Comparative planetary infrared aurorae

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Planetary aurorae, like the Earth's aurora borealis and aurora australis, are formed by energetic charged particles streaming along the planet's magnetic field lines into the upper atmosphere from the surrounding space environment. However, differences in the source of these charged particles means there are considerable variations between the aurora of different planets. On Earth's, the aurorae are dominated by the effect of interactions between the magnetic field and the solar wind. Jupiter and Saturn are far more massive and also rotate more quickly than the Earth and as a result the magnetic fields of these planets are much stronger. This results in currents that connect between the planet and the nearby space environment, shielded from the solar wind. Uranus has a magnetic field that is even more different to the Earth's and this too produces currents that connect with the planet. These currents produce aurora that differ significantly from those usually seen on Earth. Our work uses telescopes on the ground and in orbit around Saturn to observe the aurorae of the gas giants at infrared wavelengths. This allows us to not only measure the varying brightness of the aurora across the planet, but also show how the ions in the upper atmosphere of the planets move as a result of the currents which cause the aurora. Using these observations we have been able to show that the aurorae seen on the gas giants are, in fact, produced by a variety of current systems, some connected to the solar wind as at Earth and others with processes within the local space environment. However, we have also observed many features that remain unexplained by either process. In this programme of research we intend to use observations of the gas giant aurorae to significantly improve our understanding of the different processes that lead to the formation of these aurora by better characterising them directly. In the past year, a significant programme of observations were made of the aurorae of both Jupiter and Saturn, in support of the New Horizons flyby in February 2007. This consist of highly detailed ultraviolet (UV) images taken by the Hubble Space Telescope, together with a ground-based infrared (IR) campaign, which will allow us to directly compare the fine-scale images directly with the spectral data for the first time. As a part of the international Uranus at Equinox observing campaign, we also took the first-ever image of the upper atmosphere of a gas giant, correcting for the distortional effect of looking through the Earth's atmosphere. This allows us to observe the upper atmosphere of Uranus at an unprecedented accuracy. In addition to this, we have led the anaylsis of new images of Saturn's aurora taken by the VIMS instrument aboard the Cassini spacecraft, in orbit around around the planet. These have given us views of the aurora, presenting us with a whole new set of emission that looks unlike anything seen before on Saturn, or any other planet. We intend to analyse this new wealth of data to understand the differences between the aurorae of different planets. In showing the extent to which the solar wind controls the aurora on each of these planets and comparing this with what we know about the surrounding space environment, it will be possible to understand the way solar wind influences on a planet's upper atmospheres change with the planetary configuration. Some of these effects, that are too small to properly distinguish on the Earth, are far more powerful and dominant on other planets Thus, our observations will also allow us to better understand the details of how energy flows from the Sun to the Earth.