Observations of the magnetopause reconnection ion diffusion region with high-density O+ ions during the May 2024 superstorm

Based on high-resolution measurements from NASA’s Magnetospheric Multiscale mission (MMS), we present the first direct observation of an ion diffusion region (IDR) with high number density O⁺ ions within the dayside magnetopause reconnection during the May 2024 superstorm. The O⁺ ion density reaches a high value of ~3.3 cm⁻³. It helps study heavy-ion dynamics in dayside magnetopause reconnection. In the vicinity of IDR, O⁺ ions exhibit distinct acceleration to 300 km/s along the normal direction caused by the enhanced Hall electric field (E_Nmax≈ 80 mV/m). The distorted ion velocity distributions reveal the complex energization processes in the ion diffusion region. Crucially, these O⁺ ion dynamics can reduce the reconnection rate by ~ 25.3%, providing the result that heavy-ion dominance alters magnetopause reconnection physics during the superstorm. This study advances our understanding of magnetopause reconnection by demonstrating that storm-enhanced O⁺ populations modify the structure of diffusion regions, particle energization, and the reconnection rate.