Ordinary Meeting, 2009 January 28
Rotation, Magnetism and Mass Loss at Saturn
Dr Achilleos opened his talk with an overview of Saturn's magnetosphere, which he explained was the region of space around the planet where its magnetic field was strong enough to disrupt the flow of the solar wind; he added that in practice, this region extended out to around 20 times the planet's radius. He described the morphology of this region, explaining that it had a sharp boundary on Saturn's sunward side, termed a bow shock, where the outward streaming flow of the solar wind impacted upon it. He explained that the exact position of this shock front changed over time: at times of intense solar activity, Saturn's magnetosphere tended to become a little compressed and so the shock front moved inwards, but at times when the solar wind was weaker, its magnetosphere tended to expand outwards. By contrast, he explained that its boundary on the side of the planet facing away from the Sun was rather more weakly defined, because here there was much less pressure being exerted on it from the solar wind. Consequently, the magnetosphere stretched out into a long and variable comet-like tail called a magnetotail, which gradually tapered away as it moved away from the planet. As the magnetotail grew narrower and narrower, the solar wind particles surrounding it gradually reformed into a steady flow having passed around the planet.
The speaker added that although it was not generally possible for solar wind particles to penetrate into Saturn's magnetosphere – the combination of the electrical charge of the particles and the magnetic field of the planet made this impossible – there were two points on the magnetosphere's boundary immediately above the planet's north and south poles where the planet's magnetic field was relatively weak, and where charged solar wind particles could penetrate it; these were termed cusps. The speaker explained that it was the flow of particles through these 'holes' which gave rise to aurorae on Saturn as on the Earth, but that the detailed behaviour of the magnetospheres of the two planets differed considerably. In particular, the high density of ionised particles in Saturn's magnetosphere suggested that it must have its own internal source of such particles, meanwhile almost all of the charged particles in the Earth's magnetosphere seemed to have leaked in from the solar wind.
The speaker explained that recent data from the Cassini spacecraft had identified eruptions of water from vents on Saturn's icy moon Enceladus as the source of both the ice particles in Saturn's E ring, and also of the ionised particles which seemed to populate the plane of the ring and moon system. Current models suggested that at present Enceladus was ejecting a steady flow of around 10 to 100 kg of such material per second, but that the total mass of plasma in the ring plane was remaining roughly constant as a result of a corresponding leakage of material down Saturn's magnetotail, whence it was lost into the solar wind.
The speaker then turned to describe Jupiter, which he explained provided a good benchmark against which to compare the other gas giant planets. He explained that the Jovian magnetosphere also had its own source of plasma in the form of the planet's innermost moon, Io. This moon was well known for its extremely active volcanism, and, as in the case of Enceladus, it seemed to be venting some material into space. Perhaps unsurprisingly on account of its vigorous volcanism, Io was in fact very much more active as a source of plasma than Enceladus, venting around 1000 kg of sulphurous compounds to space per second. The speaker added that one of the most compelling demonstrations of this leakage of material were ultraviolet images of Jovian aurora taken by the Hubble Space Telescope (HST), which showed that, in addition to the solar-triggered auroral ovals around the poles, there was also a bright auroral spot on Jupiter's surface at a significantly lower latitude, which followed Io's orbit around the planet. It was apparent from this that electrical currents were flowing between Jupiter and Io through some conductive medium – undoubtedly a sea of plasma.
The speaker closed his talk with a discussion of his current research into the interaction of the magnetospheres of Jupiter and Saturn with the plasma that they contained. He explained that whilst the Cassini probe was returning a wealth of valuable information, the complex feedback mechanisms and internal regulation which seemed to be at work required some very detailed mathematical modelling.
Following the applause, the President introduced the evening's second speaker, Dr John Rogers, Director of the Association's Jupiter Section.