The Solar Cycle
The Sun's core is made of dense, electrically charged gas called plasma. This roiling, boiling plasma generates the Sun's powerful magnetic field. Earth has a magnetic field too. Like Earth's magnetic field, the Sun's magnetic field has a north pole and a south pole. However, unlike Earth, on the Sun the magnetic field is complicated—you might even say messy!
About every 11 years, the Sun's magnetic field does a flip. In other words, the North Pole becomes the South Pole, and the South Pole becomes the North Pole.
The Sun also does something else every 11 years. Its storminess builds up to a maximum, then it settles back down to a minimum. This repeated behavior is called the solar cycle. When the Sun is the most stormy, that's when its magnetic field flips. Scientists are not sure what the storminess has to do with the magnetic field flipping, if anything.
This series of 11 X-ray images of the Sun's atmosphere was made between 1991 (at solar maximum) and 1995 (near solar minimum). When the Sun is most stormy, it emits the most X-rays. So, in X-rays, the Sun looks about 100 times brighter at solar maximum than at solar minimum. Credit: NOAA.
Sunspots are areas of very strong magnetic forces on the Sun's surface. They look darker than their surroundings because they are cooler. Even so, when there are lots of sunspots, the Sun is actually putting out MORE energy than when there are fewer sunspots. During solar maximum, there are the most sunspots, and during solar minimum, the fewest.
Through special filters, sunspots may look like the picture on the left. The sunspot groups are as big as the giant planet Jupiter! On the right is a close-up of some other sunspots. The larger sunspot on the right is bigger than Earth! Credit: SOHO (NASA & ESA) and the Royal Swedish Academy of Sciences.
Solar flares happen because of the constantly moving magnetic fields in the Sun's atmosphere. As the Sun approaches solar maximum (the most active part of its 11-year cycle), its magnetic fields become messier and messier. The magnetic fields loop around, and cross over each other, cutting each other off, and reconnecting.
Have you ever tried sprinkling iron filings on a bar magnet? The iron filings line up along the magnetic lines of force. See the picture in the sidebar above.
Similarly, the hot plasma on the Sun's surface is at the mercy of the magnetic lines of force. Sometimes the plasma gets disconnected from the magnetic fields when the fields come together. Then particles in the hot plasma can speed up greatly and send powerful radiation into space. This is a solar flare.
When the solar cycle is at a minimum, active regions are small and rare and solar flares do not occur very often. They occur more often as the Sun gets near the maximum part of its cycle.