficient decay of the ring current and the rapid early recovery phase observed during the May 2024 superstorm.