Voyager 2 constraints on plasmoid‐based transport at Uranus.

A magnetosphere controls a planet's evolution by suppressing or enhancing atmospheric loss to space. In situ measurements of Uranus' magnetosphere from the Voyager 2 flyby in 1986 provide the only direct evidence of magnetospheric transport processes responsible for this atmospheric escape at Uranus. Analysis of high‐resolution Voyager 2 magnetic field data in Uranus' magnetotail reveals the presence of a loop‐like plasmoid filled with planetary plasma traveling away from the planet. This first plasmoid observation in an Ice Giant magnetosphere elucidates that (1) both internal and external forces play a role in Uranus' magnetospheric dynamics, (2) magnetic reconnection contributes to the circulation of plasma and magnetic flux at Uranus, and (3) plasmoids may be a dominant transport mechanism for mass loss through Uranus' magnetotail. Plain Language Summary: Uranus possesses an intrinsic magnetic field that encircles the planet and influences the local space environment. The solar wind plasma, made up of charged particles, flows away from the Sun and interacts with Uranus' magnetic field to form what is called a "planetary magnetosphere." By understanding dynamics of the magnetosphere, we are able to learn how changes in the Sun can impact the planet's space environment but also how magnetic fields and plasma are circulated throughout the system. In this work, we analyze data from the Voyager 2 spacecraft during the Uranus flyby in 1986. The data revealed a helical bundle of magnetic flux containing planetary plasma, known as a "plasmoid," in the tail of the magnetosphere. This first observation of a plasmoid in an Ice Giant magnetosphere elucidates processes that occur in the magnetosphere of Uranus and suggests that plasmoids may play a large role in transporting plasma. Key Points: Observations of a tailward moving plasmoid confirm that magnetotail reconnection contributes to magnetic flux circulation at UranusThe plasmoid's loop‐like structure, with a decrease in field magnitude, suggests that internal forces play a role in mass transportEstimates indicate that plasmoids may serve as a major transport mechanism for mass loss through the Uranus magnetotail [ABSTRACT FROM AUTHOR]