Transition regions in the jovian and kronian magnetodiscs and star-disc systems

Jupiter and Saturn are the largest, most massive planets in the solar system, and also the most rapidly rotating. A day on either planet is only lasts for around ten hours. Jupiter's magnetic field is the most intense in the solar system and is second only to the Sun. Like all magnetised planets, this internal magnetic field caves-out a cavity or 'bubble' in the solar wind known as the 'magnetosphere'. As a consequence of its large magnetic moment (20000 times that of the Earth) and its distant location in the solar system, Jupiter also has the largest magnetosphere. If the jovian magnetosphere were visible from Earth with the naked eye it would appear to be the same size as the Moon in the sky. In fact, Jupiter's magnetosphere is the largest physical structure in the solar system. Whilst Saturn's magnetic field is not as strong as Jupiter's, its location at 9.5 astronomical units from the Sun also ensures it also has a large magnetosphere, with a volume ten million times larger than Earth's magnetosphere. Planetary magnetospheres are natural plasma laboratories which can be used for in situ studies of fundamental physical processes in the Universe such as the flow of momentum and the role of magnetic phenomena in the flow of ionised gas (plasma) and energy through the Universe. The magnetospheres of Jupiter and Saturn contain disc-like magnetic field structures called magnetodiscs which are sites of plasma and energy storage, and are very important in the movement of plasma. These magnetic fields are like elastic bands which can be stretched. These magnetodiscs are like elastic bands that have been highly stretched and we intend to explore this stretching. In this proposal we seek to study two particular aspects of the jovian and kronian magnetodiscs: the effect of the solar wind on the formation and outer extents of the magnetodisc, and the transition region near the planet where the magnetic field deforms from a dipolar to a disc-like structure. Essentially we are interested in how much force we need to put on the elastic band before it no longer looks like a circular band, and what other factors can affect how much the elastic band is stretched. To do this we will use observations of Saturn gathered by the Cassini spacecraft which has been orbiting Saturn since 2004, observations of Jupiter collected by the Galileo spacecraft which explored Jupiter between 1995 and 2003, and measurements made of both planets by the Voyager 1 and 2 spacecraft from the early 1980's. These observations will be supplemented with computer calculations of the forces in these magnetodiscs. The third part of the proposal concerns objects known as Accretion discs. These are regions around compact stars, such as neutron stars, where the gravity of the star is sucking material out of a neighbouring object, which may be another star. This matter then circles the compact star forming an accretion disc, much like water circling a plug-hole. Transition regions also occur around accretion discs and they are crucial in controlling how the matter flows onto the star from the accretion disc (how it goes down the plug-hole). This flow of mass can cause the star to spin faster (spin 'up') or slower (spin 'down'); whether it spins up or down is controlled by the flow of mass from the disc to the star through the transition region. The transition region is the key in this process however it is a very poorly understood region. Since these accretion discs are so far away from Earth, spacecraft cannot be sent to study them in situ and so we do not have in situ observations to guide our understanding. In the final part of this proposal we will take knowledge gained from the in situ study of the transition regions at Jupiter and Saturn and use it to identify the correct physics involved in star-disc magnetic coupling. This will allow us to improve our understanding of mass and momentum transport in a star-disc system.