Understanding the Solar Cycle
An Explanation of Sunspots and the Sun's Magnetic Field
What is the solar cycle and how long does it last? What are the effects of the sun's changing cycle?
The solar cycle is sometimes called the solar magnetic activity cycle or the sunspot cycle.
Definition of a Solar Cycle
A solar cycle is the time period from solar minimum to solar minimum, as measured by sunspots. Every 11.1 years on average, the sun reverses polarity and sunspots move from mid-latitudes toward the equator over the course of a cycle. For example, a new solar cycle (number 24) began January 4, 2008, as a new sunspot appeared on the sun that was at a higher latitude and opposite polarity of the sunspots that preceeded it. The sunspot that signaled the start of the new solar cycle was labeled number 10,981 and the leading half of the sunspot showed negative polarity.
In the gray image, sunspots from the previous solar cycle, number 23, can still be seen near the equator of the sun. The solar cycle is usually referred to as lasting 11 years (although it can vary a year or two in either direction), but occasionally the solar cycle is referred to as a 22-year cycle, in regards to the polarity returning to what it was previously as the sun's magnetic poles reverse again.
Understanding Sunspots
Sunspots, which are dark, cool, active regions on the sun that tend to occur in clusters, generally have two main locations of opposite polarity. The half that leads the spot across the sun is called the preceeding and usually is a bit closer to the sun's equator also. The other half is called the following spot. It is these two sections that swap polarities when a new solar cycle begins. It is also this opposite polarity region that causes violent flares and prominences that can leap up from the sun, snapping off and hurtling themselves toward Earth.
The Butterfly Diagram
When a new solar cycle begins, sunspots start forming at higher latitudes in both the north and south hemispheres, farther from the equator. This is the solar minimum, when there are fewer spots and therefore less activity on the sun. Over the course of the cycle the sunspot number and solar activity will increase as the spots more closer to the equator. This time of more solar storms is called solar maximum. As the activity slowly dies down, the sunspots continue to move toward the equator and taper off in numbers until solar minimum is reached again. A diagram of the positions of the sunspots on the sun looks a bit like a butterfly flying to the left, as the outstretched wings mark the furthest north and south reaches of sunspots. The movement of the sunspots from mid-latitudes toward the equator is known as Sporer's Law.
Maunder MInimum
Astronomers of old noticed a strange event between 1645 and 1715. Fewer sunpots were seen on the sun, with the periods of solar maximum being much quieter than years that came before and after.
