Sunspots, Solar Cycles, and the Little Ice Age

Sunspots, Solar Cycles, and the Little Ice Age

via One-Minute Astronomer de admin em 16/09/09

In the mid 19th-century, after many years of diligent solar observation, the German astronomer Samuel Schwabe noticed the number of sunspots visible on the sun's disk rises and falls in nearly regular 11-year cycle.

In peak years of the cycle, he found, there were spots visible on the sun most days, and hundreds of spots and groups of spots during the course of a year.

In lean years, roughly 5.5 years after the peak, there were weeks or months when astronomers saw not a single sunspot, with fewer than a dozen spots observed during the year.

This regular cycle was traced back to the earliest telescopic observation of the sun in the 1600's, and has been recorded in increasing detail by modern astronomers right up to the present day. (The diagram below shows a measure of the sunspot number over time since the 17th century. The 11-year period is clearly visible).

The number of sunspots over time since the year 1600. On average the number of spots varies with an 11-year period

What's even more interesting is where the sunspots occur on the solar disk. As the number of sunspots rises from a minimum, the spots appear north and south of the solar equator, usually in the mid solar latitudes around 40 degrees.

As the cycle continues, sunspots appear closer to the equator until at the next minimum, most spots appear in a narrow band at the equator. Then, it all starts over again. A plot of the position of sunspot position over time, shown below, is called the Maunder diagram or the "Butterfly Diagram".

The solar "Butterfly Diagram", which shows the solar latitude of new sunspot groups over time in a number of solar cycles

This periodic change in the number and position of sunspots was a striking discovery. And it's not just the number of sunspots that varies every 11 years. The sunspot cycle also matches up with other activity on the sun, including violent mass ejections, the size and extent of the outer reaches of the sun called the corona, and the intensity of light and charged particles the sun blasts out into space to affect the atmospheres and magnetic fields of many planets in the solar system, especially Earth.

This near-regular 11-year variation in sunspots and other solar activity is called the solar cycle. It seems to be but a symptom of a complex and dimly understood dynamic process inside the sun called a "solar dynamo" that generates the sun's global magnetic field. To make matters even stranger, the magnetic field seems to flip its polarity once every two sunspot cycles, or 22 years, which means the magnetic polarity of the sunspot pairs on the solar disk flips direction from cycle to cycle.

Right now, we're coming out of a period of minimum sunspot activity, when very few sunspots were seen. The last cycle was labeled Cycle 23, and it peaked in mid-2000. The next is Cycle 24. (By convention, the sunspot cycle that began in the years 1755-56 is called Cycle 1). However, Cycle 24 has been slow to get started… there have been far fewer sunspots than expected. Many astronomers expect we'll see many more sunspots in this cycle quite soon.

But others suspect we're due for a period of relatively few sunspots. It's happened before, as you can see in the first diagram above. The period from 1645-1715, for example, had a sudden and unexplained disappearance of sunspots. The time is called the Maunder Minimum, and it coincided almost exactly with a chilly period in our climatic history called the "Little Ice Age".

Is it a coincidence the Maunder Minimum overlapped with the Little Ice Age? Perhaps not!

The effect of sunspots and solar activity on Earth's atmosphere and climate will be the subject of our next article in this series on the sun.