Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope

Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high-latitude magnetosphere near noon. The Cassini data indicate that the noon aurora lies in the boundary between open- and closed-field lines, where a layer of upward-directed field-aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the open-closed-field line boundary.