SunLearn About Sunspots, the Sun's Cool, Dark Regions

sunspots and loops
Magnetic field lines extend out from sunspots, channeling superheated plasma out from the subsurface of the Sun. Image Credit: NASA

When you look at the Sun you see a bright object in the sky. Because it's not safe to look directly at the Sun without good eye protection, it's difficult to study our star. However, astronomers use special telescopes and spacecraft to learn more about the Sun and its continual activity.

We know today that the Sun is a multi-layered object with a nuclear fusion "furnace" at its core. It's surface, called the photosphere, appears smooth and perfect to most observers.

However, a closer look at the surface reveals an active place 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. Over time, the rotation of the Sun causes the magnetic fields to become twisted, which interrupts the flow of thermal energy to and from the surface. The twisted magnetic field can sometimes pierce through the surface, creating an arc of plasma, called a prominence, or a solar flare.

Any place on the Sun where the magnetic fields emerge has less heat flowing to the surface. That creates a relatively cool spot (roughly 4,500 kelvin instead of the hotter 6,000 kelvin) on the photosphere. This cool "spot" appears dark compared to the surrounding inferno that is the Sun's surface. Such black dots of cooler regions are what we call sunspots.

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 depends 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 they were.)

As part of this cycle, the field becomes more twisted, leading to more sunspots. Eventually these twisted magnetic fields get so tied up and generate so much heat that the field eventually snaps, like a twisted rubber band. That unleashes a huge amount of energy in a solar flare. Sometimes, there's an outburst of plasma from the Sun, which is called a "coronal mass ejection". These don't happen all the time on the Sun, although they are frequent. They increase in frequency every 11 years, and the peak activity is called solar maximum.

Nanoflares and Sunspots

Recently solar physicists (the scientists who study the Sun), found that there are many very tiny flares erupting as part of solar activity. They dubbed these nanoflares, and they happen all the time. Their heat is what is essentially responsible for the very high temperatures in the solar corona (the outer atmosphere of the Sun).

 

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".

Solar observers have noticed another slowdown of activity during the most recent solar cycle, which raises questions about these variations in the Sun's long-term behavior. 

Sunspots and Space Weather

Solar activity such as flares and coronal mass ejections send huge clouds of ionized plasma (superheated gases) out to space.

When these magnetized clouds reach the magnetic field of a planet, they slam into that world's upper atmosphere and cause disturbances. This is called "space weather". On Earth, we see the effects of space weather in the auroral borealis and aurora australis (northern and southern lights). This activity has other effects: on our weather, our power grids, communication grids,and other technology we rely on in our daily lives. Space weather and sunspots are all part of living near a star. 

Edited by Carolyn Collins Petersen