Ordinary Meeting, 2004 March 31
The Winds Within Jupiter's Storms as Tracked by Amateurs around the World
Dr Rogers opened with a series of images of Jupiter by Damian Peach – known to many amateurs already for the unprecedented resolution achieved by his work. He was one of a number of amateurs who had now reached a critical threshold in the detail of their work, allowing them to monitor a range of features previously only visible in the images of the Voyager probe or in more recent times the Hubble Space Telescope. Modern photographic and CCD observations had the advantage over traditional visual work that not only could surface details be examined qualitatively, but also measured with great accuracy. It was of particular interest to trace the evolution of features, and their motions in the local and zonal winds. The most straightforward technique used was to compare images taken after an exact integer number of rotations of the planet, and pick out subtle changes and the motions of features by 'blinking' them. This had first been done with Voyager images, 25 years ago to the month.
However, Jupiter's ten-hour rotation period prevented any single observer from obtaining images after exactly one rotation period, except very close to opposition. Images at two or three rotation intervals were easier to obtain, but features tended to move too rapidly for such temporal resolution to be sufficient. For this reason, it was not only valuable to have observers achieving superbly high-resolution, but also to have such observers stationed at widely spread longitudes. The Jupiter Section now frequently received images from members on several continents – Europe, Australia, Eastern Asia and North America – and for the first time since Voyager, features could often be imaged after only a single rotation period.
The speaker went on to discuss a number of features of interest which could be traced by visual observers working with only medium-sized telescopes, and firstly, the Great Red Spot (GRS). Known to be a giant anticyclone, this could be observed to have a moderate degree of reddish colour at present. High-resolution imagery allowed narrow streaks to be tracked as they circulated around its rim, and their rotation speed could be determined. Secondly, the speaker discussed the white oval BA, the second largest anticyclone on the planet, and the only survivor of three great white ovals in the South Temperate Region which merged a few years ago.
Next, the speaker moved on to rifts in the Equatorial Belts, once again using a large number of images from Section members to illustrate the discussion. Covering first those in the South Equatorial Belt (SEB), it was explained that the rifts were bright turbulent streaks in cyclonic regions, known to be the expanding cloud-tops of giant thunderstorms. In the SEB, there were always such features following the GRS. Comparing images by 'blinking' revealed them to change rapidly within one or two rotation periods. In 2003 September, it became apparent that a separate large outbreak of rifts had occurred in the SEB during solar conjunction. In the North Equatorial Belt (NEB), similar streaks were also seen. Amateur images of such rifts revealed intricate turbulence and wind shear across the belt.
Moving on, the speaker discussed the familiar dark projections on the southern edge of the NEB, understood to be great waves in a rapid jet-stream. This year's amateur images had confirmed a discovery previously made using visual observations during the Voyager encounters, that these projections are sometimes intensified or disrupted upon passing a rifted region. This earlier discovery had been published in 1988.1
Finally, the speaker discussed a disturbance that had recently appeared in 2004 on the northern edge of the South Equatorial Belt. This was reminiscent of the 'South Equatorial Disturbance', a remarkable feature which was visible in the same region between 1999 and 2002. The new feature was possibly a revival of this old disturbance. Soon after passing the GRS, it manifested itself as a bright rift in the northern SEB, and white clouds could be traced emerging from the mouth of the rift and accelerating into the jet-stream.
As a general reflection, Dr Rogers remarked that the Section presently traced the evolution of surface features by selecting the best images, processing them appropriately, and examining them by eye. It seemed that this analysis could perhaps be substantially improved and automated. In this respect, however, the work of the Section was growing towards the complexity of a professional project, and it seemed likely that the development of such image processing tools would be worthy of a PhD project.
Dr Rogers closed by explaining what amateurs were now achieving in wavebands outside the visible. Methane filters, though still expensive, were now accessible to amateurs as well as professionals, and provided an insight into the chemistry of the clouds. Amongst other results, it was observed that Southern Equatorial Disturbances became bright in methane images around six months after erupting in the optical. By using CCDs with appropriate filters, amateurs were also now working in the near-infrared and ultraviolet, which probed cloud structures at varying depths within the atmosphere. Though time did not permit any discussion of the results of such work, many of the features previously mentioned were being studied.
Following the applause for Dr Rogers' well-illustrated talk, the President adjourned the meeting until the fifth Observers' Workshop, to be held on 2004 April 24 at Nottingham High School for Girls.
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Dominic Ford