Magnetospheres of the Outer Planets: Auroral X-Ray Emission

Auroral emission is an important diagnostic of magnetosphere-ionosphere (MI) coupling at Earth and other planets. The observed spatial and temporal morphology and the emission spectrum of an aurora can be used to estimate field-aligned currents linking ionosphere and magnetosphere. Jupiter's aurora is a powerful source of radio, infra-red, visible, ultraviolet (UV), and x-ray emission. Jovian x-ray emissions with a total power of about 1 GW have been observed by several x-ray observatories including Chandra and XMM-Newton. Most of the x-ray power is in soft x-ray emission from the polar caps, although some harder x-rays have been observed from the main auroral oval and attributed to electron bremsstrahlung emission. Ultraviolet aurorae have been observed from the other outer planets (Saturn, Uranus, and Neptune) but not x-ray aurorae, which has implications for MI coupling at these planets. The Jovian x-ray emission has been attributed to the precipitation of heavy ions that have been accelerated to several MeV energies. Particle measurements made by the JEDI and JADE instruments onboard the Juno spacecraft during some recent passages over Jupiter's polar caps might have for the first time identified the ions responsible for the x-ray aurora. Relativistic electrons are expected to be associated with energetic ion precipitation and Juno might have also observed these electrons. The source of the Megavolt field-aligned potentials needed to explain these ions has not yet been explained. Such large potentials are evidently not present at Saturn, given the lack of auroral x-ray emission from this planet. A review of recent work on these topics will be given.