The rise and fall of solar activity follows an approximately 11-year cycle. At the beginning on the cycle, activity on the Sun is common; sunspots dot the surface, flares erupt, and coronal mass ejections blast out into space. And then solar activity slows down over 5.5 years (on average) to the point that the surface of the Sun can be relatively clear of sunspots, and weeks go by without a powerful flare. And then, about 5.5 years later, the Sun is active again.
Astronomers have learned that the amount of solar activity is tied to the Sun’s twisting, turning magnetic field. The polarity of the magnetic field follows a 22-year cycle. Halfway through the cycle, the Sun flips its magnetic field; the north pole becomes the south pole, and vice versa. Why this happens is a bit of a mystery, but it could have something to do with the Sun’s interaction with the planets.
Some examples of large-scale solar activity: flares and coronal mass ejections, filament eruptions, and jets. These occur when the Sun’s magnetic field lines suddenly snap to a new configuration.
Perhaps one of the best ways to measure solar activity is to count sunspots. The sunspots are relatively cooler regions on the surface of the Sun, where the magnetic field lines pierce the surface. They come in pairs, with a magnetic loop connecting them. This is where the flares and coronal mass ejections emanate from.
The last solar maximum, the time of greatest solar activity, happened in 2000, and the next is due for 2012.
Want to know what the space weather’s doing right now? Check out the space weather website.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.