Comparative planetary infrared aurorae

行星极光，就像地球的北极光和南极光一样，是由高能带电粒子沿着行星的磁力线从周围的太空环境中进入高层大气形成的。然而，这些带电粒子来源的不同意味着不同行星的极光存在相当大的差异。在地球上，极光主要受磁场和太阳风相互作用的影响。木星和土星的质量比地球大得多，自转速度也比地球快得多，因此这些行星的磁场要强得多。这导致了连接地球和附近空间环境的电流，不受太阳风的影响。天王星的磁场与地球的更不一样，这也产生了与地球相连的电流。这些电流产生的极光与通常在地球上看到的极光有很大不同。我们的工作是利用地面和土星轨道上的望远镜，在红外波段观察气态巨行星的极光。这使我们不仅可以测量整个行星上极光的亮度变化，而且还可以显示行星上层大气中的离子是如何由于引起极光的电流而运动的。通过这些观测，我们已经能够证明，在气态巨行星上看到的极光实际上是由各种电流系统产生的，其中一些与地球上的太阳风有关，另一些则与当地空间环境的过程有关。然而，我们也观察到许多特征仍然无法解释这两种过程。在这个研究项目中，我们打算利用对气态巨极光的观测，通过更好地直接描述极光，来显著提高我们对导致这些极光形成的不同过程的理解。在过去的一年里，为了支持2007年2月的新视野号飞掠，对木星和土星的极光进行了一个重要的观测项目。这包括由哈勃太空望远镜拍摄的非常详细的紫外线（UV）图像，以及基于地面的红外（IR）活动，这将使我们能够第一次直接将精细图像与光谱数据进行直接比较。作为国际天王星秋分观测活动的一部分，我们还拍摄了有史以来第一张气体巨星上层大气的图像，纠正了透过地球大气层观察的扭曲效应。这使我们能够以前所未有的精度观察天王星的上层大气。除此之外，我们还对卡西尼号航天器上的VIMS仪器拍摄的土星极光的新图像进行了分析，这些图像是在环绕土星的轨道上拍摄的。这些卫星让我们看到了极光，向我们展示了一组全新的辐射，看起来与以前在土星或任何其他行星上看到的都不一样。我们打算分析这些新的数据财富，以了解不同行星极光之间的差异。通过展示太阳风在多大程度上控制着这些行星上的极光，并将其与我们对周围空间环境的了解进行比较，就有可能了解太阳风对行星上层大气的影响是如何随着行星结构的变化而变化的。其中一些效应在地球上太小而无法正确区分，但在其他行星上却强大得多，占主导地位。因此，我们的观测也将使我们更好地了解能量如何从太阳流向地球的细节。

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.