via New Scientist.
The Sun undergoes a solar cycle on average once every 11 years, but this number can vary by as much as four years in either direction. During the solar cycle, the differential rotation of the Sun’s body – the fact that the equator spins on the solar axis faster than the poles – means that the solar magnetic field, which is embedded in the plasma that makes up the Sun, gets twisted. Over the course of the cycle, the twisting gets worse and worse, with loops of magnetic field sticking out of the body of the Sun, until the entire thing gives up and rearranges itself, turning north into south. Then it starts all over again.
The loops of magnetic field sticking out of the body show up as sunspots. Where each foot of the loop sticks into the surface of the Sun, the plasma is contained by the high magnetic field and forced to gyrate less – a phenomenon known as magnetic cooling. As a result, there’s a slightly lower amount of light sent out, making the sunspot appear darker than the rest of the solar disc.
Sunspots first appear midway between the equator and the poles in either hemisphere. Over the course of a cycle, new sunspots appear closer to the equator itself, each sunspot pair fading away after a few days. Graphs of sunspot locations with time are known as butterfly graphs for this reason, they seem to trace the outer edges of a butterflies wings.
Swiss astronomer Rudolf Wolf made it his business to count the number of sunspots occurring over time. This Wolf number gave the frequency of sunspots, but not their location. As there are more sunspots near the end of a cycle than there are at the beginning, Wolf used his numbers to determine when solar cycles began and ended. Compiling historical data, he determined that there was an extra long cycle between 1784 and 1799, lasting 15.5 years.
Even at the time this was published in the 19th century, there were doubts that there was such a long cycle then, and suggestions abounded that there were two shorter cycles in that period, but with nothing more definitive than Wolf’s frequencies of sunspots to go on, there wasn’t hard evidence either way.
However, Wolf got his numbers from somewhere and a team led by Ilya Usoskin of the University of Oulu in Finland went back to the original data – sunspot drawings by the Austrian Johann Staudecher. To these, the team added further drawings from the same period by James Archibald Hamilton and his assistant at Armargh Observatory. Now armed with both numbers and locations, a butterfly diagram could be constructed for the period.
Hamilton only drew sunspots between 1795 and 1797 and Staudecher made relatively few drawings in the period of interest, both of which increases statistical sampling errors from the new data. But Hamilton’s 1795 sunspots start of at around 15 degrees solar latitude, fairly high up for a late cycle sunspot. Staudecher’s sunspots suddenly started appearing at twenty degrees in 1793, not as high as they could be at the start of a fresh cycle, but certainly higher than they were. So it is possible that two solar cycles happened in the 1784-1799 period, one lasting nine years, the other lasting seven. But it is also possible that this is merely an exaggerated wobble that more data points might have ironed out.
The search goes on.