A SECOND look at sunspot drawings from the 1700s has clarified a puzzling episode in the sun's history, and could lead to more accurate forecasts of dangerous solar outbursts.
The sun sometimes hurls clouds of plasma our way, which can fry satellites and knock out power grids on Earth. The outbursts are most common during solar maxima, when the dark blemishes of sunspots appear in greatest abundance on the sun.
Although there is an average of 11 years between solar maxima, predicting the exact timing and height of each peak is difficult as there is little historical data to plug into models. About two dozen solar cycles have occurred since reasonably complete records began. Now an analysis of historic sunspot drawings suggests that this patchy record had omitted a solar cycle from the late 1700s.
Sunspot numbers for this period were compiled by Swiss astronomer Rudolf Wolf in the 19th century, who based his tallies on drawings of the sun made by Austrian amateur astronomer Johann Staudecher. Wolf's numbers suggested there was a 15.5-year-long solar cycle between 1784 and 1799, the longest on record. However, astronomers have long questioned the reliability of these numbers, since Staudecher's observations from this period are sparse - he made just two drawings in the second half of 1793, for example. Even in the 19th century, some suspected that it might have actually been two short solar cycles.
To clear up the debate, a team led by Ilya Usoskin of the University of Oulu in Finland re-examined Staudecher's original drawings, currently held at the Astrophysical Institute in Potsdam, Germany. They also added information from a handful of sunspot drawings made by astronomer James Archibald Hamilton and his assistant at Armagh Observatory, Ireland, between 1795 and 1797.
The team looked at both the number of sunspots and their location on the sun - important as sunspots tend to appear 20 ° to 30 ° from the sun's equator when a new cycle begins, but gradually show up closer to the equator as the cycle progresses, rarely straying more than a few degrees from it by the cycle's end. "Wolf used [Staudecher's] drawings, but only to count the number of spots," says Usoskin. "He didn't use the location information."
The team's analysis suggests a new, weak solar cycle began around 1793 - the sunspots in Staudecher's drawings started appearing about 20 ° from the equator that year, and one of Hamilton's 1795 drawings shows a sunspot at 15 °. This suggests that in place of one unusually long solar cycle, there were actually two, lasting about nine and seven years, respectively (Astrophysical Journal Letters, DOI: 10.1088/0004-637x/700/2/l154).
"That would have an effect certainly on some of the [forecast] models," says Douglas Biesecker of the National Oceanic and Atmospheric Administration in Boulder, Colorado. For example, solar cycle statistics had previously suggested long cycles tend to be followed by weak ones. But the significance of that link now appears to have been exaggerated, since it was based partly on the observation that a very weak cycle followed what was thought to be a very long cycle ending in 1799.
Still, Biesecker cautions against drawing a firm conclusion about the extra solar cycle. "Personally, I find the observations too sparse, even with the new data, to be convinced," he says.