What Are Sunspots? | The Old Farmer's Almanac

What Are Sunspots?

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Sunspots and Sunspot Activity

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The Old Farmer’s Almanac uses sunspot activity to help in making its weather predictions. What are sunspots? We explain.

What Are Sunspots?

Sunspots are more active, intense areas on the surface of the Sun where the magnetic force is so bundled up that heat can’t escape. As a result, the Sun’s surface at these areas is darker and cooler than its surroundings, although still very bright. 

For perspective, the surface of the Sun called the photosphere has a temperature of 5,800 degrees Kelvin. Sunspots have temperatures of about 3,800 degrees K. They look dark only in comparison with the brighter and hotter regions of the photosphere around them.

Solar Activity

To understand solar activity, one analogy is a rubber band.  The magnetic fields can get twisted like a rubber band, twisting and pulling and twisting, building energy.

Sometimes, when the magnetic field suddenly breaks around a sunspot, energy is released and plasma explodes into space. Think of that rubber band snapping! These explosions, called solar flares and coronal mass ejections, can cause magnetic storms that can temporarily knock out Earth’s satellites, electricity, and communications, and pose a hazard to astronauts.

Image: Solar Flare. NASA

Another analogy is a soda pop bottle being shaken. The pressure builds and builds inside the soda can until … pop! The energy explodes!

Astronomers study sunspots to determine when these magnetic storms will occur. Scientists also examine how sunspots may affect Earth’s climate. They look at …

Sunspot Size

Some sunspots are big as Earth while others are more than ten times that! They can get as large as 80, 000 km in diameter. Sunspots expand and contract as they move across the surface of the Sun.

Sunspot Number

The count varies; sunspots often appear in groups. We know that sunspot counts generally rise and fall every 11 years on the average.

Interestingly, sunspots often appear in pairs that are aligned in an east-west direction. One set will have a positive or north magnetic field while the other set will have a negative or south magnetic field.

The field is strongest in the darker parts of the sunspots—called the umbra. The field is weaker and more horizontal in the lighter part—the penumbra.

Overall, sunspots have a magnetic field that is about 1000 times stronger than the surrounding photosphere.

Sunspot Cycle

Sometimes the Sun contains a large number of sunspots, while at other times, few or none are seen. However, there does seem to be a fairly regular cycle in that sunspot counts rise and fall every 11 years on the average. 

The part of the solar cycle with low sunspot activity is referred to as “solar minimum.” It’s a period of relative calm on the Sun’s surface.

Conversely, the portion of the cycle with high activity is known as “solar maximum” or “solar max.”

Learn more about the current sunspot cycle and solar activity.

Life Span

Each sunspot can last from a few hours to a few months.

See Two Weeks in the Life of a Sunspot

Credit: NASA

Read about how The Old Farmer’s Almanac makes weather predictions.