Abstract Earth’s main magnetic field forms the magnetosphere, shielding the planet from solar wind plasma. Southward interplanetary magnetic fields erode the magnetosphere, a process known as magnetopause erosion, allowing solar wind penetration close to the Earth. While magnetic reconnection is considered a key driver, the mechanism during super geomagnetic storms remains unclear due to limited in situ observations. Here, we report a magnetopause crossing by Arase at 4.96 RE during the May 2024 superstorm, one of the innermost magnetopause crossings ever detected by magnetospheric spacecraft. We observe an anomalous and global reduction in magnetic field intensity extending from the inner magnetosphere to the magnetopause, suggesting global weakening by magnetospheric currents. This weakened magnetic field is responsible for the enhanced magnetopause erosion.