The 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. (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. 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. 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.


Teacher Notes:


How do scientists use patterns to determine the cycle of the Sun?

Patterns of change are important for predicting what will happen. This is important for the design of technological systems. A sequence of changes that happens over and over again is cyclical. The Sun's cycle is characterized by how large the range of variation is from maximum to minimum, how long the cycle takes (11 years) and when it peaks. The extent of variation during a cycle can disrupt the system.

(K-2) Things change with time in different ways, such as in size, weight, color, and movement. Some small changes can be detected by taking measurements.

(3-5) Things change in steady, repetitive, or irregular ways---or sometimes in more than one way at the same time. Often the best way to tell which kinds of change are happening is to make a table or graph of measurements.

(6-8) Cycles, such as the seasons or body temperature, can be described by their cycle length or frequency, what their highest and lowest values are, and when these values occur. Different cycles range from thousands of years down to less than a billionth of a second.

(9-12) Graphs and equations are useful (and often equivalent) ways for depicting andanalyzing patterns of change.



Science Nuggets:

2000...Ulysses discovers that high speed solar wind streams seen during solar minimum in 1994 over the south pole, are absent during solar maximum conditions in 2000.

1999...Ulysses, Voyager, and IMP-8 discover that solar wind pressure changes during the solar activity cycle occur at all solar lattitudes.

1997...Yohkoh satellite observes a global change in the Sun's magnetic field during a sunspot cycle., by imaging the X-rays from the Sun.

1997...SOHO discovers the "magnetic Carpet" on the Sun; a small-scale field that is constantly being regenerated every 2-3 days.