Background:
Sunspots have been observed on the surface of the Sun for thousands of years beginning with the meticulous records kept by ancient Chinese observers. It wasn't until the advent of the telescope in the early 1600's and its use by Galileo that they were 're-discovered' by western science since the Sun was presumed to be a perfect orb, free of any flaws! Sunspots are a student's most familiar contact with the idea that the Sun changes its appearance in time in a regular way, and that these changes mark times when the Sun is active and capable of producing hazardous storms.
Sunspots are actually regions of the solar surface where the magnetic field of the Sun becomes concentrated over 1000-fold. Scientists do not yet know how this happens. Magnetic fields produce pressure, and this pressure can cause gas inside the sunspot to be in balance with the gas outside the sunspot...but at a lower temperature. Sunspots are actually several thousand degrees cooler than the 5,770 K surface of the Sun, and contain gases at temperature of 3000 to 4000 K. They are dark only by contrast with the much hotter solar surface. If you were to put a sunspot in the night sky, it would glow brighter than the Full Moon with a crimson-orange color!
The ebb and flow of solar activity is traced by the rise and fall of the number of sunspots you see on the surface in what is called the Sunspot Cycle. Although one cycle lasts about 11 years and we are currently at the beginning of Cycle 23 since records were kept, A full magnetic activity cycle takes 22 years for the magnetic field of the Sun to make a complete flip in polarity from North to South and back again in each hemisphere. This 22 year cycle of magnetic field reversal is also reflected in the Earth's magnetic field which takes about 400,000 years to make a complete cycle.
The sunspot cycle seems to be correlated with Earth's climate in some way scientists are still trying to understand. Between 1600-1700 there were very few sunspots seen and no cycle was at all apparent. Europe experienced what was called the Mini Ice Age during this 'Maunder Minimum'. Then in the early 1700's the first traces of the sunspot cycle began with increasing strength. The sunspot cycle we have known for the last 200 years seems not to be a permanent feature of the Sun...and when it is absent...some regions of the Earth experience very cold climates. We think that, like the proverbial miner's canary, sunspots indicate how active the Sun is, and that this is influenced by the Sun's brightness. An almost unmeasurable 1 percent reduction in the Sun's solar energy output may be enough to stop the sunspot cycle, and bring on cooler weather on the Earth. There seems to be no forewarning of when this can happen, with disastrous consequences for human activity on Earth!
Web Resources:
| The Sun's surface today! | Here is what the Sun looks like today as seen by a variety of space satellites from X-ray to visible wavelengths! |
| The Exploratorium's Sunspot Primer | An illustrated guide to the history of sunspot studies and what they mean. |
| Sunspot Numbers | Have a look at this site to learn about sunspots, and also see their plot of sunspots since 1600 showing the cycles. |
| NOAA Archive of Sunspot Numbers | Archives of sunspot numbers in ASCII format for each day, month and year since 1600, suitable for plotting to show the cycles. |
| Mr. Sunspot's Answer Book | National Solar Observatory web resource with great FAQs about the Sun and sunspots among many other related topics. |
| Stanford Solar Center Sun Pages | Great introductory material, FAQs and other resources and links. |