In the mid-1800s astronomers discovered from hundreds of sunspot sightings that, when they tabulated and graphed them, their numbers increased and decreased over time in a repeatable cycle. These extremes represent the amplitude of the cycle. We now call this the solar activity cycle or the sunspot cycle. Heinrich Schwabe published his first measurements of the sunspot cycles between 1826-1843 in 1843. He had counted these spots every possible clear day, and found two peaks in 1828 and 1837, with minima in 1833 and 1843. Subsequent historical studies by astronomers such as Rudolph Wolf uncovered recognizable cycles since 1700. A curious absence of cyclic behavior was noted by Gustav Spörer to the dawn of telescopic observations in ca 1610. This period is known as the Maunder Minimum, which coincides with an unusually cool period in European history called the Little Ice Age. Further attempts at developing an historical record for sunspot cycles have yielded suggestions for cycles between 1600-1650, making the Maunder Minimum a verified 70-year absence in sunspot activity. Astronomers today count the sunspot cycles beginning with Cycle 1 whose maximum occurred in 1760 which is the first year follwing the MAunder Minimum when a sunspot cycle had a distinch beginning and ending. We have just completed Cycle 23 (1996-2007) and wil lsoo nstart Cycle 24 (2008-2019).
During the last 200 years, the time between years of maximum activity, which is called the period of the cycle, has been about 11 years, but sunspot cycles can be as short as 9 or as long as 15 years. During sunspot minimum conditions, such as the year 1996, astronomers counted fewer than 5 sunspots on the surface of the Sun at any one time. During sunspot maximum conditions, as many as 250 could be seen. On September 20, 2000 one very large sunspot group could be seen with the naked eye with the proper safety precautions. (You should never look directly at the Sun without proper shielding to avoid eye damage!).
Ancient Chinese astronomers also kept track of naked-eye sunspots 4000 years ago, and that's how we know that sunspots have been a common feature of the Sun for millennia. In fact, solar activity is mirrored in the rise and fall of Carbon-14 isotopes found in biological matter. As solar activity increases, the solar magnetic field becomes more distended, reducing the inflow of cosmioc rays into the atmosphere. This reduces the production of atmospheric carbon-14 which is ultimately ingested by biological systems. By looking at carbon-14 in tree rings, the rise and fall of sunspot activity is detectable in the carbon-14 record, allowing the recovery of the solar activity record thousands of years into the past.
Scientists don't fully understand the connection between the sunspot cycle and weather conditions here on Earth, but there does seem to be something going on between them. Could it be that the sunspots block out light from the Sun and make the Earth cooler as the mini-Ice Age example might suggest? Curiously, if you were to measure how bright the Sun is during sunspot maximum when it has the most spots, it is actually slightly brighter, not dimmer! This is because the magnetic fields in the sunspots are so stiff that they prevent the gas from convecting and transporting energy from the lower layers to the surface. Energy that would have flowed out of the dark spots is actually re-directed around them like a broken car blocking traffic on a busy highway.
Predicting Sunspot Cycles
Can the details of the sunspot cycle be predicted? It seems so. In March, 2006 solar physicist Mausumi Dikpati and her colleagues at the National Center for Atmospheric Research in Boulder Colorado announced the results from their 'Predictive Flux-transport Dynamo Model '. The newly developed model simulated the strength of the past eight solar cycles with more than 98% accuracy. The forecasts are generated, in part, by tracking the subsurface movements of the sunspot remnants of the previous two solar cycles. The actual and predicted cycles are shown in the figure below.
It appears that the next sunspot cycle, Cycle 24, will be stronger than Cycle-23 now ending. This implies that the 21,000 solar flares and 13,000 coronal mass ejections we have just experienced during Cycle-23 may be substantially exceeded during the 2007-2019 time period. This will have even more serious implications for satellite systems, astronauts working in space, the global electric power grid, and even passengers flying in jet aircraft. All of these systems have known vulnerabilities to solar storms and space weather.
During the annual Space Weather Workshop held in Boulder, CO in May, 2008, the Solar Cycle 24 Prediction Panel released (June 27, 2008) an update to the prediction for the next solar cycle. In short, the update is that the panel has not yet made any changes to the prediction issued in April, 2007. (See Consensus Statement below.)
The panel expects solar minimum to occur in March, 2008. The panel expects the solar cycle to reach a peak sunspot number of 140 in October, 2011 or a peak of 90 in August, 2012. 
CONSENSUS STATEMENT OF THE SOLAR CYCLE 24 PREDICTION PANEL
March 20, 2007
The Solar Cycle 24 Prediction Panel anticipates the solar minimum marking the onset of Cycle 24 will occur in March, 2008 (±6 months). The panel reached this conclusion due to the absence of expected signatures of minimum-like conditions on the Sun at the time of the panel meeting in March, 2007: there have been no high-latitude sunspots observed with the expected Cycle 24 polarity; the configuration of the large scale white-light corona has not yet relaxed to a simple dipole; the heliospheric current sheet has not yet flattened; and activity measures, such as cosmic ray flux, radio flux, and sunspot number, have not yet reached typical solar minimum values.
In light of the expected long interval until the onset of Cycle 24, the Prediction Panel has been unable to resolve a sufficient number of questions to reach a single, consensus prediction for the amplitude of the cycle. The deliberations of the panel supported two possible peak amplitudes for the smoothed International Sunspot Number (Ri): Ri = 140 ±20 and Ri = 90 ±10. Important questions to be resolved in the year following solar minimum will lead to a consensus decision by the panel.
The panel agrees solar maximum will occur near October, 2011 for the large cycle (Ri=140) case and August, 2012 for the small cycle (Ri=90) prediction.
- Solar Cycle 24 Prediction Issued April 2007, Presented by the NOAA Space Weather Prediction Center (SWPC), National Oceanic and Atmospheric Administration (NOAA).
- Heinrich Schwabe, 1843, Astronomische Nachrichten, vol. 20., no. 495.
Related EoC Articles
- Solar Cycle Prediction (Updated 2009/04/02) - Solar Physics, Marshall Space Flight Center, NASA.
- Solar Cycle Progression - NOAA/Space Weather Prediction Center.
- Space Weather Prediction Center, National Weather Service, National Oceanic and Atmospheric Administration (NOAA).
- The Sunsprt Cycle, 1610-1976 - Solar Results From Skylab, NASA.
- The Sunspot Cycle (Updated 2009/04/02) - Solar Physics, Marshall Space Flight Center, NASA.
- This image shows the variation of sunspot number over time for solar cycle 11 (which began around 1867) and cycle 23, which began around 1996. The graphical representation of the sunspot cycles shows that the two cycles have similar shape and size and a long decrease to minimum. However, the numbers of sunspots in cycle 12 increased much more quickly than our new cycle, cycle 24. Sunspot cycle 23, peaked in 2001 and produced some of the largest flares on record (the "record" approximately equates to the space age, when we began observing X rays from the Sun). The background image is from Hinode's X-ray Telescope (XRT). View full-size image. (Source: NASA-HINODE - Current Update.)