Ordinary Meeting, 2007 November 24


The STEREO Mission

Prof. Harrison opened by remarking that the Sun was unusual among astronomical objects in the degree to which its three-dimensional structure could be readily appreciated. Whereas the surfaces of the planets appeared largely two-dimensional, and our view of galaxies was even more restricted – their orientations did not appear to change from one century to the next – movies of the solar atmosphere revealed a writhing dynamic sea of super-imposed structures which had self-evident three-dimensional depth.

The diffuse uppermost layers of the solar atmosphere, its corona, were usually observed using a coronagraph – a kind of telescope designed especially for the purpose, which had an occulting plate obscuring the central solar disk to stop its light from entering the optics, allowing features which were normally lost in the Sun's glare to be resolved. Images from such telescopes revealed that the Sun's surface occasionally underwent massive explosions, blowing large pockets of gas outwards into the solar system; these so-called coronal mass ejections (CMEs) were the most powerful explosions to take place anywhere in the solar system.

Prof. Harrison explained that these events were not merely scientific curiosities. When they were directed towards the Earth, and our planet was consequently impacted a few days later by the ejected material, the result was a so-called geomagnetic storm. Often such storms were benign, and merely resulted in an auroral display triggered by the impact of high-velocity ionised solar wind particles with the upper atmosphere. Occasionally, however, they could be more harmful. Space-based electronic circuits could be damaged by exposure to ionised particles, and several satellites had been lost as a result of such damage. The varying magnetic fields associated with such storms could generate power surges in long-distance telecommunications and power lines, potentially triggering wide-scale electrical blackouts.

Turning to outline the recent history of solar observation, the speaker explained that there had been many space-based solar observatories active over the past decade. To summarise their work, he showed a view of an active region of the Sun's surface undergoing a CME, as seen at a wavelength of 195 Å (far-UV). This image indicated the region to be incredibly hot; emission at 195 Å was produced by highly ionised iron atoms which could only be formed at temperatures of several tens of millions of °C. For comparison, most of the Sun's surface was at a temperature of only a few thousand °C. The speaker also pointed out that the 195-Å emission was concentrated into loop-like structures and explained that this provided compelling evidence for strong magnetic fields in the vicinity of active regions on the Sun's surface.

Despite the immense amount that could be learnt from such images, all past observations had been made either from the ground or from satellites in Earth orbit, and here lay a significant weakness in them. They had all been made from the vicinity of the Earth, from which vantage point it was easiest to see those ejection events which were directed at 90° to the Earth–Sun line and which, being directed in the plane of the sky, appeared with good angular separation from the Sun. Any events which had been directed towards the Earth would most likely not have been recorded in these observations at all: they would have appeared to have been coming directly out of the solar disk, and would have been lost in its glare. However, it was these events which were arguably the most important to observe, because they were the progenitors of geomagnetic storms, and needed to be measured in order to better quantify how the magnitudes of events on the solar surface corresponded to measurable effects on Earth.

The STEREO mission had been designed to overcome this limitation. It consisted of two nearly identical spacecraft, both roughly following the Earth's orbit around the Sun. The first – labelled 'A' for 'Ahead' – lay slightly inside the Earth's orbit and moved a little faster than the Earth, drifting ahead of it at a rate of 22°/yr. The second – labelled 'B' for 'Behind' – lay slightly outside the Earth's orbit and moved slower than the Earth, drifting behind it at the same rate. An observer on the Sun would see the two satellites each moving away from the Earth at the same rate of 22°/yr, but in opposite directions.

The two spacecraft had been launched on 2006 October 25 aboard a single Delta II rocket, and had commenced scientific operations in 2007 April. At the time of the meeting, they had drifted to distances of around 20.5° ahead and behind the Earth respectively. They had already captured images of several CMEs travelling towards the Earth, and together they had been able to produce a three-dimensional mapping of each event, since they had yielded simultaneous images from two widely-separated vantage points. The power of STEREO in this regard would grow in coming months as the two spacecraft moved further apart.

Although the STEREO spacecraft were nominally solar physics observatories, the speaker added that the optimisation of their coronagraphs for picking out faint CMEs had also made them exceptionally good at imaging other diffuse astronomical objects. In the background of all STEREO images, the Milky Way and nearby galaxies stood out especially prominently. The view of Mercury was also rather appealing: the whole of its 88-day orbit was contained within the coronagraphs' field-of-view, and so unbroken movies of its orbital passage could be obtained. The speaker presented a puzzle to the audience: astute observers would notice that Mercury appeared to be circling the Sun's north pole in the wrong direction in these images – counter-clockwise. He went on to explain that Mercury appeared brightest when in full phase, on the far side of its orbit, and faintest when on the near side of its orbit. The eye was tricked into inverting the orbit by assuming that Mercury was brightest when nearest to the observer.

Prof. Harrison continued on this theme by showing some of the earliest images captured by STEREO. Prominent features in these included the highly-structured curving tail of Comet McNaught, which appeared rich with ray-like striations. The speaker explained that whilst the STEREO science team was composed exclusively of solar physicists and had not itself been able to analyse these images, all STEREO data was freely available on the web, and comet observers had taken an interest in this data. It had thus turned out that, rather surprisingly, the first scientific paper produced by STEREO's flight had been about Comet McNaught.

Looking ahead, cometary imaging would undoubtedly continue to form a substantial auxiliary part of STEREO's science programme. The LASCO coronagraph aboard its forerunner, the SOHO satellite, had discovered in excess of 1,000 new comets since starting scientific operations in 1996 – mostly so-called sungrazing comets, which brightened substantially as they made close approaches to the Sun's surface. STEREO would likewise be a powerful tool for comet discovery, with the advantage over SOHO of having binocular vision, allowing instantaneous three-dimensional positions to be obtained for comet nuclei, and much more rapid determinations of their orbital elements as a result.

The speaker added that most of SOHO's discoveries had been made by amateur comet hunters; the most dedicated among them had developed an exceptionally good sense for picking out faint objects from noise. They would be encouraged to continue their good work using STEREO, and the speaker envisaged that their dominance of the art would continue: the STEREO team would make all of their images publicly available on the web. Prof. Harrison welcomed any members who were interested in becoming involved to visit the STEREO website1.

Following the applause for Prof. Harrison's talk, the meeting broke for tea. The President then welcomed the afternoon's second speaker, Dr David Berghmans, from the Solar Influences Data analysis Center (SIDC) of the Royal Observatory of Belgium.




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