The Magnetosphere of Uranus

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Planetary Science. Please check back later for the full article. A magnetosphere is formed by the interaction between the magnetic field of a planet and the high-speed solar wind. Those planets with a magnetosphere have an intrinsic magnetic field such as Earth, Jupiter, and Saturn. Mars, especially, has no global magnetosphere, but evidence shows that a paleo-magnetosphere existed billions of years ago and was dampened then due to some reasons such as the change of internal activity. A magnetosphere is very important for the habitable environment of a planet because it provides the foremost and only protection for the planet from the energetic solar wind radiation. The majority of planets with a magnetosphere in our solar system have been studied for decades except for Uranus and Neptune, which are known as ice giant planets. This is because they are too far away from us (about 19 AU from the Sun), which means they are very difficult to directly detect. Compared to many other space detections to other planets, for example, Mars, Jupiter, Saturn and some of their moons, the only single fly-by measurement was made by the Voyager 2 spacecraft in the 1980s. The data it sent back to us showed that Uranus has a very unusual magnetosphere, which indicated that Uranus has a very large obliquity, which means its rotational axis is about 97.9° away from the north direction, with a relative rapid (17.24 hours) daily rotation. Besides, the magnetic axis is tilted 59° away from its rotational axis, and the magnetic dipole of the planet is off center, shifting 1/3 radii of Uranus toward its geometric south pole. Due to these special geometric and magnetic structures, Uranus has an extremely dynamic and asymmetric magnetosphere. Some remote observations revealed that the aurora emission from the surface of Uranus distributed at low latitude locations, which has rarely happened on other planets. Meanwhile, it indicated that solar wind plays a significant impact on the surface of Uranus even if the distance from the Sun is much farther than that of many other planets. A recent study, using numerical simulation, showed that Uranus has a “Switch-like” magnetosphere that allows its global magnetosphere to open and close periodically with the planetary rotation. In this article, we will review the historic studies of Uranus’s magnetosphere and then summarize the current progress in this field. Specifically, we will discuss the Voyager 2 spacecraft measurement, the ground-based and space-based observations such as Hubble Space Telescope, and the cutting-edge numerical simulations on it. We believe that the current progress provides important scientific context to boost future ice giant detection.