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Solar Activity Cycle

In the mid-1800s astronomers discovered from thousands 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.

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.

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. We also know from graphs of the sunspot cycle that sometimes the Sun just stops making them altogether. This happened in the 1600s, and this was also the time when Europe was in the grip of what they called a mini-Ice Age.

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.

Source:NASA

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