Solar Cycle Stranger Things

I’ve reached the halfway point through my Introduction to Astronomy class. This week marks the eighth week of fifteen, sixteen if you count the first week where we just spent time getting to know each other and exploring the textbook and getting the lab software, Starry Night, installed and licensed. Last week, we reached the outer limits in the Kuiper Belt and Oort Cloud of our solar system where only comets and Voyagers I and II have ventured. Now we’ve snapped back to study our closest star, Sol, or more commonly just the Sun. My topic for discussion responds to the following question:

Why is the solar cycle said to have a period of 22 years, even though the sunspot cycle is only 11 years long?

Some surface features on our active Sun vary periodically in an eleven year cycle.  The Sun’s magnetic fields which cause the surface changes vary over a twenty-two year cycle.  The relatively cool and slightly darker regions, commonly called sunspots, are produced by local concentrations of the Sun’s magnetic field piercing the photosphere.  The latitude and number of sunspots on average vary during the same eleven year cycle.  But the hemisphere where the Sun’s north magnetic pole anchors during one eleven year cycle will have south magnetic poles during the next.  Because it takes a full twenty-two years for the magnetic poles to return to their original orientation astronomers refer to the entire solar cycle.   The magnetic dynamo model posits that many transient features of the solar cycle are caused by the effect of differential rotation and convection on the Sun’s magnetic field.  The Sun’s differential rotation (different speeds at different latitudes) causes its magnetic field to become increasingly stretched like a rubber band.  The bands can’t break so they periodically untangle themselves with the help of trapped gases which leak out (sunspot) and gradually settle back under the photosphere, when the sunspot disappears.  The most recent reversal of the Sun’s magnetic field occurred in 2013.  We are currently at the tale end of Solar Cycle 24.  (Comins, 2015, p.  272-83)

From the NASA Solar Cycle Primer:

The sunspot cycle happens because of this pole flip — north becomes south and south becomes north—approximately every 11 years. Some 11 years later, the poles reverse again back to where they started, making the full solar cycle actually a 22-year phenomenon. The sun behaves similarly over the course of each 11-year cycle no matter which pole is on top, however, so this shorter cycle tends to receive more attention. (Fox, 2011)

Trends and Predictions:
In 1893, British astronomy E. Walter Maunder compiled historical observations to conclude that no sunspots occurred from 1645 through 1715 (now commonly referred to as the Maunder minimum) and coincided with a period of cold so extreme in Europe it is now known as the Little Ice Age. Back home here in North America, a severe drought occurred during that time frame. (Comins, 2015, p. 279)

By Cycle 25 or 26, magnetic fields may be too weak to punch through the solar surface and form recognizable sunspots at all, spelling the end of the sunspot cycle phenomenon, and the start of another Maunder Minimum cooling period perhaps lasting until 2100. (Odenwald, 2016)