Solar Cycle

The number of sunspots on the Sun is not constant. In addition to the obvious variation caused by the Sun's rotation (sunspots disappear from view and then re-appear), over time new sunspot groups form and old ones decay and fade away. When viewed over short periods of time (a few weeks or months), this variation in the number of sunspots might seem to be random. However, observations over many years reveal a remarkable feature of the Sun: the number of sunspots varies in a periodic manner, usually described as the 11 year cycle (in actuality, the period varies, and has been closer to 10.5 years this century). The 11 year sunspot cycle is related to a 22 year cycle for the reversal of the Sun's magnetic field. In 1848 Johann Rudolf Wolf devised a method of counting sunspots on the solar disk called the Wolf number. Today the Wolf number (averaged from many observing sites) is used to keep track of the solar cycle. While the cycle has been relatively uniform this century, there have been large variations in the past. From about 1645 to 1715, a period known as the Maunder minimum, apparently few sunspots were present on the Sun. During the solar cycle,the migration of sunspots in latitude has a ``butterfly pattern.''

Although the number of sunspots is the most easily observed feature, essentially all aspects of the Sun and solar activity are influenced by the solar cycle. Because solar activity (such as coronal mass ejections) is more frequent at solar maximum and less frequent at solar minimum, geomagnetic activity also follows the solar cycle. Why is there a solar cycle? No one knows the answer to this question. A detailed explanation of the solar cycle is a fundamental physics problem still waiting to be solved.

Last modified April 16, 2008 by Jennifer Bergman.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Sunspots

Sunspots are dark spots on the Sun. They may look small, but they are actually as bigas a planet like Earth or Mars! Sunspots are "dark" because they are colder than the areas around them. Of course, they...more

Maunder's Butterfly Diagram

Throughout the solar_cycle, the latitude of sunspot occurrence varies with an interesting pattern. The plot on the left shows the latitude of sunspot occurence versus time (in years). Sunspots are typically...more

Sunspots and Magnetic Fields

You probably have magnets on your refrigerator holding up some of your artwork or a photograph of a friend or family. Did you know that it is magnetic fields (which all magnets have) that make sunspots...more

Solar Cycle Variations and Effect on Earth's Climate

Did you know that the Sun has spots on it? The number of spots on the Sun changes. Sometimes there are lots of spots and other times there few spots. Scientists have wondered for a long time if the number...more

Scientists Discover Connections among the Solar Cycle, the Stratosphere and the Ocean

Scientists used information they got from over a 100 years of weather observations and from three powerful computer models to try and answer a very difficult question about the weather. They wanted to...more

Solar Cycle Linked to Global Climate

Scientists have discovered that changes that happen on the Sun have an impact on weather here on Earth. When the sun shines a lot in areas over the Pacific Ocean that do not have a lot of clouds, it heats...more

Lower Solar Activity Linked to Changes in Sun's Conveyor Belt

The sun goes through cycles that last approximately 11 years. These solar cycle include phases with more magnetic activity, sunspots, and solar flares. They also include phases with less activity. The...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA