1 Amateur contributions to the study of the atmospheric dynamics of Jupiter and Saturn.

The past decade or so has witnessed a revolution in amateur studies of Jupiter and Saturn. This has come about through the new capability of fast frame-rate cameras, generically known as 'webcams', coupled with fast home computers and image-processing software, to record images of these planets at a sensitivity and resolution beyond that of the visual observer. These technologies have allowed amateurs to transcend the seeing limitations previously imposed by the atmosphere as well as the spectral range and colour sensitivity of the human eye. The result has been that images recorded today with modest amateur equipment far exceed anything achieved by professional telescopes 20 years ago, and rival the early space probe results.

These images provide those interested in analysing the long-term changes in the atmospheres of Jupiter and Saturn with a vast body of data. The strength of the amateur contribution to this field lies in the fact that we have long-term and continuous coverage from our global network of observers. Clearly the finer structure of what is going on in these atmospheres will only be elucidated by space probes, but they only provide short-term coverage, and even when present, they do not cover the whole planet. Imaging of Jupiter and Saturn by professional telescopes and the HST is rare (though very valuable when it occurs), so amateur observations provide the longer-term context for understanding what is seen in the space images. Particularly in the case of Jupiter, interesting and novel developments can occur very fast, showing processes on a scale which has not been observable before the webcam era. Then the amateur coverage by observers situated at every longitude comes into its own, as changes can be followed hour by hour using images from all over the world.

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Traditionally, feature positions on Jupiter and Saturn were determined by transit timings, but these required the feature to be on the central meridian at the time, and provided no information on latitude, which was difficult to determine accurately in the visual era. Transit timings are now superseded. Measuring feature positions from accurately-timed images has been rendered straightforward through the development of the JUPOS software by Grischa Hahn, a German amateur. (1) This software works essentially by allowing the user to overlay a calculated outline of the planet on to an image, to then click on features on the image, and read off latitude, and longitude in any of the standard rotation systems. JUPOS provides reliable measures of feature positions provided the features are not too close to the limb. Being no longer restricted to the central meridian, we can get far more measures of features than used to be the case; thus the quantity of data at our disposal and its accuracy is far greater.

The positional data is conventionally plotted on charts with a vertical time axis, known as drift charts. These are the basis of deductions about wind speeds and other patterns. In addition, JUPOS facilitates the production of global cylindrical projection maps, such as the one given here of Jupiter in 2008.

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Notable pieces of work published by the BAA Jupiter Section in recent years have included an analysis of the influence of the South Equatorial Disturbance (SED) on jet-stream speed, which proved for the first time that the speed of features on the 7[degrees]S jet are modulated by the visible presence or absence of the SED, (2) and a study of the changing size and rotation rate of the Great Red Spot over time, which proved that the spot is accelerating in its internal circulation and also shrinking. (3) Both these studies collated amateur work with professional telescope and space-probe results and resolved some issues which could not have been dealt with without the amateur data.

The same methods of study are applicable to Saturn, but the amateur data is much more scanty here as images showing spots and other measurable features are only obtained by a few observers and only from time to time. However, there is far more data on these features than there used to be, and bright spots on Saturn are now quite commonly imaged.

Reduction of images supplied to the Saturn Section over the past two apparitions has demonstrated the very wide range of wind speeds observable in Saturn's atmosphere. Over this period spots in the South Equatorial Belt Zone have demonstrated negative drift (faster movement) against System 3 longitude by 7[degrees] per day, while an unusual barge feature on the north edge of the South Equatorial Belt discovered just this year seems, from preliminary results, to have a positive drift of 11[degrees] with respect to System 1. These fast drift rates could never have been demonstrated in the visual era owing to the paucity of visual observations of such features. Work on analysing the images of Saturn being submitted to the BAA is at a relatively early stage, and this work should involve collating the results with Cassini observations, which are just now doing much to elucidate the structure of the winds on the planet.

http://www. davidarditti.co.uk

References

(1) See http://jupos.privat.t-online.de/index.htm

(2) Rogers J. H. & Mettig H-J., J. Brit. Astron. Assoc., 118(6), 326334 (2008)

(3) Rogers J. H., J. Brit. Astron. Assoc., 118(1), 14-20 (2008)