The Role of Planetary Rotation in Polar Cusp Localization

Juno observations have revealed that Jupiter's polar cusps are displaced toward dusk and even the nightside, contradicting the Earth-derived paradigm that is confined to narrower ranges near noon. These findings underscore the overlooked influence of planetary rotation on magnetospheric dynamics, exposing a critical gap in current theoretical frameworks. Using global magnetohydrodynamic (MHD) simulations, we show that high-altitude cusps reside on the dusk hemisphere at both Saturn and Jupiter, consistent with Juno data at Jupiter and distinct from Earth's near-noon-centered cusps. Controlled experiments with artificially increased Earth rotation rates demonstrate a systematic duskward shift of cusp local time, transitioning from noon to afternoon (Saturn-like) and dusk (Jupiter-like) sectors. We propose the corotation-to-convection speed ratio as a unifying parameter that captures this duskward displacement across different planets. This unified framework explains gas-giant magnetospheres, offering potential applications to other rapidly rotating systems, such as ice giants, early Earth, and exoplanets.