The surface of the Sun, called the photosphere, appears smooth and perfect to most observers. But a closer look at the surface through a high powered telescope or other scientific instrument reveals a violence unlike anything we experience on Earth. One of the key, defining features of the surface is the occasional presence of sunspots.
What are Sunspots?
Beneath the Sun's photosphere lies a complex mess of plasma currents, magnetic fields and thermal channels. Overtime, the rotation of the Sun causes the magnetic field to become twisted, causing the flow of thermal energy to and from the surface to be interrupted.
It's all a bit confusing, to be sure, but the end result is that the twisted magnetic field can sometimes pierce through the surface, creating an arc of plasma, called a prominence, or a solar flare.
The point(s) on the sun where the magnetic field emerges prevents heat from flowing to the surface, creating a relatively cool spot (roughly 4,500 kelvin instead of the usually balmy 6,000 kelvin) on the photosphere.
This cool "spot" appears dark compared to the surrounding inferno that is the Sun's surface. And it is this Black dot that we call a sunspot.
How Often Do Sunspots Occur?
The appearance of sunspots is entirely due to the war between the twisting magnetic fields and plasma currents beneath the photosphere. So the regularity of sunspots is therefore dependent on how twisted the magnetic field has become (which is also tied to how quickly or slowly the plasma currents are moving).
While the exact specifics are still being investigated, it seems that these subsurface interactions have a historical trend.
The Sun appears to go through a solar cycle about every 11 years or so. (It's actually more like 22 years, as each 11 year cycle causes the magnetic poles of the Sun to flip, so it takes two cycles to get things back to the way there were.)
As part of this cycle the field becomes more twisted, leading to more sunspots, until the field becomes so twisted that it completely unravels. The peak activity is known as solar maximum.
Once the magnetic field is unraveled, the activity drops again, leading to solar minimum. There have also been periods in history where solar activity has dropped for an extended period of time, effectively staying to solar minimum for years or decades at a time.
A 70 year span from 1645 to 1715, known as the Maunder minimum, is one such example. It is thought to be correlated with a drop in average temperature experienced across Europe. This has come to be known as "the little ice age".
There is also some evidence to suggest that we are heading to another extended period of solar inactivity, similar to the Maunder minimum.
