Ordinary Meeting, 2007 November 24

 

The History of the Sunspot Index

Dr Berghmans explained that whereas the previous talk had described efforts to understand the Sun's present behaviour, his would look at how historical records, and in particular sunspot counts, could be used to infer how it had behaved over the past few hundred years. He explained that sunspots were cool regions on the face of the Sun, which appeared as dark blemishes. They were understood to be magnetic in origin, forming in places where the Sun's magnetic field had locally grown so strong that it disrupted the normal convectional flow of the solar atmosphere.

Sunspot activity showed variability on many timescales. Individual spots, typically similar in size to the Earth, were transient phenomena which lasted for only a few days. The statistical average number of sunspots visible followed a regular 11-year cycle, decreasing to almost zero between cycles. On still longer timescales, the number of sunspots visible in each cycle showed substantial variation, following consistent trends over periods of centuries. At the time of the meeting, the Sun was almost completely devoid of sunspots because sunspot cycle 23 – the 23rd 11-year cycle since the 1755-1766 cycle – had just drawn to a close, and the first spots of the new cycle were not expected to be seen until early 2008.

The speaker explained that trends in sunspot numbers were rather more than simply curiosities for solar observers; they affected a surprisingly broad range of sciences. Experience showed that they were correlated with many other aspects of the Sun's behaviour, and through the constant streaming of the solar wind out through the solar system, this in turn affected the Earth's atmosphere. To illustrate his point, the speaker showed a plot of the quantity of a radioactive isotope of carbon, 14C, found in ancient organic remains as a function of their age, and compared it to a plot of historical sunspot counts over the same period. It was apparent that the two were highly correlated: those organic samples which dated from periods of weak solar activity contained consistently more 14C than those dating from other periods. The speaker explained that the production of 14C in body tissue was usually triggered by cosmic ray impacts, and that the Earth's magnetosphere seemed to perform much better as a shield against cosmic rays when it was being bombarded by a strong solar wind.

As a second illustration, he added that the Little Ice Age (LIA) of the 18th century was widely linked by climatologists to the lull in solar activity which had been observed at around the same time – the Maunder Minimum. Their supposition was that the Sun had been fractionally less luminous during this period. Understanding such connections between solar activity and our climate would be a vital step in distinguishing between the topical issues of man-made versus natural global warming.

Turning to the history of sunspot observation, Dr Berghmans explained that it was not possible to date the earliest observations of sunspots. In misty conditions, when the Sun's disk was sufficiently dimmed by the atmosphere, it was possible to see large sunspots even with the naked eye, and so their existence seemed to have been recognised since prehistoric times. It was also impossible to name the first astronomer to have begun systematic counts, although it was clear that by the early 17th century, Galileo, Christoph Sheiner, Thomas Harriot, and David and Johannes Fabricius had all begun keeping records. However, these first efforts at counting sunspots had not lasted long. Within a few years, the Maunder Minimum had begun, sunspots had almost completely disappeared from the Sun's face, and inevitably, interest in them had waned – a situation which had prevailed right through until the early 18th century. The speaker stressed, however, that it would be unfounded to suggest that this so-called 'minimum' was merely the result of a lack of observations. Johannes Hevelius and John Flamsteed, amongst others, had occasionally recorded small sunspots during this period, and would surely also have recorded larger spots if they had been present.

In the early 18th century, after the end of the Maunder Minimum, professional interest in sunspots had remained rather limited: the perception had broadly been that they were inconsequential curiosities. It was therefore not surprising that the first person to have compiled consistent counts over a long enough period to notice that they followed an 11-year cycle had not been a professional, but an Austrian amateur, Samuel Heinrich Schwabe. Even Schwabe, an apothecary by trade, had not been especially interested in the spots themselves: he had been searching for transits of a hypothetical planet called Vulcan, which had been thought at the time to lie in a close orbit about the Sun, inside the orbit of Mercury. Vulcan's existence had been proposed to explain the strange non-elliptical orbit of Mercury, which seemed to defy Kepler's Laws, but which could be perfectly explained by the gravitational influence of another nearby planet. Searching for transits had seemed to Schwabe the easiest way to detect a planet which might lie so close to the sun that it was always hidden in twilight.

In light of this, Schwabe had compiled 17 years of sunspot observations by 1843, at which point he had accumulated enough data to note with confidence that sunspot numbers seemed to have changed markedly over his years of recording them, apparently following an 11-year cycle. Soon after, others had noted that this period corresponded rather precisely with the appearance of aurorae, and professional interest had begun to be pricked. Dr Bergmans added that Schwabe had, of course, been disappointed in his search for Vulcan; in fact Mercury's strange orbit had remained unexplained until the formulation of Einstein's General Theory of Relativity in 1915, although similar observations of non-ellipticity in Uranus' orbit had led to the discovery of Neptune in 1846.

The era of professional sunspot observation had dawned, and in 1849, Johannes Rudolph Wolf, a Swiss professional, had begun to collate historical sunspot observations into a systematic catalogue, with the hope of constructing from them a consistent measure of past sunspot activity. To this end, he had defined the Wolf Number, NW, as:

NW = k(10NG + NS)

where NG was the number of sunspot groups visible and NS was the number of large sunspots visible. The scaling constant k had been introduced to compensate for the inferiority of the telescopes used in historical observations as compared to Wolf's own, which would lead to their having shown fewer small sunspots. To continue his catalogue into the present, he had added his own observations, setting k = 1 for these. Seeking to minimise the methodological difference between new and old observations, he had, in addition to this rescaling, also opted not to include the very smallest spots – which would not have been seen by historical telescopes – in his own counts. The work of making these daily observations had been taken over by the Zürich Observatory upon its foundation in 1864, when Wolf had been appointed its inaugural Director.

Reflecting on Wolf's work, the speaker explained that whilst the Wolf Number seemed at first sight to be an entirely arbitrary formula, it was possible to see how it had been motivated. Typically, NS ≈ 10NG, and so the formula gave approximately equal weight to numbers of individual sunspots versus numbers of sunspot groups as two measures of solar activity. He added that it was Wolf's insight in devising an objective counting scheme which was retrospectively extensible right back to the earliest systematic counts which gave the modern International Relative Sunspot Number, Ri, still based upon Wolf's metric, its unique value. It was the only measure of solar activity which could be traced back consistently to the early 18th century.

Since Wolf's time, the counting scheme had undergone various modernisations. After Wolf's retirement as Director of the Zürich Observatory in 1882, his successor, Alfred Wolfer, had decided that future counts should include all visible spots. The exclusion of small spots had been, in Wolfer's view, subjective, as there had been no well-defined threshold size. To highlight this break away from Wolf's methodology, the new counts had been renamed the Zürich Sunspot Number, NZ, but in order to place them on a consistent scale with the Wolf Number, the scaling factor in Wolf's metric had been set to k = 0.8 for all new observations, accounting for the inclusion of more sunspots in the new counts.

Wolfer had also built up a network of around 30 stations, spread across Europe and Asia, each making their own counts to supplement those made from the single Zürich observatory used by Wolf. Though this had been a labour-intensive development – counts now needed to be collated from many widely-spread observatories – it had meant that observations could be made even during periods of poor seeing, cloud cover, or even night-time in Zürich. This step was now understood to have halved the uncertainty in the resulting sunspot counts.

After this time, the Zürich Sunspot Number had continued to be compiled in essentially the same way for almost a century, until 1980, when the Zürich Observatory had controversially decided to abandon sunspot counting altogether, under pressure from its parent institution, Zürich's Federal Institute of Technology (ETH). There had been numerous reasons for this decision. Sunspot counting was costly, labour-intensive work, and whilst it was valuable for posterity, funding councils tended to favour more glamorous or groundbreaking projects. In addition, the site of the Zürich Observatory was no longer ideal for the purpose: what had once been the outskirts of Zürich was now surrounded by the city, and its sky visibility and seeing conditions had deteriorated.

But perhaps most importantly, several rival measures of solar activity had grown into widespread use by 1980. During World War II, the American military had started their own sunspot count – the American Relative Sunspot Number, Ra – realising the value of sunspot counts for forecasting radio propagation conditions, but lacking easy communications links with Switzerland. Ever since 1945, the American Association of Variable Star Observers (AAVSO) had continued to compile these counts. Another easily obtainable estimate of solar activity came in the form of the Sun's 10.7 cm radio flux, which correlated well with NZ.

After much debate, it had been agreed that the Zürich Observatory would cease compiling the Zürich Sunspot Number, but would strive to find another institution to take over the work, which was willing to retain the old methodology as closely as possible. It was agreed that the name of the sunspot count would, from 1980, be changed to the International Relative Sunspot Number, Ri, to reflect its geographical move. The Royal Observatory of Belgium in Brussels had agreed to become the sunspot count's new home, and over the course of 1980-1, the Sunspot Index Data Center (SIDC) had been formed there, later to be renamed in 2000 the Solar Influences Data analysis Center, to reflect its widening rôle in monitoring space weather.

Whilst remaining loyal to the methodology formerly used at Zürich, the SIDC had sought to modernise its procedures. The handling of data reports was now largely computerised, and this had allowed for an expansion of the number of its stations from 30 to a present figure of nearly 80. These were still heavily concentrated in Europe and Asia – at a recent count, 71% had been in Europe – but there were now a handful of stations in the Americas.

The speaker closed with an invitation for members to help the SIDC by considering becoming a station in their network. The SIDC's newly computerised analysis system could take data from new stations with no extra work, and amateur astronomers were warmly encouraged to get involved. More details could be found on the centre's website2.

Following the applause for Dr Berghmans' talk, the President invited the Director of the Association's Solar Section, Lyn Smith, to give an update on the section's activity.

Ashburn

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