The unusual radiation belts of the ice giant planets

2022年4月，美国公布了最新的行星科学与天体生物学十年调查结果，建议将航天器送入天王星轨道的任务应成为美国国家航空航天局未来十年的首要任务。这次任务将彻底改变我们对这类“冰巨星”的认识，也有造访另一个冰巨星--海王星的计划。这个项目建立在我们在定义地平线上这些开创性任务的科学目标方面的领导地位。目的是评估这些冰巨人内部产生的不寻常的磁场是否可以解释它们同样不寻常的“辐射带”，所有这些都是在20世纪80年代旅行者2号飞越这些行星时观察到的。辐射带是磁化行星周围的空间区域，高能带电粒子被困在周围的磁场结构中。我们提出，与地球等其他磁化行星相比，冰巨型辐射带的一些不同寻常的方面可能是因为它们的磁场更复杂。这项工作将涉及到对带电粒子如何通过两个行星磁场结构的简单数值模拟，以最终确定观察到的辐射带特性可以在多大程度上由额外的磁场复杂性来解释。随着该项目的推进，将有可能极大地扩展数值模拟工作，比较太阳系所有的磁化行星，修改现有的理论以应对冰巨星，甚至扩展到太阳系之外，对多极磁场支持的辐射带进行更全面的评估。

In April of 2022 the results of the latest Planetary Science & Astrobiology Decadal Survey were announced in the USA, recommending that a mission to put a spacecraft in orbit around the planet Uranus should be NASA's highest priority flagship over the next decade. This mission will revolutionise our understanding of such "ice giants", and there are also plans to visit the other ice giant, Neptune. This project builds on our leadership in defining the science goals of these pioneering missions on the horizon.The aim is to assess whether the unusual magnetic fields generated inside these ice giants could explain their equally unusual "radiation belts", all observed by Voyager 2 when it flew by these planets in the 1980s. Radiation belts are regions of space around a magnetised planet where high-energy charged particles are trapped within the surrounding magnetic field structure. We have proposed that some of the unusual aspects of the ice giant radiation belts could be due to their more complex magnetic fields, compared to other magnetised planets like the Earth.The work will involve simple numerical modelling of how charged "test" particles move through the two planetary magnetic field structures, to ultimately establish the extent to which observed radiation belt properties can be explained by the additional field complexity. As the project moves forwards there will be potential to greatly expand the numerical modelling effort, to compare all the Solar System's magnetised planets, to modify existing theory to the ice giants, and even extend beyond the Solar System with a more general assessment of radiation belts supported by multipolar magnetic fields.