Time-dependent MHD Simulation of the Lunar Limb Compression Caused by the Core-induced Magnetic Field

Using a time-dependent magnetohydrodynamic simulation, we demonstrated the lunar limb compression caused by the core-induced magnetic field. When a sudden change occurs in the upstream interplanetary magnetic field (IMF), an induced response is generated within the Moon’s conductive core. When the direction of the IMF jump is the same as the IMF direction after the jump, a diamagnetic current is driven inside the core, strengthening the magnetic field beneath the lunar surface but outside the core. In most dayside regions, this induced subsurface magnetic pressure is suppressed by the strong solar wind dynamic pressure. However, in regions near the terminator, where the solar wind dynamic pressure is lower, this induced subsurface magnetic pressure can compress the solar wind above the lunar surface, forming limb compression structures around the Moon. This work not only clarifies a new physical mechanism for the source of the lunar limb compression (driven by the core-induced magnetic field rather than the crustal magnetic field) but also ascertains the starting position of the limb compression, as well as its temporal variation during the IMF jump. These results can greatly improve our understanding of the Moon’s electromagnetic environment and have potential applications to constrain the conductive state of lunar core.