Identification and comparative analysis of polar cusp region at Earth and the giant planets

Xu Yan; Yao Zhonghua; Ye Shengyi; Zhang Binzheng; Chen Junjie; Zeng Zhili; Sun Jiuwen

doi:10.19975/j.dqyxx.2025-048

Abstract：The polar cusp region serves as a critical gateway for magnetosheath and solar wind plasma to enter planetary magnetospheres,playing a fundamental role in shaping solar wind-magnetosphere interactions.The mor-phology,location,and dynamics of the cusp not only reflect underlying magnetopause reconnection processes but also reveal the diversity of magnetospheric structures across different planetary environments.This paper provides a systematic review and comparative analysis of recent advances in the study of cusp regions at Earth,Saturn,and Jupiter,with a focus on identification criteria,spatial distribution,and formation mechanisms. Studies revealed that,while the cusp regions of these three planets share key microphysical characteristics—such as the injection of magnetosheath-like low-energy electrons(～100 eV),ion energy dispersion from velocity filtering effects,and the presence of auroral hiss emissions—their large-scale spatial distributions and dynamic behaviors differ markedly.At Earth,cusp regions are typically centered near the noon sector,with recon-nection at the magnetopause primarily controlled by the north-south component of the interplanetary magnetic field(IMF).Under southward IMF,reconnection occurs at low latitudes,shifting the cusp equatorward;under north-ward IMF,reconnection shifts to high-latitude lobe regions,moving the cusp poleward. In contrast,Saturn's cusp exhibits periodic latitudinal oscillations modulated by the planet's～10.7-hour rota-tion,reflecting the influence of rapid rotation and internal plasma sources,particularly from Enceladus.These cusp motions are closely linked to planetary period oscillations within Saturn's magnetosphere,with amplitudes excee-ding 1° in latitude.This behavior highlights Saturn's rotation-dominated systems,and presents a dynamic picture distinct from Earth's. Jupiter,meanwhile,displays the most unconventional cusp configuration.Recent Juno observations have shown that its cusp can persist stably in the dusk sector(17-20 MLT),challenging the traditional Earth-based paradigm of a noon-centered cusp.This unique structure arises from Jupiter's complex three-dimensional magneto-spheric topology,shaped by its rapid～9.9-hour rotation and distinct solar wind environment.At Jupiter's orbit(～5 AU),the IMF spiral angle and clock angle approaches±90°,making the dawn-dusk(IMF By component)compon-ent dominant.Consequently,magnetopause reconnection is primarily governed by By of IMF.Jupiter's strong coro-tation electric field further drives a spiral-shaped open flux region,directly responsible for the cusp's duskward dis-placement. Through this cross-planetary comparison,the study highlights a fundamental transition in cusp behavior—from solar wind-dominated control(as at Earth)to increasing dominance by internal magnetospheric dy-namics(as at Jupiter).The findings demonstrate how the balance between planetary rotation and solar wind forcing systematically shapes cusp characteristics across giant planets.This comparative framework not only advances our understanding of magnetospheric processes within the solar system but also provides a foundation for interpreting magnetospheres at ice giants(Uranus and Neptune)and in exoplanetary systems.